TE Enriched Sample (E)

Photoelectric Effect 1 30 Solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (a) (i) Curve A , because the saturation photocurrent should increase with the light intensity for the same frequency (or wavelength). At a fixed frequency, light of higher intensity delivers more photons per second. Hence, more electrons can absorb photons and escape from the metal plate, and the photocurrent is larger. (ii) The horizontal intercept is the stopping potential − V s . If the frequency of the light is unchanged, the energy of the photons will be unchanged too. Hence, the photoelectrons emitted will have the same maximum KE and stopping potential. The intensity only affects the number of photons delivered per second but not their energy. (b) (i) Max KE of the photoelectrons 1.6 eV K eV max s = = Energy of the photon . eV hf hc 350 1243 3 55 m = = = Equating K eV max s = and K hf max z = - , we get eV hf s z = - So, the work function is . . . eV hf eV 3 55 1 6 1 95 s z = - = - = (ii) No. of photons hitting the surface N ( . ) ( . ) ( . ) ( ) 7.04 10 s 3 55 1 60 10 0 01 4 10 19 4 12 1 # # # = = - - - Max photocurrent . ( . ) ( . ) ( . ) 1. 10 A I Ne 0 001 0 001 7 04 10 1 60 10 13 max 9 12 19 # # # = = = - - (c) Curve Y . Since green light has a lower frequency, its photons have less energy. The maximum KE of the photoelectrons is lower too. The photoelectrons are therefore stopped by a smaller potential, which is the horizontal intercept. φ = hf 0 ≠ eV s One may suggest that a beam of higher frequency (but equal intensity) will give a smaller saturation photocurrent: I t Ne h E t e f f 1 constant P $ # = = = assuming the photoelectric efficiency remains the same for different frequencies (see Enrichment on the next page). But the assumption is unrealistic because this factor varies greatly with frequency. 0 I p / μA V / V f 1 f 2 f 2 > f 1 (See DSE 2014 Q2.6) Teaching notes Ensure Ss understand the graphs on p.34 bottom right and do not mix them up. (See DSE 2019 P2 Q2.2) (Also see DSE 2022 Q2aii: finding number of photoelectrons) 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.