Homework answers / question archive / I will send you the pictures of the questions May 10, 2021 at 8:00 AM Pacific Time (US)

I will send you the pictures of the questions May 10, 2021 at 8:00 AM Pacific Time (US)

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I will send you the pictures of the questions May 10, 2021 at 8:00 AM Pacific Time (US). The questions has a time limit of 2 hours and it will have 25-30 multiple choice questions !! So it needs to be done in that time limit. Please review the uploaded documents so that way you have an idea of what the questions will be about! The questions is based on 6 units I will also upload to you the overall summary of the units and also some sample questions.

Also make sure you look over the links and review ALL of the HTML because they are all about units and because it will help you prepare for the questions!

In this link are the 2 units Unit EM and Unit PEF NextGenPET Student Resources (sdsu.edu)

In this link are the 2 units FM and CF (NextGenPET Student Resources (sdsu.edu))

In this link are the unit M and SE (NextGenPET Student Resources (sdsu.edu)

Sample  Questions-­UNIT  EM     1. A professor does a demonstration where he gives a block a quick shove along a track. After it leaves his hand the block slows down and comes to a stop after moving for about one meter. Students in the class are asked to explain in terms of energy why the block slowed down after the shove, and to include all the relevant interactions and energy changes in their explanation. Here is an explanation from one student. (1) After the block leaves the professor’s hand, there is a [friction-type] contact push/pull interaction between the block and the track. (2) Energy is transferred from the block to the track, causing the block to slow down and the track to warm up. (3) Because the track warms up there are heat interactions between the track and the surroundings, transferring energy to the surroundings, causing the surroundings to warm up somewhat. Evaluate this explanation for accuracy. Consider each of the three sentences. A. B. C. D. It is accurate as it is. It is not accurate because just sentence #1 is incorrect. It is not accurate because just sentence #2 is incorrect. It is not accurate because just sentence #3 is incorrect. 2. Alicia is preparing to go play a game of soccer. She grabs a cooler, fills it with very cold water (because she has no ice), puts a warm can of soda in it, but leaves the cooler open to the air. [The soda can is totally immersed in the water.] Thirty minutes later, Alicia enjoys a cool, refreshing drink. Which G/R energy diagram below best describes the interactions between the soda can, water and the surroundings during the time that the water, soda can and surroundings are still changing their temperatures? Below is a G/R energy diagram describing interactions between a dry cell battery, a small, but noisy, heater, the sound receivers and the surroundings, during a period of time when the battery and noisy heater have already reached their constant operating temperatures. You can assume this G/R energy diagram is accurate. 3. Suppose during a short period of time the battery decreased in CPE by 80 joules, and the heater transfers 64.8 joules of energy to the surroundings via heat interactions. Assume the battery has an efficiency of 90%. During this same period of time, what was the total increase in thermal energy in the surroundings? A. B. C. D. 4. 64.8 joules 72 joules 72.8 joules 80 joules For the same scenario described above, what is the efficiency of the heater? A. 10% B. 81% C. 90% D. 20%   Sample  Questions-­UNIT  FM  &  CF   1. Sometime in the future two small lunar buggies are having a drag race on the Moon (meaning there is no air). Buggy A has a total mass of 700 kg and its motor generates a force of 2800 N to push it forward, while Buggy B has a total mass of 800 kg with its motor generating a forward force of 3200 N. Assuming the effects of friction are negligible, which of these buggies will win the race? A. Buggy A will win. B. Buggy B will win. C. The race will be a tie. D. It is impossible to tell. 2. Here is a force diagram for an object at a particular moment in time. Which of these speed-time graphs would represent the motion of this object, assuming the same forces continue to act on it for the whole time period shown on the graph? 3. The diagram below shows a speed-time graph for a cart that was moving along four segments of track. The graph consists of four straight-line segments labeled I through IV. During which segment(s) was the force acting on the cart in the direction of motion stronger than the force acting on it in the opposite direction to its motion? A. I only B. I and III C. I, III, and III D. III only E. All four segments 4. Here is a force diagram for an object at a particular moment in time. Which of these speed-time graphs would represent the motion of this object, assuming the same forces continue to act on it for the whole time period shown on the graph? A. C. B. D. 5. As Carlos and Manual both push a 4 kg box to the right across a rough floor, the box speeds up at a rate of 1.2 m/s2. If you assume that Manual pushes with a force strength of 8.0 N and the friction force of the floor on the box is 5.0 N, then what is the strength of the force that Carlos pushes on the box? [In the diagram below the force arrow for Carlos is drawn of arbitrary length, so do not assume it is correctly drawn.] A. 0.33 N B. 1.8 N C. 4.6 N D. 6.0 N 6. A low-friction cart has two fan units mounted on it that push in the same direction with the same strength forces. The cart is released so it will move along a track. After 1.5 seconds the blades on one of the fan units jams and the fan stops turning immediately, so it is not pushing the cart any more. The other fan continues working. Assuming the effects of friction can be ignored, which of the speed-time graphs below best represents the motion of the cart? A. B. C. D. Sample  Questions-­?UNIT  PEF   1.  A  person  holds  two  carts  with  a  spring  between  them.           The   person   pushes   the   two   carts   together,   compressing   the   spring,   and   then   lets   go   of   the   carts.     Which   of   the   following   G/R   energy   diagrams   most   accurately   describes   the   interaction   during   the   time   when   the   spring   is   uncompressing   and   the   carts   are   speeding   up   as   they   move   away   from   each   other?     (Ignore   any   effects  due  to  friction.)       2. Below is a snapshot from a simulation showing a skateboarder (a girl) ready to slide down a track. She starts at rest in the upper left (as shown) and begins to move along the track. Points I and III on the track are at the same height, and point IV is much higher than the starting point. Assume there is a negligible amount of friction, so neither the skateboarder nor the track warms up. Which one of the following statements would be accurate in this situation? A. When she reaches point IV and when she does she will have the maximum amount of gravitational potential energy. B. She has the same amount of kinetic energy at point III as she does at point I. C. Her kinetic energy at point II is greater than her kinetic energy at either point I or point II. D. Her total energy at point III is less than than her total energy at point II. 3. Below, on the left, is a compass with its needle pointing straight upward. To the right is a magnet. If the compass were placed at each of the four positions shown (X, Y, Z and W), at which position, if either, would the compass needle point upwards? A. B. C. D. E. F. Position X Position Y Position Z Position W At more than one of the positions At none of the positions G.   Sample  Questions     1. The North Pole of a bar magnet is slid across a nail from its tip end to its head end. The same process is repeated several times. How would you now describe the nail? A. The nail is not magnetized. B. The nail is magnetized with its tip end a South Pole and its head end a North Pole. C. The nail is magnetized with its tip end a North Pole and its head end a South Pole. 2. A group of students are in the process of developing a model to explain magnetism phenomena. They know about the Law of Magnetic Poles; like poles repel and unlike poles attract. At one point they are considering the following model, suggested by a group member. Inside the nail there are many tiny magnets, each with a North and South Pole. In the unrubbed nail, the tiny magnets are oriented randomly, with just as many N Poles pointing to the right as to the left, as shown in the picture below to the left. When the nail is rubbed from its tip to its head with the North Pole end of a magnet, all the tiny magnets become aligned, with their S Poles pointing towards the head end of the nail, and they are dragged to the head end of the nail, as shown in the picture below to the right. Before Rubbing After Rubbing Which of the following observations, if any, could this model explain? A. The head end of the rubbed nail acts like a North Pole and the tip end acts like a South Pole. B. If two nails are rubbed exactly in the same way, and the head ends are brought near each other, they repel each other. C. If the nail is cut anywhere along its length, the two pieces each still behaves like a rubbed nail. D. This model can explain all the above three observations. E. This model cannot explain any of the above three observations.   3.  Which  of  the  following  diagrams  best  shows  how  a  paperclip  is  magnetized  after   being  rubbed  with  the  North  end  of  a  magnet  from  one  end  to  the  other  as  in  the   picture  below?   