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Here I think, one beam will knock out just one electron. So, I am not able to even understand what the question says. Please someone give a hint as to what the question asks...

As source of the question isn't reliable, it may be wrong too (?)

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closed as too localized by Manishearth May 1 '13 at 8:04

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Twin question by the same user. User has created another account..! –  Waffle's Crazy Peanut May 1 '13 at 6:25
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1 Answer 1

I would guess the question is asking you what the maximum charge on the sphere is.

Suppose the photon energy ($hc/\lambda$) is $E$, then the kinetic energy of the electron leaving the surface (in electron volts) is $E$ - 4.47. As you increase the positive charge on the sphere you increase the work needed to remove an electron to infinity, and for some charge $Q$ the work will be equal to $E$ - 4.47. At this point you can't charge the sphere any further because the photoelectrons can no longer escape.

So calculate the potential energy, $V$, of an electron just outside the surface of the ball, calculate the energy, $E$, of a 250nm photon, set $V = E = 4.47$ and you can calculate the maximum charge.

Later: I've just done the calculation above, and it does match one of the answers given.

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