Homework answers / question archive / University of Calgary Department of Physics and Astronomy PHYS 259, Winter 2021 Lab 1: Electric Charges and the Forces Between Them Electrostatics is the study of the forces between objects that carry an electric charge

University of Calgary Department of Physics and Astronomy PHYS 259, Winter 2021 Lab 1: Electric Charges and the Forces Between Them Electrostatics is the study of the forces between objects that carry an electric charge

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University of Calgary

Department of Physics and Astronomy

PHYS 259, Winter 2021

Lab 1: Electric Charges and the Forces Between Them

Electrostatics is the study of the forces between objects that carry an electric charge. Electric charges can either be positive or negative, and these charge can neither be created nor destroyed, but simply flow from one object to another. Any two charged objects exert an electric force on each other equal in magnitude and opposite in direction; opposite charges attract, like charges repel. The magnitude of the electric force between two point charges is given by Coulomb’s law, from fisitech.wordpress.com)

An important application of the electric force is the electrostatic scrubber, shown in the figure on the right. It is used to remove dust, smoke, bacteria and other micrometer or submicrometer size particles from exhaust gases or indoor air: charged airborne particles are attracted by the oppositely charged scrubbing droplets. ( Image from www.imp.gda.pl, where you can learn more about this procedure.)

Goals:

To understand how a macroscopic object acquires a net charge, and what that means on a microscopic level. To practice how to find the resulting force on a charge by using vector addition.

Preparation:

Halliday, Resnick, and Walker, “Fundamentals of Physics” 10th edition, Wiley: 21.1–21.3. Complete the Pre-lab assignment before coming to the lab.

Equipment:

Scotch tape; ruler; the applet: https://www.compadre.org/physlets/electromagnetism/illustration23 2 .cfmu.

Pre-Lab Assignment

In this pre-lab assignment, you will explore the interactions between charged and uncharged objects. You will need a piece of paper, scissors, and strips of transparent tape (duct tape, electrical tape, etc. will not work as well). You may find it useful to watch the Lab 1 Introduction Video posted to D2L prior to completing this pre-lab assignment, but it is expected that you watch this video before showing up to your lab so that you come prepared.

Question 1: Cut off two strips of paper (1 cm by 20 cm each). Take one paper strip and hang it from the edge of a table (or other non-metal furniture) using a small piece of transparent tape so that it is at rest. Bring the second paper strip close to the hanging one. What do you observe?

 

Instructions for making strips of transparent tape to interact electrically

1. On a flat non-metal surface, stick down a 30 cm transparent strip of tape and flatten it with your finger. This strip will not be used in the demonstration, but serves as a foundation.
2. Fold the end of the tape roll (to make small handle) and cut a 20 cm strip. Add it sticky-side-down on the top of the foundation strip and flatten with your finger. This will be Strip 1: label it using a permanent marker or pen.
3. Repeat Step 2 to get another 20 cm strip of tape with a small handle and add it sticky-side-down on the top of Strip 1. This will be Strip 2: label it using a permanent marker or pen.
4. Pull the combined Strips 1 and 2 slowly from the foundation strip. Gently rub the combined strip between your fingers to make sure they are neutral. This will allow you to get more consistent results.
5. Hang the combined Strips 1 and 2 from the edge of a non-metal table. Quickly pull Strip 2 off Strip 1.
6. Hang the strips approximately 50 cm apart (if you did not label the strips with the marker, use sticky notes or another method to mark which is Strip 1 and which is Strip 2.

Question 2: Bring the free strip of paper close to the hanging Strip 1 (but be careful not to touch them together). What do you observe and does it depend on the distance between them?

 

Question 3: Bring the free strip of paper close to the hanging Strip 2 (but be careful not to touch them together). What do you observe and does it depend on the distance between them?

 

Question 4: Repeat Steps 2–5 above to get two more of each type of strip (1 and 2). Bring the second Strip 1 close to the hanging Strip 1. What do you observe and does it depend on the distance between them?

 

Question 5: Bring the second Strip 2 close to the hanging Strip 2. What do you observe and does it depend on the distance between them?

 

Question 6: Bring the second Strip 1 close to the hanging Strip 2. What do you observe and does it depend on the distance between them?

 

Question 7: Bring the second Strip 2 close to the hanging Strip 1. What do you observe and does it depend on the distance between them?

 

 

One of the tape strips is negatively charged, and the other is positively charged. The paper strips are electrically neutral. Use this definition and your observations recorded above to answer the following questions:

Question 8: Is there an electrostatic force between two positive (or two negative) charges? If so, sketch the two charged objects and the forces acting on them in the space below.

 

Question 9: Is there an electrostatic force between a positive and a negative charge? If so, sketch the two charged objects and the forces acting on them in the space below.

 

Question 10: Is there an electrostatic force between two neutral insulating objects? If so, sketch the two objects and the forces acting on them in the space below. If not, explain why not.

 

Question 11: Does the electrostatic force depend on the distance between the charges? If so, does it increase or decrease with distance? How did you come to this conclusion?

 

End of Pre-Lab Assignment

Submit your pre-lab assignment to the appropriate dropbox for your lab section on D2L by 5:00 PM the day before your scheduled lab section. Please see the instructions posted to D2L about submitting pre-lab assignments.

Start of Lab 1

1. Electric forces between two point charges

Question 12: The figure below shows two point charges of −1µC and +1µC. Draw the force vectors on each charge, making sure to accurately represent the relative lengths of the force vectors. How would the direction and magnitude of the force vectors change if the distance between the two charges were doubled?

 

Question 13: This time, consider two point charges of −1µC and +2µC. Draw the force vectors on each charge. Is the force on the −1µC charge greater than the force on the +2µC charge? Why or why not?

 

Question 14: For the configuration of charges in Question 12, what would the distance between the charges have to be in order to double the magnitude of the force?

 

 

2. Superposition of electric forces in one dimension

Question 15: The figure shows four configurations of three charges in one dimension. For each configuration, draw the forces acting on the middle charge, then use a different colour to draw the net force on the middle charge. Note: the relative magnitudes must be correct.

 

Question 16: A charge q₁ = +2.0 nC is located at the origin, and a second charge q₂ = +4.0 nC is located at position x = d.

1. At what position x will a q = −1 nC charge experience zero net force?
2. In part (a), you should have found two mathematical solutions. Using a sketch, explain how you know which one is the physical solution. (You may use the applet “mixed charges” to study the two situations.)
3. What physical situation does the other mathematical solution correspond to?

 

3. The Electric Force in Two Dimensions

Question 17: The electric force between the electrons and protons in an atom is what holds the atom together. In Bohr’s model of the hydrogen atom, the electron orbits the proton on a circular path with a diameter of approximately 10⁻¹⁰m. Use the Coulomb force to calculate the speed with which the electron orbits the proton in this model.

 

Question 18: In larger atoms, there are several protons inside the nucleus, and they are all positively charged. Calculate the minimum electrostatic force between two protons inside a nucleus of diameter 4×10⁻¹⁵ m.

 

 

Question 19: Is the force calculated in the previous question repulsive or attractive? How do you know?

 

Question 20: If there is such a strong repulsive force between the protons, there must be an even stronger attractive force between them to keep the nucleus together. Is this the gravitational force between the protons? Support your conclusion with a calculation.

 

Question 21: Calculate the ratio of the Coulomb force to the gravitational force, F_C/F_g, between two protons separeted by a distance d.

Does the ratio change if the distance d changes from 10⁻¹⁰ m to 100 km?