HW11 - Homework 11 PDF

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Homework 11...

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Homework Assignment 11 Physics 7C, UC Berkeley Due Thursday, August 6th, at 11:59PM Problem 1 A photocell is based upon the photoelectric effect. A light beam illuminates a metallic plate, and in some conditions electrons are ejected from the metal. In order to measure the maximum kinetic energy of the electrons, another plate is kept at a negative enough potential that the emitted electrons are slowed down and stopped. (a) Explain how the particle-like aspect of light can explain (i) the absence of delay in the observation of the current, (ii) the existence of a cut-off frequency which is independent of the intensity of light, (iii) and the fact that the maximum kinetic energy of the electrons only depends of the frequency, not the intensity. (b) Sketch on the same graph the maximum kinetic energy of the ejected electrons as a function of the frequency of the incident light for Cs and Al (φAl > φC s ). What physical quantities do the x-intercept and the slope correspond to? What is the y-intercept and why? (c) Assuming the frequency f is high enough to allow electrons to be ejected from a metal of work function φ, determine the stopping voltage Vs that needs to be applied between the two plates in order to stop the fastest electrons. (d) Considering the fastest ejected electrons as non-relativistic particles of mass m, express the magnitude of their linear momentum p and their de Broglie wavelength λ in terms of f , m, h and φ. Electrode

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