S       N             A   B   S  N     N  S   NS     S  N     S  N     N  S   S  N     N  S     N S   S   S     N   N N     S   S   N     S  N     S  N     N  S   N  S   N  S   S  N     SN   NS       NS             C   D   S  N     S  N     S  N     S  N     S  N     N  S   N  S     N  S   N  S   N  S     S  N     NS     S  N     N  S     S  N     S  N     S  N     N  S   N  S   N  S   N  S     S  N     S  N     N  S           UNIT EM Summary Ideas ENERGY MODEL of INTERACTIONS In this unit, you developed your understanding of a model that explains the effects of interactions in terms of ideas about transfers of energy between objects and how they cause different types of energy associated with them to change. Below we give a historical perspective on scientists’ development of similar ideas. Historical Perspective Up until the end of the 18th century, many of the models of different types of interactions were largely based on the idea of a special substance that flows from one object to another. For example, phenomena involving temperature changes were explained using a model in which an invisible, weightless, fluid called caloric flowed between objects with different temperatures. In 1798, Count Rumford rejected this idea, arguing that it could not account for the enormous amount of heat produced in frictiontype contact push/pull interactions. Instead he suggested that heat was really a form of something called ‘energy’. Throughout the 19th century, Sir James Prescott Joule spent much of his time trying to understand the relationship between the energy associated with contact push/pull interactions and the changes in temperature that occur as a result of frictional effects. He developed a simple apparatus that allowed him to measure the relationship between changes in kinetic energy and changes of the temperature of water that was stirred through a contact push/pull interaction. He found that the change in energy of the moving objects during the experiment was equal to the energy necessary to change the temperature of the water from its initial to its final value. Based on work by Sadi Carnot (1842) and Rudolph Clausius (1850) the ideas emerged that: i) there are many forms of energy, and that they can be transformed from one form to another; and ii) energy can be transferred between objects, but only while they interact with each other. In a lecture in 1846, William Thompson (Lord Kelvin), referring to the work of Joule, announced that in his view, energy had become the primary concept on which physics was to be based. In 1847, Hermann von Helmholtz used mathematics to express that the effects of contact push/pull, light, heat, electric, and magnetic interactions were different manifestations of energy. In 1852 and 1855, W. J. Rankine declared that the term ‘energy’ could be applied to “ordinary motion and mechanical power, chemical action, heat, light, electricity, magnetism, and other powers, known or unknown, which are convertible or commensurable with these.” Just a few years later, Michael Faraday published an essay called ‘On the conservation of force.’ He understood this to mean the transformability and indestructibility of natural powers. In the essay, Faraday discussed the ambiguities of the phrase because he understood that force can be applied and removed. Rankine argued that a better way to express what Faraday was trying to say is the phrase ‘conservation of energy’ which was not ambiguous, for it is energy that is not created or destroyed. ©2017 Next Gen PET EM-S1 Unit EM Basic Energy Model of Interactions Idea E1 – Energy Model of Interactions During an interaction, two objects act on or influence each other to cause some effect. During the interaction, energy is transferred between the two objects. One object is the energy giver (where the energy comes from) and the other object is the energy receiver (where the energy goes to). Because of this transfer of energy, there is a decrease in some type of energy within the giver and an increase in some type of energy within the receiver. Idea E2 – The energy model of an interaction can be described using a giver/receiver (G/R) energy diagram. The energy model for a single interaction can be represented using a diagram like this. By convention: i) The type of interaction is identified. ii) The names of the interacting objects are included within rectangles. _________ Interaction Energy Receiver Energy Giver Giver object Energy Decrease in Receiver object Increase in __________ __________ energy energy iii) The energy transferred is indicated by a broad arrow iv) The energy changes within the objects are included in ovals attached to the objects. In this unit, you examined several types of interactions and associated energy changes. These are summarized in the tables below and described in more detail later in this document. Interaction Conditions under which it occurs Contact Push/Pull (CPP) Objects in contact push or pull on each other. Heat Conduction (HC) Objects in contact have different temperatures. Infrared Radiation (IR) Non-touching objects have different temperatures. Convection (Con) Fluid (liquid or gas) flows between non-touching objects with different temperatures. Heat Interactions (HI) A collective name used whenever two objects interact due to a temperature difference between them. [This can be used instead of the individual HC, IR, or Con types of interactions, since in almost all cases at least two of these types of interactions are occurring simultaneously.] Electric Circuit (EC) An electrical energy source is connected to an electrical device in a circuit. EM-S2 Summary Ideas: Energy Model of Interactions Light Light moves from a source to a light receiver. Sound Sound moves from a source to a sound receiver. Energy Type Evidence for change Kinetic (KE) Change in speed Chemical Potential (CPE) Use of muscles, battery powering an electrical device, or burning of some type of fuel (such as gasoline) Thermal (TE) Change in temperature ‘Energy’ Used as a generic term when changes are complex or varied, such as for light and sound receivers Energy Model of Contact Push/Pull (CPP) Interactions Idea C1a - Definition of a CPP interaction A CPP Interaction occurs when any two touching objects push or pull each other. In the absence of an equally strong opposing CPP interaction, there is a change in the speed (and/or direction) of at least one of the objects involved. Idea C1b - Duration of CPP interactions and changes in speed CPP interactions have a definite duration in time, and changes in speed take place gradually and continuously all the time the interaction continues. Idea C1c – Energy transfer and changes during a CPP interaction In terms of energy, during a CPP interaction there is a transfer of energy between the interacting objects. The type of energy associated with an object’s motion is called kinetic energy, thus when the speed of an object changes during a contact push/pull interaction, so does its kinetic energy. We identified two types of CPP interactions, which are described in ideas C2 and C3. These ideas assume that the particular interaction in question is acting in isolation. That is, there is no opposing CPP interaction of equal strength that would cause there to be no change in speed (and/or direction) of the objects involved. EM-S3 Unit EM Idea C2 - CPP interactions involving rigid objects One type of CPP interaction occurs when two touching non-elastic (i.e., rigid or stiff) objects push or pull on each other. In such CPP interactions, the speed (and/or direction) of at least one of the non-elastic objects changes while they are touching and push or pull on each other. As an example, here is a G/R energy diagram describing the CPP interaction between a person’s hand and a cart that causes the cart to speed up. (Assume the effects of friction can be ignored.) In terms of energy, there is a change in the kinetic energy of at least one of the objects involved. If a human person is the source of the CPP interaction, then as energy is transferred out from the person, the chemical potential energy in the person decreases. Idea C3 – CPP interactions involving friction: A CPP interaction also occurs when two surfaces rub against each other. The evidence of such a friction-type CPP interaction is that at least one of the objects involved decreases in speed while the temperature of both objects increases. In terms of energy, during a friction-type CPP interaction there is transfer of energy between the interacting objects and a decrease in the kinetic energy of at least one of the objects involved. There is also an increase in the thermal energy of both objects. As an example, here is a G/R energy diagram describing the CPP interaction in which a block slides across a table, slowing down as it does so. Though friction can never be totally eliminated, in some situations its effects can be neglected for simplicity. This is especially true during periods when other CPP interactions (such as strong pushes and pulls) are having a strong enough effect on an object that including the effects of friction would make little or no difference. EM-S4 Summary Ideas: Energy Model of Interactions Energy Model of Electric Circuit Interactions Idea EC1 – Definition of an Electric Circuit Interaction An electric circuit interaction occurs whenever a source of electrical energy (battery, solar cell, generator) is connected to an electrical device (bulb, buzzer, motor/fan, heater, etc.) in a complete circuit. Idea EC2 – Energy Model of Electric Circuit Interactions During an electric circuit interaction, energy is transferred from the electrical energy source to an electrical device. All electrical devices (sources and receivers) warm up when they first start to operate, meaning they increase in thermal energy. Different types of electrical energy source operate in different ways. A dry cell battery has no energy input, but is able to transfer energy to a device because there is a decrease in the chemical potential energy stored in the battery itself. Other devices, such as a solar cell or a generator, are able to transfer energy to a device because energy is transferred into them via another interaction. Energy Model of Heat Interactions Idea H1 – Definition of a Heat Interaction A heat interaction occurs between objects that have different temperatures. During such interactions, energy is transferred from the warmer object to the cooler object. When such interactions occur in isolation, the warmer object decreases in thermal energy (hence, it decreases in temperature) and the cooler object increases in thermal energy (hence, it increases in temperature). The interaction stops when the two objects reach the same temperature. (However, if energy is being transferred into the warm object at the same time via another interaction, it is possible that it will not decrease in thermal energy.) Scientists identify three specific types of heat interactions. Idea H1a - A heat conduction interaction occurs when energy is transferred between any two objects that are touching and have different temperatures. Idea H1b - An infrared interaction occurs when energy is transferred between any two objects that are near one another and have different temperatures. EM-S5 Unit EM Idea H1c - A convection interaction occurs when heat energy is transferred between any two objects that have different temperatures and a fluid (gas or liquid) is able to flow between them. Idea H2 – Multiple Heat Interactions Under most circumstances, more than one type of heat interaction can occur between two objects with different temperatures. (For example, when a cold can of soda sits on a warmer table there is a heat conduction interaction at the points where they actually touch, an infrared interaction between the nontouching parts, and a convection interaction because air can flow between them.) In such cases we simply group all the types of interaction together and call