A photoelectric device contains two separate cathodes, C₁ and C₂
PHYSICS · Multi-part · A Level · Physics · Particles and radiation · Particles and radiation
Problem
A photoelectric device contains two separate cathodes, C₁ and C₂, made of different metals that share a common anode and are connected in parallel to the same adjustable external voltage supply. Both cathodes are simultaneously illuminated by an ultraviolet source of fixed frequency f0 and uniform intensity. The threshold frequencies of the metals are fth,1 for C₁ and fth,2 for C₂, with f0>fth,1>fth,2. The external supply can impose either a retarding or an accelerating potential on the anode with respect to the cathodes.
(a) Derive, in terms of the given symbols, expressions for the maximum kinetic energies of electrons emitted from each cathode.
(b) Using energy conservation, obtain expressions for the minimum external potential (retarding) required to reduce the photo-current from each cathode to zero individually.
(c) As the external potential is varied from a large negative value (strong retardation) to a large positive value (strong acceleration), sketch and label qualitatively the combined current contributed by both cathodes as a function of the external potential. Clearly indicate the key potential values at which changes in slope or saturation occur, and justify the overall shape of your graph.
(d) Suppose C₁ is replaced with a new cathode C₁′ whose threshold frequency equals f0. Predict and explain how your graph from part (c) changes, paying particular attention to the onset of current from each cathode.