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Chapter38

# Chapter38 - 38.1 Model The photoelectric current depends on...

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38.1. Model: The photoelectric current depends on the potential V between the two electrodes, the nature of the cathode metal, and the intensity of the light. Visualize: (a) When V > 0, all the emitted electrons are attracted to and collected by the anode. This means a further increase in the voltage cannot change the number of electrons arriving per second and thus cannot increase the current. (b) The work function E 0 is the minimum energy an electron needs to escape from the metal. Some electrons, such as those a bit further from the surface, need more than E 0 to escape. There is a range of escape energies, so the escaped electrons have a range of kinetic energies and not a single kinetic energy. (c) If the anode potential is V , an electron leaving the cathode with kinetic energy K arrives at the anode with kinetic energy K = K + eV . A negative V causes a decrease in kinetic energy. K cannot become negative, so for eV K the electron is repelled by the anode and turned back toward the cathode. The emitted electrons have a maximum kinetic energy K max . When eV = K max , all electrons are turned back and the current drops to zero. If the current reaches zero at V = V stop , then V stop = K max / e . The stopping voltage measures the maximum kinetic energy by causing the most energetic electrons, those with K = K max , to be turned back from the anode.

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38.2 Visualize: According to classical physics, the photoelectric current is not dependent on the light’s frequency. If the light intensity remains constant (same energy per second falling on the metal), the photoelectric current should be constant with no threshold as the frequency is changed. Thus, the graph would be a horizontal line, starting from f = 0 Hz.
38.3. Visualize: The graphs would still be horizontal for V > 0 V because all the electrons are being collected. Also, more intense light would still give a larger current. But classical physics postulates a thermal cause for the photoelectric effect. If this were true, more intense light would heat the electrons more and thus eject the electrons with more kinetic energy. In this case, more intense light would have a larger stopping voltage . This would appear on the graph as a more negative x -intercept.

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38.4. Model: The photoelectric current depends on the potential difference V between the two electrodes, the nature of the cathode metal, and the intensity of the light. Visualize: Solve: (a) According to classical physics, there is no dependence on the light’s wavelength. If the light intensity remains constant (same amount of energy falling on the metal cathode), the photocurrent will be unchanged. (b) The maximum kinetic energy of the electrons emitted from a cathode is K max = E elec E 0 . If E 0 is smaller for a different metal, the emitted electrons will have a higher kinetic energy and thus the stopping potential will be larger.
38.5.

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