them ‘heat interactions’. Here is a G/R energy diagram for the cold soda can sitting on the warmer table. Idea H3 – Interactions with the surroundings All electrical energy sources and devices warm up when they begin to operate. Also, during any real-world CPP interaction between objects, the temperature of the objects will increase at least slightly due to the effects of friction. Because of this, some energy is always transferred from these warm objects to the surroundings (that is, all nearby and touching objects) via the three types of heat interactions. It is these interactions that are responsible for warm objects eventually cooling down. Depending on the particular situation, the amount of energy transferred to the surroundings via heat interactions may be small (even negligible) or it may be large, but in principle it always happens. The greater the temperature difference between the object and its surroundings, the greater the rate at which heat energy is transferred from a warm object to its surroundings. Idea H4 – Transient and Equilibrium States Any object can be both an energy receiver and an energy giver in different interactions at the same time. For example, consider a hand holding a hot cup of coffee. It is the energy receiver in a heat conduction interaction with the cup, but also an energy giver in heat interactions with the surroundings. EM-S6 Summary Ideas: Energy Model of Interactions What happens to the thermal energy of the hand in this case depends on the relative rates of energy transfer into and out of it. If the rate of energy input from the hot cup is higher than the rate of energy output to the surroundings, then the thermal energy of the hand will increase. If the rate of energy input is less than the rate of energy output, then its thermal energy will decrease. In both of these cases, the temperature of the hand changes, and it is said to be in a transient state with respect to its thermal energy. However, if the rates of energy input and energy output are exactly equal, then the thermal energy of the hand will not change. This means it will stay at a constant temperature, and it is then said to be in a state of dynamic equilibrium with respect to its thermal energy (usually just ‘equilibrium’ for short). All electrical devices (sources and receivers) warm up when they start to operate, but quickly reach a constant temperature that is warmer than the surroundings. (This constant temperature is sometimes called their operating temperature.) While they are warming up, they are in a transient state, but once the their temperature stabilizes, they then remain in an equilibrium state until the circuit is disconnected. This idea can also apply to devices with moving parts, such as a generator or motor fan. While they are speeding up after first being connected, they are in a transient state with respect to their kinetic energy, but they rapidly reach a constant speed, which is an equilibrium state with respect to kinetic energy. Note that it is possible for a device to be in an equilibrium state with respect to one form of energy, but not another. When we refer to a dry cell battery being in equilibrium, we mean this in regard only to its thermal energy. All the time it is operating it is still decreasing in chemical potential energy. Ideas Involving Energy Conservation Idea CON1 - Efficiency All electrical devices (and other manufactured items) are designed with a definite purpose in mind. The energy output associated with that purpose is useful energy. Any other energy outputs are not useful. (For example, the useful energy output from a light bulb is the energy is associated with light interactions. However, a bulb also has an energy output associated with heat interactions that is not useful.) EM-S7 Unit EM The energy efficiency of a device is defined as the fraction (converted to a percentage) of the total energy input that is output as useful energy: ?????????? ?? %=?????? ?? ?????? ?????? ??? ?????? ?????????????????? ?? ?????? ?????×100 For example, if a light bulb has an input of 50 J of energy, but only 4 J are output via light interactions, then the efficiency is: Eff = 4 × 100 = 8% 50 A few energy source devices, such as a dry cell battery, have no energy input while they operate, but instead produce an energy output from a decrease in some form of stored (potential) energy within them. In such cases€we define the efficiency of the device in a slightly different way: ?????????? ?? %=?????? ?? ?????? ?????? ??? ?????? ?????????????????? ?? ???????? ?? ?????? ????????? ??????×100 For example, if the chemical potential energy of a battery decreases by 60 J, but only 54 J are output via an electric circuit interaction, then the efficiency is: Eff = 54 ×100 = 90% 60 Idea CON2 – Keeping track of energy through a system A ‘system’ is any set of objects that we want to study. It can consist of a single object or a group of interacting objects, and it may, or may not, include the surroundings. We can draw a G/R energy diagram showing all the energy inputs, outputs, transfers, and changes associated with the objects in the system. Using the idea of efficiency, we may also be able to track amounts of energy through the system. Here is an example of such a diagram for a system consisting of a solar battery, a buzzer, some sound receivers, and the surroundings. EM-S8 Summary Ideas: Energy Model of Interactions 85 Light & Heat Increase in thermal energy 12 Interactions 85 Light Interaction Energy (from sun) 100 Energy Surroundings Heat Interactions Increase in thermal energy 12 Solar Battery (15%) Energy Energy Surroundings Energy Sound Receivers Buzzer (20%) 15 Electric Circuit Interaction 3 Sound Interaction Increase in energy 3 The input to this system is 100 J of energy from the Sun via a light interaction. The useful energy output from a solar battery is that which powers an electric circuit; since its efficiency is 15%, this means only 15 J are associated with the energy transfer to the buzzer. (The other 85 J is transferred to the surroundings via heat interactions, and since the surroundings have no output, this is the amount by which their thermal energy increases.) Now consider the buzzer. It has an energy input of 15 J (from the solar battery), but its efficiency of 20% means that only 3 J of its output are associated with making sound. This 3 J is transferred to some sound receivers and, since they have no energy output, their energy increases by this amount. (The other 12 J is transferred to the surroundings via heat interactions, and since the surroundings have no output, this is the amount by which their thermal energy increases.) When considered together, the solar battery + buzzer system has an energy input of 100 J, but a useful energy output of only 3 J. Thus the efficiency of this system is: Eff = 3 × 100 = 3% 100 Idea CON3 - Law of Conservation of Energy: € Energy cannot be created or destroyed, but only changed from one form to another and transferred between objects. In this unit we considered two situations where we EM-S9 Unit EM applied the Law of Conservation of Energy (COE). In one case we considered a system of interacting objects with a certain amount of energy being transferred into the system. In that case, according to COE, the amount of energy transferred into the system must equal all the increases in energy of the interacting objects. For example, suppose the system consists of a solar cell, motor, air and the surroundings, and that energy is transferred from the Sun to the solar cell (which is part of the system). If we consider the time period during which the solar cell and motor are still warming up and the motor blades are still speeding up (the transient state), then the Law of Conservation of Energy would take the form: Energy transferred into the system from the Sun = Increase in thermal energy of (solar cell + motor + surroundings) + Increase in kinetic energy of (motor + air) If, instead, we consider the time period after the solar cell and motor have reached their operating temperatures and the motor blades have reached their maximum speed (the equilibrium state), then the conservation of energy equation takes a simpler form: Energy transferred into the system from the Sun = Increase in thermal energy of surroundings + Increase in kinetic energy of air We also considered the case where no energy is transferred into the system, but instead there is a decrease in energy in one of the interacting objects. In that case, according to COE, the decrease in energy of the one object must equal the sum of all the energy increases in all the interacting objects. For example, suppose the system consists of a battery, bulb, light receivers and the surroundings during a time when the battery and bulb are still increasing in temperature (transient state). Then the Law of Conservation of Energy would take the form: Decrease in chemical potential energy in the battery = Increase in thermal energy of (battery + bulb + surroundings) + Increase in energy in the light receivers If, instead, we consider the time period after the battery and bulb have reached their operating temperatures (equilibrium state): Decrease in chemical potential energy in the battery = Increase in thermal energy of surroundings + Increase in energy in the light receivers EM-S10 Unit FM Summary Ideas Force Model of Interactions When studying contact push/pull interactions between objects, it is sometimes convenient to analyze and explain them in terms of a model that links the forces being applied to them to their motion, rather than in terms of the energy transfers between them. The link between the forces acting on an object and its motion fascinated scientists for a long time. Historical Perspective In the 4th century BC, the Greek philosopher Aristotle concluded, from watching the world around him, that in order for an object to move at a constant speed, it was necessary to maintain a constant push on it. As a consequence, he said that if no such push acted, it was the natural motion of an object to slow and stop. Amazingly, Aristotle’s ideas stood almost unchallenged for some 2000 years! It was not until the early 17th century AD that Aristotle’s ideas were seriously challenged, primarily by the Italian scientist Galileo. Among Galileo’s many contributions to science was his assertion that moving objects tend to slow and stop because of some outside resistance to their motion (usually friction), not because it is their natural tendency to do so. In the late 17th century Sir Isaac Newton, building on the work of Galileo and others, published his ideas, which have come to be known as Newton’s Laws. Written in a form relevant to this unit, Newton’s first two laws state: 1ST LAW: If no forces act on an object then, if it is at rest, it will remain at rest, and if it is in motion, it will continue to move in a straight line path at a constant speed. 