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Shine a Light Purpose To investigate the nature of the photoelectric effect and (H) identify an unknown metal

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Shine a Light Purpose To investigate the nature of the photoelectric effect and (H) identify an unknown metal. Discussion When light shines on a polished, unoxidized metal surface, electrons can be ejected from the metal. This is the photoelectric effect, a cornerstone of our understanding of light as a particle. When we describe the particle nature of light, we refer to light particles as “photons.” The use of photons to generate an electric signal is used in light-activated circuits and in the soundtrack strip of cinematic films. It was Einstein’s explanation of the photoelectric effect, not his work on relativity, that was honored in his Nobel Prize! PART A: EXPLORATION Step 1: Open the Photoelectric Effect simulation. Set the metal to Sodium and the wavelength to 400 nm. In the simulation window’s Options menu, select “Show photons”. Hypothesis Suppose you set up the experiment so that the plate is ejecting electrons. Predict which of the following changes to the experiment could increase the maximum initial kinetic energy of the ejected electrons. (Select all that apply) ? Increasing the intensity of the light beam ? Decreasing the intensity of the light beam ? Increasing the wavelength of light ? Decreasing the wavelength of light ? Increasing the frequency of light ? Decreasing the frequency of light ? Increasing the voltage of the battery ? Decreasing the voltage of the battery Step 2: Check all the boxes on the right for ? Current vs light intensity ? Electron energy vs light frequency Slowly move the intensity slider from 0% to 100% and observe the photoelectric effect. The electrons emitted from the metal have the same mass and charge as any other electron; the only difference is the kinetic energy they leave the metal with. Electrons with higher kinetic energy should travel ____________ . Nature speaks! What happens to the number of electrons as intensity increases i.e. the current? What happens to the speed of the electrons as intensity increases? (If there are too many electrons to track, try clicking the ? Show only the highest energy electrons option) Sketch or put a screen shot of the Current vs light intensity graph: what does it show is happening to the number of electrons as intensity increases? Sketch the Electron energy vs light frequency graph: what does it show is happening to the speed (KE) of electrons as intensity increases? Step 3: Leave the intensity at 100%. Move the wavelength selector back and forth and observe the results. Nature speaks! What happens to the number of electrons -the current- as wavelength increases? What happens to the speed of the electrons as wavelength increases? (If there are too many electrons to track, try clicking the ? Show only the highest energy electrons option) Sketch the Current vs light intensity graph: what does it show is happening to the number of electrons as frequency increases? Sketch the Electron energy vs light frequency graph: what does it show is happening to the speed (KE) of electrons as frequency increases? Step 4: Carefully find the threshold wavelength for sodium; the wavelengths are most conveniently in billionths of a meter (×10-9 m), or nanometers (nm). What is the wavelength of the lowest energy light at which electrons are emitted, even at maximum intensity? Threshold wavelength = ____________________ nm (×10-9 m) Use the threshold wavelength (λ0) to calculate the threshold frequency (f0). Optical frequencies are in the hundreds of trillions of Hertz, or 1×1012 Hz, so are written in TeraHertz (THz). Show the calculation and solution in the space below: Threshold frequency = ____________________ THz (×1012 Hz) Repeat for the other metals, and note any aberrant behavior when moving the intensity and wavelength sliders. Make a table of threshold wavelengths and frequencies for all the metals metal Threshold wavelength (nm) Threshold frequency (THz) (×1012 Hz) Sodium Zinc Copper Platinum Calcium ? Step 5: Now let’s see what effect that battery has! Change your metal back to Sodium. Check the ?Current vs battery voltage box on the right Reset the wavelength to 400 nm and intensity to 100%. Notice the value of the current in the circuit. Step 6:Adjust the voltage on the battery slowly to 8 V: what happens to the speed of the electrons? What happens to the number of electrons -what happens to the current? Step 7:Now adjust the setting on the battery to cut off the current. That is, set the voltage so that electrons just barely make it to the other side; the current is just barely brought to zero. cut-off potential at 400 nm = ____________________ V Does the amount of current change below the cut-off potential? Sketch the current vs battery voltage graph. You are probably used to current increasing in a simple circuit as voltage increases; is that what happens here? Step 8: Without changing the battery, change the wavelength by 100 nm in either direction in such a way