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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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Twin question by the same user. User has created another account..! –  Waffle's Crazy Peanut May 1 '13 at 6:25
Welcome to Physics! Please see our homework policy. We expect homework and homework-like problems to have some effort put into them, and deal with conceptual issues. If you edit your question to explain (1) What you have tried, (2) the concept you have trouble with, and (3) your level of understanding, I'll be happy to reopen this. (Flag this message for ♦ attention with a custom message, or reply to me in the comments with @Manishearth to notify me) –  Manishearth May 1 '13 at 8:04
Besides, we don't do interpretations of questions here. Please see the faq and How to Ask, a question should be useful to a wider audience. –  Manishearth May 1 '13 at 8:04
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closed as too localized by Manishearth May 1 '13 at 8:04

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1 Answer

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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