2nd LAW: If a single force acts on an object, its speed and/or its direction will change. How quickly the speed (and/or direction) changes is directly related to the strength of the force and inversely related to the object’s mass. These statements of Newton’s Laws contain many ideas that likely correspond closely to the ideas you have developed in this unit, based on evidence you gathered yourself. ©2017 Next Gen PET FM-S1 Unit FM Idea N1 – Interactions described in terms of forces The interaction between two objects may be described in terms of the force (a push or pull) that one object exerts on the other. To describe a force fully, we need to know its strength and its direction. The strength of a force is measured in units of newtons (N). Forces act between objects only while they interact, and are not transferred from one object to another. (However, if an object is in motion while a force acts on it, then energy may be transferred to, or from, the object.) A force diagram uses arrows to represent the forces acting on an object at a particular moment. The length of the arrows represents the relative strengths of the forces. When an object is in motion, its force diagram should also include a speed arrow to show in which direction it is moving. Two examples are shown below: Force diagram for a cart moving to the right and being pushed in the direction of motion by a fan unit. Force applied to cart by fan unit Force diagram for a cart moving to the right but being pushed to the left by a frictional force that opposes its motion. Frictional force applied to cart by track Idea N4– Force on an object at rest If a single force acts on an object at rest, the object will begin to move in the direction of that force. Idea N5 –Force in direction of motion If a single force acts on an object in the same direction as its motion, the object’s speed will increase. (When the speed of an object changes scientists say it is accelerating.) Idea N6 –Force in direction opposite to motion: If a single force acts on an object in the direction opposite to its motion the object’s speed will decrease. (Scientists call this accelerating too, since a decrease in speed is also a change.) If the force continues to act, the object may eventually stop and even reverse direction. FM-S2 Summary Ideas: Force Model of Interactions Friction An example of a force that always opposes motion is friction. It is caused by the contact push/pull interaction between tiny bumps and irregularities in the surfaces of objects as they rub together. Idea N7 - Effect of no forces If a no forces act on an object at rest, then the object will remain at rest. If no forces act on an object in motion, then the object will remain in motion at a constant speed in a straight line. Idea N8 – Rate of change in speed When a force acts on an object, the greater the strength of the force, the higher the rate of change of the object’s speed. When a force acts on an object, the more mass the object has, the lower the rate of change of the object’s speed. (Scientists call this property of objects their inertia.) The effects of both force-strength and mass on the rate of change of speed of an object can be combined into a single relationship: ???? ?? ?????? ?? ?????= ???????? ?? ????????? ?? ?????? If the strength of the force is measured in units of newtons (N) and the mass in kilograms (kg), then the units of ‘rate of change in speed’ will be (m/s)/s. Scientists define the rate of change in speed of an object (that is, how much it’s speed changes in one second) to be its acceleration. Idea N9 - Force from the side If a force acts on a moving object in a direction that is neither along the line of the object’s motion nor directly opposed to it, then the object’s path will change direction. Idea N10 - Curved path: If a continuous force acts on a moving object in a direction that always points toward a common center, the object’s path may curve around the center. FM-S3 Unit FM Gravitational Force The Earth exerts a downward gravitational force on all objects. Idea G1 – Strength of gravitational force and mass The strength of the gravitational force that the Earth exerts on an object is proportional to the mass of the object: the greater the mass of an object, the stronger the gravitational force acting on it. Idea G2 – Falling objects (ignoring air resistance) When the only force acting on objects is the gravitational attraction of the Earth, all objects fall at the same rate of change of speed. (We are ignoring the effects of air resistance.) FM-S4 Unit CF Summary Ideas Combined Forces When studying contact push/pull interactions between objects, it is sometimes convenient to analyze and explain them in terms of a model that links the forces being applied to them to their motion, rather than in terms of the energy transfers between them. The link between the forces acting on an object and its motion fascinated scientists for a long time. Historical Perspective In the 4th century BC the Greek philosopher Aristotle concluded, from watching the world around him, that in order for an object to move at a constant speed, it was necessary to maintain a constant push on it. As a consequence, he said that if no such push acted, it was the natural motion of an object to slow and stop. Amazingly, Aristotle’s ideas stood almost unchallenged for some 2000 years! It was not until the early 17th century AD that Aristotle’s ideas were seriously challenged, primarily by the Italian scientist Galileo. Among Galileo’s many contributions to science was his assertion that moving objects tend to slow and stop because of some outside resistance to their motion (usually friction), not because it is their natural tendency to do so. In the late 17th century Sir Isaac Newton, building on the work of Galileo and others, published his ideas, which have come to be known as Newton’s Laws. Written in the language we have been using in this course, Newton’s Laws state: 1st LAW: If a balanced combination of forces acts on an object, then, if it is at rest, it will remain at rest, and if it is in motion, it will continue to move in a straight line path at a constant speed. (A situation in which no forces acting on an object is just the simplest case of balanced forces.) 2nd LAW: If an unbalanced combination of forces acts on an object, its speed and/or its direction will change. How quickly the speed (and/or direction) changes is directly related to the strength of the net force and inversely related to the object’s mass. (A situation in which only a single force acts on an object is just the simplest case of unbalanced forces.) 3rd LAW: During a contact push/pull interaction, the two objects involved exert a force on each other. These two forces are equal in strength and opposite in direction. Only one of these two forces acts on each of the objects involved. ©2017 Next Gen PET CF-S1 Unit CF These statements of Newton’s Laws contain many ideas that likely correspond closely to the ideas you have developed yourself in this unit, based on evidence you gathered yourself. Idea N1 – Interactions described in terms of forces The interaction between two objects may be described in terms of the force (a push or pull) that one object exerts on the other. To describe a force fully, we need to know its strength and its direction. The strength of a force is measured in units of newtons (N). Forces act between objects only while they interact, and are not transferred from one object to another. (However, if an object is in motion while a force acts on it, then energy is transferred to, or from, the object.) A force diagram uses arrows to represent the forces acting on an object at a particular moment. The length of the arrows represents the relative strengths of the forces. When an object is in motion, its force diagram should also include a speed arrow to show in which direction it is moving. Two examples are shown below: Force diagram for a cart moving to the right and being pushed in the direction of motion by a fan unit. Force applied to cart by fan unit Force diagram for a cart moving to the right but being pushed to the left by a frictional force that opposes its motion. Frictional force applied to cart by track Idea N2 – Forces between interacting objects When two objects interact, they exert forces on each other that are equal in strength and opposite in direction. Only one force acts on each object involved. Force exerted on Cart A by Cart B Cart A CF-S2 Force exerted on Cart B by Cart A Cart B Summary Ideas: Combined Forces Idea N3 - Combinations of forces When more than one force acts on a single object (which means it is involved in more than one interaction at the same time), the effect they have on its motion is due to the combination of these forces and not just the strongest one. If the strengths and directions of the individual forces are such that their combination results in an exact balancing of forces in opposite directions, the forces are said to be balanced. (Note that a situation in which no forces act on an object is just the simplest case of a balanced combination of forces.) If the strengths and directions of the individual forces are such that their combination does not result in an exact balancing of forces in opposite directions, the forces are said to be unbalanced. (Note that a situation in which only a single force acts on an object is just the simplest case of an unbalanced combination of forces.) When the forces acting on an object are added and/or subtracted to give an equivalent single force, scientists call the result the net force. Unbalanced forces on an object in motion 150 N 200 N Equivalent to single force Balanced forces on an object at rest 200 N 200 N Equivalent to no forces 50 N Net force = 50 N to the left Net force = 0 N CF-S3 Unit CF Idea N4 – Net force on an object at rest If a non-zero net force acts on an object at rest, the object will begin to move in the direction of the net force. Idea N5 – Net force in direction of motion If a non-zero net force acts on an object in the same direction as its motion, the object’s speed will increase. (When the speed of an object changes scientists say it is accelerating.) Idea N6 – Net force in direction opposite to motion If a non-zero net force acts on an object in the direction opposite to its motion, the object’s speed will decrease. (Scientists call this accelerating too, since a decrease in speed is also a change.) If the net force continues to act, the object may eventually stop and even reverse direction. Friction An example of a force that always opposes motion is friction. It is caused by the contact push/pull interaction between tiny bumps and irregularities in the surfaces of objects as they rub together. Idea N7 - Effect of balanced forces If a balanced combination of forces acts on an object at rest, then the object will remain at rest. If a balanced combination of forces acts on an object in motion, then the object will remain in motion at a constant speed. If a balanced combination of forces acts on an object in motion, then the path of the object will be a straight line. Idea N8 – Rate of change in speed When a non-zero net force acts on an object, the greater the strength of the net force, the higher the rate of change of the object’s speed. CF-S4 Summary Ideas: Combined Forces When a non-zero net force acts on an object, the more mass the object has, the lower the rate of change of the object’s speed. (Scientists call this property of objects their inertia.) The effects of both net force-strength and mass on the rate of change of speed of an object can be combined into a single relationship: ???? ?? ?????? ?? ?????