that current once again flows. New cut-off potential wavelength = ____________________ nm Did you have to increase or decrease the wavelength? PART B: INVESTIGATE THE ENERGY QUANTA OF PHOTONS Step 1: Switch the metal to zinc. Set your lamp to zinc’s threshold wavelength. Step 2: Reduce the wavelength to a smaller value by no fewer than 20 nm and no more than 40 nm. Record the new wavelength. Step 3: Adjust the stopping potential of the battery so that it just barely stops the current. Remember that you can type values into the battery. When the condition is met, electrons are ejected from the zinc and a few make it to the opposite electrode. But most return to the zinc, and the current remains zero. Record the minimum stopping potential for this wavelength. Step 4: Repeat the process with zinc of reducing the wavelength by 20—40 nm and changing the stopping potential. Mix it up a bit! Repeat until you have four new data sets. Nature Speaks! Wavelength (nm) Frequency (x1012 Hz) Stopping potential (V) Step 5: Make a graph of zinc’s Stopping Potential vs Frequency and compare with the sim’s built-in Electron energy vs light frequency graph Analysis: 1. If light were purely a wave, then increasing the intensity should have increased the energy of the escaping electrons. What did it increase instead? 2. As the frequency increases, what happens to the energy of the escaping electrons? Does it depend on the intensity? What model of light best explains this behavior? Conclusion: get a KLEW! Of the hypotheses you checked off at the beginning, which were confirmed and which were busted? What did you learn and what is your evidence? Connections with the real world? HONORS GOING FURTHER: MYSTERY METAL What is the metal labeled “?????”? Describe the method used, record appropriate data, and show calculations. https://phet.colorado.edu/sims/cheerpj/nuclear-physics/latest/nuclear-physics.html?simulation=alphadecay → 7.2 PhET LAB: Alpha Decay ______________________ + Name: OBJECTIVE: explain the alpha decay process; complete radioactive decay equations; define and analyze half-life through applying the PhET “Alpha Decay” simulation at http://phet.colorado.edu. Open/Run Alpha Decay at http://phet.colorado.edu. Take 5 minutes to freely explore the sim. Investigating Alpha Decay 1. Start on the SINGLE ATOM tab. Observe the decay of Polonium-211. After each decay, press the RESET NUCLEUS button to watch the process again. Write a description of alpha decay for Po-211. _______________________________________________________________________________________ _______________________________________________________________________________________ _______________________________________________________________________________________ Complete the following alpha decay equations, using http://www.chemicalelements.com as a resource: 211 2. Polonium-211: 84?? → 207 82?? + __ _____ 226 3. Radium-226: 88?? 4. Uranium-238: _____ ______ → 234 90?? + 42? 240 5. Plutonium-240: 94?? 6. Americium-241: 241 95?? → 237 93?? → ____ ________ + 42? ____ ______ + 42? → + __ _____ 7. How is alpha decay used in everyday life? (give at least two uses) _______________________________________________________________________________________ _______________________________________________________________________________________ Investigating Half-Life of Alpha Decay 8. Click the MULTIPLE ATOMS tab. Execute five trials to determine the number of parent and daughter nuclei at one half-life. Complete the table below. Parent Nuclei Po-211 100 80 60 Parent Nuclei (remaining nuclei) [yellow] Daughter Nuclei (decayed nuclei) [black] 40 20 9. Define half-life. _______________________________________________________________________________________ _______________________________________________________________________________________ 10. Suppose a substance has a half-life of 0.52 s. Create accurate pie charts showing the number of remaining parent nuclei and decayed daughter nuclei (shade slightly) starting with 40 total nuclei. PREDICTION with VALUES: ? = ?. ?? ? ? = ?. ?? ? ? = ?. ?? ? ? = ?. ?? ? 11. Use the PhET alpha decay simulation to test your scenario copying each pie chart. SIMULATION with VALUES: ? = ?. ?? ? ? = ?. ?? ? ? = ?. ?? ? ? = ?. ?? ? 12. How does your prediction match with the results of the simulation? Convey with actual values from the simulation and a calculation of percent difference on 0.52 seconds. _______________________________________________________________________________________ _______________________________________________________________________________________ _______________________________________________________________________________________ 13. Run three trials and complete the data table below. Include uncertainty. Number of Decayed Daughter Nuclei (n/nuclei) Time (t, s) ± ?. ?? ? 0.52 1.04 1.56 2.08 Trial 1 Trial 2 Trial 3 Average ± ______ ?????? 2.60 14. Share and discuss one aspect of your table with another student in the class. 15. Use MS Excel to make a graph of average decays v. time including vertical uncertainty error bars. Staple this to this graph worksheet.