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Nuclei
Question
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A photocell is illuminated by a small bright source placed 1 m away. When the same source of light is placed 1 /2 m away, the number of electrons emitted by photocathode would

  • decrease by a factor of 4

  • increase by a factor of 4

  • decrease by a factor of 2

  • increase by a factor of 2

Answer

B.

increase by a factor of 4

On doubling the distance the intensity becomes one fourth i.e. only one-fourth of photons now strike the target in comparison to the previous number. Since the photoelectric effect is a one photon-one electron phenomenon, so only one-fourth photoelectrons are emitted out of the target hence reducing the current to one fourth the previous value.

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Some More Questions From Nuclei Chapter

The above is a plot of binding energy per nucleon Eb, against the nuclear mass M; A, B, C, D, E, F correspond to different nuclei. Consider four reactions:

(i) A + B → C + ε  (ii) C → A + B + ε
(iii) D + E → F + ε and (iv) F → D + E +ε

where ε is the energy released? In which reaction is ε positive?

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