=???????? ?? ??? ????????? ?? ?????? If the strength of the net force is measured in units of newtons (N) and the mass in kilograms (kg), then the units of ‘rate of change in speed’ will be (m/s)/s. Scientists define the rate of change in speed of an object (that is, how much it’s speed changes in one second) to be its acceleration. Idea N9 - Force from the side If a force acts on a moving object in a direction that is neither along the line of the object’s motion nor directly opposed to it, then the object’s path will change direction. Idea N10 - Curved path If a continuous force acts on a moving object in a direction that always points toward a common center, the object’s path may curve around the center. Air Resistance Idea AR1 – Air Resistance As an object moves through the air, either horizontally or vertically, the air exerts a force on it in the direction opposite to its motion. This force is called air resistance or drag. The strength of the drag force depends on two factors. First, it depends on the speed of the object: the faster the object is moving, the greater the strength of the drag force. Second, it depends on the surface area of the object in the direction of motion: the greater the surface area, the greater the strength of the drag force. CF-S5 UNIT EM Summary Ideas ENERGY MODEL of INTERACTIONS In this unit, you developed your understanding of a model that explains the effects of interactions in terms of ideas about transfers of energy between objects and how they cause different types of energy associated with them to change. Below we give a historical perspective on scientists’ development of similar ideas. Historical Perspective Up until the end of the 18th century, many of the models of different types of interactions were largely based on the idea of a special substance that flows from one object to another. For example, phenomena involving temperature changes were explained using a model in which an invisible, weightless, fluid called caloric flowed between objects with different temperatures. In 1798, Count Rumford rejected this idea, arguing that it could not account for the enormous amount of heat produced in frictiontype contact push/pull interactions. Instead he suggested that heat was really a form of something called ‘energy’. Throughout the 19th century, Sir James Prescott Joule spent much of his time trying to understand the relationship between the energy associated with contact push/pull interactions and the changes in temperature that occur as a result of frictional effects. He developed a simple apparatus that allowed him to measure the relationship between changes in kinetic energy and changes of the temperature of water that was stirred through a contact push/pull interaction. He found that the change in energy of the moving objects during the experiment was equal to the energy necessary to change the temperature of the water from its initial to its final value. Based on work by Sadi Carnot (1842) and Rudolph Clausius (1850) the ideas emerged that: i) there are many forms of energy, and that they can be transformed from one form to another; and ii) energy can be transferred between objects, but only while they interact with each other. In a lecture in 1846, William Thompson (Lord Kelvin), referring to the work of Joule, announced that in his view, energy had become the primary concept on which physics was to be based. In 1847, Hermann von Helmholtz used mathematics to express that the effects of contact push/pull, light, heat, electric, and magnetic interactions were different manifestations of energy. In 1852 and 1855, W. J. Rankine declared that the term ‘energy’ could be applied to “ordinary motion and mechanical power, chemical action, heat, light, electricity, magnetism, and other powers, known or unknown, which are convertible or commensurable with these.” Just a few years later, Michael Faraday published an essay called ‘On the conservation of force.’ He understood this to mean the transformability and indestructibility of natural powers. In the essay, Faraday discussed the ambiguities of the phrase because he understood that force can be applied and removed. Rankine argued that a better way to express what Faraday was trying to say is the phrase ‘conservation of energy’ which was not ambiguous, for it is energy that is not created or destroyed. ©2017 Next Gen PET EM-S1 Unit EM Basic Energy Model of Interactions Idea E1 – Energy Model of Interactions During an interaction, two objects act on or influence each other to cause some effect. During the interaction, energy is transferred between the two objects. One object is the energy giver (where the energy comes from) and the other object is the energy receiver (where the energy goes to). Because of this transfer of energy, there is a decrease in some type of energy within the giver and an increase in some type of energy within the receiver. Idea E2 – The energy model of an interaction can be described using a giver/receiver (G/R) energy diagram. The energy model for a single interaction can be represented using a diagram like this. By convention: i) The type of interaction is identified. ii) The names of the interacting objects are included within rectangles. _________ Interaction Energy Receiver Energy Giver Giver object Energy Decrease in Receiver object Increase in __________ __________ energy energy iii) The energy transferred is indicated by a broad arrow iv) The energy changes within the objects are included in ovals attached to the objects. In this unit, you examined several types of interactions and associated energy changes. These are summarized in the tables below and described in more detail later in this document. Interaction Conditions under which it occurs Contact Push/Pull (CPP) Objects in contact push or pull on each other. Heat Conduction (HC) Objects in contact have different temperatures. Infrared Radiation (IR) Non-touching objects have different temperatures. Convection (Con) Fluid (liquid or gas) flows between non-touching objects with different temperatures. Heat Interactions (HI) A collective name used whenever two objects interact due to a temperature difference between them. [This can be used instead of the individual HC, IR, or Con types of interactions, since in almost all cases at least two of these types of interactions are occurring simultaneously.] Electric Circuit (EC) An electrical energy source is connected to an electrical device in a circuit. EM-S2 Summary Ideas: Energy Model of Interactions Light Light moves from a source to a light receiver. Sound Sound moves from a source to a sound receiver. Energy Type Evidence for change Kinetic (KE) Change in speed Chemical Potential (CPE) Use of muscles, battery powering an electrical device, or burning of some type of fuel (such as gasoline) Thermal (TE) Change in temperature ‘Energy’ Used as a generic term when changes are complex or varied, such as for light and sound receivers Energy Model of Contact Push/Pull (CPP) Interactions Idea C1a - Definition of a CPP interaction A CPP Interaction occurs when any two touching objects push or pull each other. In the absence of an equally strong opposing CPP interaction, there is a change in the speed (and/or direction) of at least one of the objects involved. Idea C1b - Duration of CPP interactions and changes in speed CPP interactions have a definite duration in time, and changes in speed take place gradually and continuously all the time the interaction continues. Idea C1c – Energy transfer and changes during a CPP interaction In terms of energy, during a CPP interaction there is a transfer of energy between the interacting objects. The type of energy associated with an object’s motion is called kinetic energy, thus when the speed of an object changes during a contact push/pull interaction, so does its kinetic energy. We identified two types of CPP interactions, which are described in ideas C2 and C3. These ideas assume that the particular interaction in question is acting in isolation. That is, there is no opposing CPP interaction of equal strength that would cause there to be no change in speed (and/or direction) of the objects involved. EM-S3 Unit EM Idea C2 - CPP interactions involving rigid objects One type of CPP interaction occurs when two touching non-elastic (i.e., rigid or stiff) objects push or pull on each other. In such CPP interactions, the speed (and/or direction) of at least one of the non-elastic objects changes while they are touching and push or pull on each other. As an example, here is a G/R energy diagram describing the CPP interaction between a person’s hand and a cart that causes the cart to speed up. (Assume the effects of friction can be ignored.) In terms of energy, there is a change in the kinetic energy of at least one of the objects involved. If a human person is the source of the CPP interaction, then as energy is transferred out from the person, the chemical potential energy in the person decreases. Idea C3 – CPP interactions involving friction: A CPP interaction also occurs when two surfaces rub against each other. The evidence of such a friction-type CPP interaction is that at least one of the objects involved decreases in speed while the temperature of both objects increases. In terms of energy, during a friction-type CPP interaction there is transfer of energy between the interacting objects and a decrease in the kinetic energy of at least one of the objects involved. There is also an increase in the thermal energy of both objects. As an example, here is a G/R energy diagram describing the CPP interaction in which a block slides across a table, slowing down as it does so. Though friction can never be totally eliminated, in some situations its effects can be neglected for simplicity. This is especially true during periods when other CPP interactions (such as strong pushes and pulls) are having a strong enough effect on