TE Enriched Sample (E)

Photoelectric Effect 1 28 (b) Combining K eV max s = and K max = hf − φ gives eV hf s z = - Dividing both sides by e , we get V e h f e s $ z = - The slope is h / e , the y -intercept is − φ / e , and the x -intercept is f 0 (the threshold frequency). (c) From (b), the slope of the V s – f graph is . J s h 6 00 10 ` # = ( ) Hz ( ) V . C slope e h h 12 4 10 3 0 1 60 10 14 19 34 # # = - - = - - (d) From the graph, the threshold frequency f 0 = 4 × 10 14 Hz . The work function is ( ) ( ) . J . eV hf 6 10 4 10 2 4 10 1 50 0 34 14 19 # # # # z = = = = - - (e) From the equation V e hf e s z = - changing the work function φ only affects the vertical intercept of the V s – f graph but not the slope. Hence, the new graph should be parallel to the old one and have a less negative vertical intercept. J . eV 1 1 6 10 1 19 # = - f / 10 14 Hz 1 4 8 12 0 2 3 V s / V new graph Watch-out The roles of f 0 and V s Do not mix up the roles of f 0 and V s in the photocell experiment. You can see the distinction more clearly by expressing the photoelectric equation in the following way: photon’s energy photoelectron’s min. escape energy (metal‘s work function) photoelectron’s max. initial KE hf φ K max = hf 0 = eV s Here, hf 0 measures the energy barrier against the emission from the metal, while eV s measures the energy barrier that stops the flight to the anode. More specifically, (1) if f < f 0 , no photoelectrons are emitted from the metal surface. (2) if | V | > V s , no photoelectrons emitted can reach the anode. In both cases, no photocurrent is produced. { } { } { } = + Teaching notes For part (e), remind Ss that the V s – f graphs of different metals are parallel to each other but have different vertical (horizontal) intercepts. The slope of the graphs is independent of the metal used. Sample © United Prime Educational Publishing (HK) Limited, Pearson Education Asia Limited 2023 All rights reserved; no part of this publication may be reproduced, photocopied, recorded or otherwise, without the prior written permission of the Publishers.