an object that including the effects of friction would make little or no difference. EM-S4 Summary Ideas: Energy Model of Interactions Energy Model of Electric Circuit Interactions Idea EC1 – Definition of an Electric Circuit Interaction An electric circuit interaction occurs whenever a source of electrical energy (battery, solar cell, generator) is connected to an electrical device (bulb, buzzer, motor/fan, heater, etc.) in a complete circuit. Idea EC2 – Energy Model of Electric Circuit Interactions During an electric circuit interaction, energy is transferred from the electrical energy source to an electrical device. All electrical devices (sources and receivers) warm up when they first start to operate, meaning they increase in thermal energy. Different types of electrical energy source operate in different ways. A dry cell battery has no energy input, but is able to transfer energy to a device because there is a decrease in the chemical potential energy stored in the battery itself. Other devices, such as a solar cell or a generator, are able to transfer energy to a device because energy is transferred into them via another interaction. Energy Model of Heat Interactions Idea H1 – Definition of a Heat Interaction A heat interaction occurs between objects that have different temperatures. During such interactions, energy is transferred from the warmer object to the cooler object. When such interactions occur in isolation, the warmer object decreases in thermal energy (hence, it decreases in temperature) and the cooler object increases in thermal energy (hence, it increases in temperature). The interaction stops when the two objects reach the same temperature. (However, if energy is being transferred into the warm object at the same time via another interaction, it is possible that it will not decrease in thermal energy.) Scientists identify three specific types of heat interactions. Idea H1a - A heat conduction interaction occurs when energy is transferred between any two objects that are touching and have different temperatures. Idea H1b - An infrared interaction occurs when energy is transferred between any two objects that are near one another and have different temperatures. EM-S5 Unit EM Idea H1c - A convection interaction occurs when heat energy is transferred between any two objects that have different temperatures and a fluid (gas or liquid) is able to flow between them. Idea H2 – Multiple Heat Interactions Under most circumstances, more than one type of heat interaction can occur between two objects with different temperatures. (For example, when a cold can of soda sits on a warmer table there is a heat conduction interaction at the points where they actually touch, an infrared interaction between the nontouching parts, and a convection interaction because air can flow between them.) In such cases we simply group all the types of interaction together and call them ‘heat interactions’. Here is a G/R energy diagram for the cold soda can sitting on the warmer table. Idea H3 – Interactions with the surroundings All electrical energy sources and devices warm up when they begin to operate. Also, during any real-world CPP interaction between objects, the temperature of the objects will increase at least slightly due to the effects of friction. Because of this, some energy is always transferred from these warm objects to the surroundings (that is, all nearby and touching objects) via the three types of heat interactions. It is these interactions that are responsible for warm objects eventually cooling down. Depending on the particular situation, the amount of energy transferred to the surroundings via heat interactions may be small (even negligible) or it may be large, but in principle it always happens. The greater the temperature difference between the object and its surroundings, the greater the rate at which heat energy is transferred from a warm object to its surroundings. Idea H4 – Transient and Equilibrium States Any object can be both an energy receiver and an energy giver in different interactions at the same time. For example, consider a hand holding a hot cup of coffee. It is the energy receiver in a heat conduction interaction with the cup, but also an energy giver in heat interactions with the surroundings. EM-S6 Summary Ideas: Energy Model of Interactions What happens to the thermal energy of the hand in this case depends on the relative rates of energy transfer into and out of it. If the rate of energy input from the hot cup is higher than the rate of energy output to the surroundings, then the thermal energy of the hand will increase. If the rate of energy input is less than the rate of energy output, then its thermal energy will decrease. In both of these cases, the temperature of the hand changes, and it is said to be in a transient state with respect to its thermal energy. However, if the rates of energy input and energy output are exactly equal, then the thermal energy of the hand will not change. This means it will stay at a constant temperature, and it is then said to be in a state of dynamic equilibrium with respect to its thermal energy (usually just ‘equilibrium’ for short). All electrical devices (sources and receivers) warm up when they start to operate, but quickly reach a constant temperature that is warmer than the surroundings. (This constant temperature is sometimes called their operating temperature.) While they are warming up, they are in a transient state, but once the their temperature stabilizes, they then remain in an equilibrium state until the circuit is disconnected. This idea can also apply to devices with moving parts, such as a generator or motor fan. While they are speeding up after first being connected, they are in a transient state with respect to their kinetic energy, but they rapidly reach a constant speed, which is an equilibrium state with respect to kinetic energy. Note that it is possible for a device to be in an equilibrium state with respect to one form of energy, but not another. When we refer to a dry cell battery being in equilibrium, we mean this in regard only to its thermal energy. All the time it is operating it is still decreasing in chemical potential energy. Ideas Involving Energy Conservation Idea CON1 - Efficiency All electrical devices (and other manufactured items) are designed with a definite purpose in mind. The energy output associated with that purpose is useful energy. Any other energy outputs are not useful. (For example, the useful energy output from a light bulb is the energy is associated with light interactions. However, a bulb also has an energy output associated with heat interactions that is not useful.) EM-S7 Unit EM The energy efficiency of a device is defined as the fraction (converted to a percentage) of the total energy input that is output as useful energy: ?????????? ?? %=?????? ?? ?????? ?????? ??? ?????? ?????????????????? ?? ?????? ?????×100 For example, if a light bulb has an input of 50 J of energy, but only 4 J are output via light interactions, then the efficiency is: Eff = 4 × 100 = 8% 50 A few energy source devices, such as a dry cell battery, have no energy input while they operate, but instead produce an energy output from a decrease in some form of stored (potential) energy within them. In such cases€we define the efficiency of the device in a slightly different way: ?????????? ?? %=?????? ?? ?????? ?????? ??? ?????? ?????????????????? ?? ???????? ?? ?????? ????????? ??????×100 For example, if the chemical potential energy of a battery decreases by 60 J, but only 54 J are output via an electric circuit interaction, then the efficiency is: Eff = 54 ×100 = 90% 60 Idea CON2 – Keeping track of energy through a system A ‘system’ is any set of objects that we want to study. It can consist of a single object or a group of interacting objects, and it may, or may not, include the surroundings. We can draw a G/R energy diagram showing all the energy inputs, outputs, transfers, and changes associated with the objects in the system. Using the idea of efficiency, we may also be able to track amounts of energy through the system. Here is an example of such a diagram for a system consisting of a solar battery, a buzzer, some sound receivers, and the surroundings. EM-S8 Summary Ideas: Energy Model of Interactions 85 Light & Heat Increase in thermal energy 12 Interactions 85 Light Interaction Energy (from sun) 100 Energy Surroundings Heat Interactions Increase in thermal energy 12 Solar Battery (15%) Energy Energy Surroundings Energy Sound Receivers Buzzer (20%) 15 Electric Circuit Interaction 3 Sound Interaction Increase in energy 3 The input to this system is 100 J of energy from the Sun via a light interaction. The useful energy output from a solar battery is that which powers an electric circuit; since its efficiency is 15%, this means only 15 J are associated with the energy transfer to the buzzer. (The other 85 J is transferred to the surroundings via heat interactions, and since the surroundings have no output, this is the amount by which their thermal energy increases.) Now consider the buzzer. It has an energy input of 15 J (from the solar battery), but its efficiency of 20% means that only 3 J of its output are associated with making sound. This 3 J is transferred to some sound receivers and, since they have no energy output, their energy increases by this amount. (The other 12 J is transferred to the surroundings via heat interactions, and since the surroundings have no output, this is the amount by which their thermal energy increases.) When considered together, the solar battery + buzzer system has an energy input of 100 J, but a useful energy output of only 3 J. Thus the efficiency of this system is: Eff = 3 × 100 = 3% 100 Idea CON3 - Law of Conservation of Energy: € Energy cannot be created or destroyed, but only changed from one form to another and transferred between objects. In this unit we considered two situations where we EM-S9 Unit EM applied the Law of Conservation of Energy (COE). In one case we considered a system of interacting objects with a certain amount of energy being transferred into the system. In that case, according to COE, the amount of energy transferred into the system must equal all the increases in energy of the interacting objects. For example, suppose the system consists of a solar cell, motor, air and the surroundings, and that energy is transferred from the Sun to the solar cell (which is part of the system). If we consider the time period during which the solar cell and motor are still warming up and the motor blades are still speeding up (the transient state), then the Law of Conservation of Energy would take the form: Energy transferred into the system from the Sun = Increase in thermal energy of (solar cell + motor + surroundings) + Increase in kinetic energy of (motor + air) If, instead, we consider the time period after the solar cell and motor have reached their operating temperatures and the motor blades have reached their maximum speed (the equilibrium state), then the conservation of energy equation takes a simpler form: Energy transferred into the system from the Sun = Increase in thermal energy of surroundings + Increase in kinetic energy of air We also considered the case where no energy is transferred into the system, but instead there is a decrease in energy in one of the interacting objects. In that case, according to COE, the decrease in energy of the one object must equal the sum of all the energy increases in all the interacting objects. For example, suppose the system consists of a battery, bulb, light receivers and the surroundings during a time when the battery and bulb are still increasing in temperature (transient state). Then the Law of Conservation of Energy would take the form: Decrease in chemical potential energy in the battery = Increase in thermal energy of (battery + bulb + surroundings) + Increase in energy in the light receivers If, instead, we consider the time period after the battery and bulb have reached their operating temperatures (equilibrium state): Decrease in chemical potential energy in the battery = Increase in thermal energy of surroundings + Increase in energy in the light receivers EM-S10 UNIT M Summary Ideas MODEL OF MAGNETISM Historical Perspective Magnetism is one of the earliest known physical phenomena. The ancient Greeks studied naturally occurring magnets (called lodestones) and the basic properties of magnetic interactions were discovered before 600 BC. By the end of the 17th century, William Gilbert had identified two different poles of a magnet. He described attraction in terms of “harmony” and repulsion in terms of “discord.” Gilbert also noticed that by cutting a magnet in half, he could produce two pieces that acted as individual magnets. His harmony and discord model could explain why magnets attract and repel other magnets but could not explain why magnets attract iron. In 1644 René Descartes published an influential work that outlined his own model for magnetism in terms of an invisible substance or fluid consisting of very small spiral particles that were always in motion. According to this model, magnets contained pores or channels that were aligned so that the magnetic substance could flow easily through the magnet in one direction, but not the other. This idea helped to explain the existence of two poles, one into which the substance flows and the other out of which the fluid emerged. According to Descartes’ model, when a magnet is brought into the presence of unmagnetized iron, the streaming particles of substance push the channels in the iron into alignment, so the iron then behaves like a magnet. Thus, this model could explain why iron is attracted to a magnet. In 1756, Franz Aepinus proposed a “magnetic fluid” which exists in all magnets. According to this model, all iron contains a magnetic fluid. A magnet was simply a piece of iron that had the magnetic fluid stuck at one end. This left it with an excess of fluid on one end and a deficit of fluid on the other. Although this model could explain the two-ended nature of the magnet, it had some difficulty explaining the “broken magnet” problem. In the late 18th century, Charles Coulomb introduced the idea of “magnetic entities” which exist within the magnet. According to this model, each entity contained the magnetic fluid and entities could not be broken apart. The fluid could move around within each entity but could not move between entities. This led to the modern model for magnetism. Summary Ideas Here are the ideas about magnetism developed by our class. These are likely very similar to those developed by scientists as they are based on much the same evidence. ©2017 NextGenPET M-S1 Unit M Idea M1 - Kinds of materials involved in magnetic interactions A magnetic interaction occurs between a magnet and another magnet, or between a magnet and an unmagnetized ferromagnetic object. Ferromagnetic objects are made from iron, nickel, or cobalt, or alloys containing one or more of those materials. (For example, steel is an alloy of iron and copper and is ferromagnetic.) There is always an attraction between an unmagnetized ferromagnetic material and a magnet (which, if either, moves as a result of this attraction will depend on whether either, or both, are being restrained by other means). A magnet is a ferromagnetic object that has become permanently magnetized. Idea M2 - Magnetic Interactions between two magnets When a magnet is allowed to rotate freely, without any other magnets nearby, the end pointing (approximately) towards the geographical North Pole of the earth is defined as the north pole of the magnet. The opposite end of the magnet is defined as its south pole. A magnetic interaction occurs between a magnet and another nearby magnet. Two magnets with like poles facing each other will repel. Two magnets with unlike poles facing each other will repel. (This idea is sometimes called the Law of Magnetic Poles.) Idea M3 - Model of Magnetism Many magnetic effects can be explained in terms of the Alignment of Domains Model. a) This model is based on the idea that inside a ferromagnetic object there are a very large number of tiny entities, sometimes called magnetic domains. Each magnetic domain is assumed to behave like a tiny bar magnet, with north (N) and south (S) poles. When drawing a model diagram of domains inside a ferromagnetic object, it is common to use small bar magnets to represent them. (A common alternative is to represent an individual domain using an arrow, the head of which represents the north pole of the domain.) The domains cannot move from place to place within the ferromagnetic material, but they can rotate around so that their poles point in different directions. S N (When a domain is cut, it simply becomes two smaller domains aligned in the same direction as the original.) b) In an unmagnetized ferromagnetic object, the domains are randomly oriented; that is, for each domain pointing in one direction, there is another domain nearby pointing in the exact opposite direction. The magnetic effects of the north and south M-S2 Summary Ideas: Model of Magnetism poles of the domains therefore cancel each other out throughout the entire object, and produce no net magnetic effect. Here is a simple representation where we assume the domains point either to the right or to the left. (It is also acceptable to show the domains pointing randomly in all directions, not only to the right or left.) S N N N SS S S N N N N S S S N or At the ends of the object, random orientation of the domains means they cancel each other’s effects. Therefore, the entire object has neither a North Pole nor a South Pole at its ends. c) When a ferromagnetic object is fully magnetized, all the domains become aligned, pointing in the same direction. The magnetic effects of all the domains reinforce each other and produce strong magnetic effects. The end of the object toward which all the north poles of the domains are aligned becomes the North Pole of the magnet; the end toward which all the south poles of the domains are aligned becomes the South Pole of the magnet. (In the example below, the right side would be the North Pole of the magnet and the left side would be the South Pole.) S N S N S N S N S N S N S N S N or d) The domains in an unmagnetized ferromagnetic material can be aligned by using the magnetic influence of a permanent magnet in different ways. i) When one pole of a permanent magnet is rubbed along the ferromagnetic material (e.g. a nail) in one particular direction, the opposite poles of all the domains are attracted to it and rotate to follow it. By the time the magnet has been rubbed along the material several times, most of the domains have rotated so that they are aligned in the same direction. ii) When one pole of a permanent magnet is held close to a ferromagnetic material, its magnetic influence causes the domains in the material to rotate by attracting their opposite poles. After being exposed to this influence for several seconds most of the domains have rotated so that they are aligned in the same direction. e) The alignment of the domains in a magnetized ferromagnetic material can be disrupted by making them move around. Two techniques that work are: i) ‘Shocking’ by vigorously hitting the material (or throwing it on the floor) and ii) Heating the material for several seconds. M-S3 UNIT PEF Summary Ideas POTENTIAL ENERGY and FIELDS Historical Perspective Fields: The observation that some objects, like magnets, can affect other objects without touching them was a puzzle to scientists for many hundreds of years. Early scientists, such as Gilbert and Descartes, tried to explain such magnetic and static electric effects using models in which streams of invisible particles or emanations were emitted by some objects and absorbed or enveloped by others. When developing his ideas about gravitational interactions, Isaac Newton was dissatisfied that he could not explain the observation that the Earth could attract objects toward it without touching them. He called this phenomenon ‘action-at-adistance’ and this terminology has been used since then to describe the different types of interactions that have this property. In the late 18th and early 19th centuries, scientists began to explain these phenomena using an idea they called ‘spheres of influence’ or ‘lines of force’ that extended out from objects. Finally, in 1845 Michael Faraday described the idea of a magnetic field that extended the influence of a magnet beyond its physical boundaries. So useful was this idea that it was soon adapted to also explain the ‘action-at-distance’ nature of static electric and gravitational interactions. Potential Energy: Before the 19th century, scientists’ thinking about energy was confined to what we now call kinetic energy – that is the energy associated with motion. In this restricted sense, they thought that when an object’s speed changed, that kinetic energy was simply created or lost. However, starting around the year 1800 various scientists began to consider the temperature of objects as an indication of another type of energy (which we call thermal energy). Carefully controlled experiments showed that these two types of energy could be converted from one to the other without any losses or gains, and from this the idea of the Law of Conservation of Energy was developed. However, for this idea to be useful, in some situations it had to be assumed that energy could be ‘stored’ in objects, ready to be ‘released’ and transformed into more evident types like kinetic and thermal energy. Around the middle of the 19 th century William Rankine described this ‘stored’ energy as having the potential to produce changes in motion and temperature, and so coined the phrase potential energy to refer © 2017 Next Gen PET PEF-S1 Unit PEF to it. The scientific community quickly adopted this idea because it allowed all interactions to be described in terms of the Law of Conservation of Energy. In the late 19th century James Clerk Maxwell used the ideas of fields and potential energy to unify the study of electricity and magnetism into electromagnetism. In order to do so he proposed the idea that the magnetic and electric fields could have forms of potential energy associated with them (magnetic potential energy – MPE, and electric potential energy – ElecPE). This idea was also adopted for use in the description of gravitational interactions by saying the gravitational field has gravitational potential energy (GPE) associated with it. On the following pages we describe some of the ideas about potential energy and fields that have been developed in this unit. They should correspond closely to ideas used by scientists, since they are based on some of the same evidence. Idea PEF1 – ‘Action-at-a-distance’ Interactions There are certain types of interactions in which the interacting objects can exert pushes and pulls on each other even though they are not touching. These types of interactions are known collectively as ‘action-at-a-distance’ type interactions. The evidence for an ‘action-at-a-distance’ interaction is a change in motion without any direct contact. Examples are: • Magnetic interactions – occur between two magnets, or between a magnet and a ferromagnetic material. • Static electric interactions – occur between two charged objects, or between a charged object and an uncharged object. • Gravitational interactions – occur between all pairs of objects (but are only noticeable when at least one of them is very, very, massive.) Idea PEF2 – Accounting for ‘Action-at-a-distance’: The phenomenon of ‘action-at-a-distance’ can be accounted for using the idea of an invisible ‘field of influence’ that surrounds the relevant objects. Any other relevant object that lies within that field feels its influence and is pushed or pulled accordingly. PEF-S2 Summary Ideas: Potential Energy and Fields • Surrounding a magnet there is a magnetic field — any other magnet, or ferromagnetic material, within that magnetic field feels a push or a pull according to how it is oriented. The direction of the magnetic field at any point is the direction of the force that would be experienced by the north pole of a small test magnet at that location. Magnetic fields point away from the north pole of a magnet and toward the south pole. The strength of the magnetic field around a magnet gets weaker as distance from the magnet increases. • Surrounding an electric charge (or charged object), there is an electric field — any other charge (or charged object, or charges within a neutral object) within that electric field feels a push or a pull according to the types of charge involved (+ or –). The direction of the electric field at any point is the direction of the force that would be experienced by a small positive test charge at that location. Electric fields point away from positive charges and toward negative charges. The strength of the electric field around a charged object gets weaker as distance from the object increases. • Surrounding all objects there is a gravitational field — any other object within that gravitational field feels a pull toward the first object. (The gravitational field of everyday objects is extremely weak and usually not noticeable except under very carefully controlled conditions. The gravitational field of a planet is much stronger and so shows noticeable effects.) The direction of the gravitational field at any point is the direction of the push/pull that would be experienced by a small mass at that location. The strength of the gravitational field around an object gets weaker as distance from the object increases. (However, the Earth’s gravitational field only gets weaker very slowly as you get further and further from the Earth. You must get a few hundred miles above the surface before the field gets noticeably weaker.) PEF-S3 Unit PEF Idea PEF3 – Electromagnetic Interactions: When electric charges move through a circuit and a compass is close by, the compass reacts by moving. This is evidence that moving electric charges create a magnetic field in the area around them and is the principle on which an electromagnet works. When a magnet moves around close to a coil of wire, an electric current flows through the wire. This is evidence that a moving magnet creates an electric field in the area around it and is the principle on which an electric generator works. These two effects are examples of electromagnetic interactions. Idea PEF4 – Potential Energy: In order that the Law of Conservation of Energy can be applied to all interactions, we assume that energy can be stored in objects/fields and that this stored energy can increase or decrease during different types of interaction. Idea PEF5 – Elastic Potential Energy: When two touching objects push or pull on each other it is possible that at least one of them is elastic (stretchy or compressible). In such CPP interactions, while the interacting objects are touching, there is a change in the speed (and/or direction) of at least one of the objects involved, while the amount of compression or extension of at least one of them also changes. In order for the Law of Conservation of Energy to be used, we must assume that there is a change in the elastic potential energy (EPE) of the elastic object(s) involved. When an elastic object gets more compressed/stretched, the amount of EPE associated with it increases, and vice versa. PEF-S4 Summary Ideas: Potential Energy and Fields Idea PEF6 – Potential energy in fields All types of fields store potential energy that can increase or decrease during the relevant type of interaction. Thus the field itself can be either an energy giver or receiver. Using the Law of Conservation of Energy, we can infer that when the kinetic energy of the interacting objects increases, the potential energy in the field decreases, and vice versa. • A magnetic field can store magnetic potential energy (MPE) that increases or decreases during a magnetic interaction if the kinetic energy of the magnets/objects involved also changes. When two magnets are attracting one another, the amount of MPE in the field increases the further apart the two magnets are. • An electric field can store electric potential energy (ElecPE) that changes during an electric charge interaction if the kinetic energy of the charges/objects involved also changes. When two charges are attracted to one another, the amount of ElecPE in the field increases the further apart the two charges are. PEF-S5 Unit PEF • A gravitational field can store gravitational potential energy (GPE) that changes during a gravitational interaction if the kinetic energy of the objects involved also changes. The amount of GPE in the field increases the further apart the two objects are. Idea PEF7 – Effect of mass When two objects interact via an ‘action-at-a-distance’ interaction, in principle they will both move. However, if the mass of one object is much larger than the mass of the other, then the one with more mass will have a smaller degree of motion. In gravitational interactions between the Earth and everyday objects (people, cars, balls, etc), the mass of the Earth is so much larger than that of the other object involved that the motion of the Earth is imperceptibly small. Idea PEF8 – Gravitational interactions on a frictionless track When an object is released from a certain height on a frictionless track, as it descends the GPE in the gravitational field decreases and so the object’s KE increases (as does the Earth’s, but imperceptibly). However, because of the Law of Conservation of Energy the total energy (GPE+KE) involved stays constant. If the track rises again, the object can never attain a height higher than that at which it started because that would require more total energy than was available as GPE when it was released. PEF-S6 My Drive - Google Drive ? Bb Online Exams - PHYS412-01-Spr X Bb Take Test: Final Exam - PHYS412- X Launch Meeting - Zoom C 2 https://blackboard.sdsu.edu/webapps/assessment/take/launch.jsp?course_assessment_id=_175421 Force Completion Once started, this test must be completed in one sitting. Do not leave the test before clicking Save and Submit. This test does not allow backtracking. Changes to the answer after submission are prohibited. Remaining Time: 1 hour, 59 minutes, 51 seconds. Question Completion Status: » A A Moving to the next question prevents changes to this answer. Question 1 What is common ideas students have regarding the force on an object, but which are not consistent with the ideas of scientists? OA As the force on an object increases in strength, the more rapidly its speed changes. O B. The speed of a moving object is proportional to the strength of the force on it. O C. The force is transferred to the object and carried with it. OD. A and B. O E A and C OF. B and C Å > A Moving to the next question prevents changes to this answer. O Type here to search O I C L M n Esc ? F1 + F3 X ? F2 B F4 + F5 F6 F7 F8 F9 DE F10 F11 $ 1 2 #3 % 5 4 & 7 * C 6 9 Q W E E R T Y U o /ATS 10T C Warnings appear when hall Multiple Attempts Not allowed. This test can only be taken once. Force Completion Once started, this test must be completed in one sitting. Do not leave the test before clicking Save and Submit. This test does not allow backtracking. Changes to the answer after submission are prohibited. Remaining Time: 1 hour, 57 minutes, 17 seconds. Question Completion Status: ctkim@sdsu.edu Question 2 of A Moving to the next question prevents changes to this answer. Question 2 3.333 points Save Ana Consider an experiment of testing temperature changes of a hot liquid in different containers over time. Using the grade level expectations you read on page TL 3-6 as a guide, what kinds of questions might you ask children in grades 3-5 to help their collection of data around temperature? Describe which container is "hotter" at specific intervals in time through observations. O A B. Write down the temperature of the liquid every minute. oc. Draw a graph of time vs temperature of each container over time. OD. A and C O E B and C w Å OF A All of the above Moving to the next question nrovonte changes to this answer o Type here to search O 8:11 AM 5/10/2021 CI L M A + F3 X C ? A Esc D ?? F10 --- 0:+ PrtSc Insert DE F4 F2 F1 F5 F9 F6 F8 F7 F11 F12 ! 7 # 3 $ 4 % 5 6 & 7 00 * C 9 + II Bac 2 6 Q W R E T T } Y U P Tab [ ] ? S D F G I J K ELK N X ? V B N. M ?. ? FN Alt ??? Alt Ctrl Home de pod 5540