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## Homework Statement

Lets say we have a silver ball with radius ##r=1cm## hanging on a string which is an isolator. What is a charge on the ball if we shine on it with a light with ##\lambda = 200nm##? The work function for silver is ##A_0 = 4.7eV##.

## Homework Equations

\begin{align}

&W - A_0 = W_k = eU\\

&W = h\nu = \frac{hc}{\lambda}

\end{align}

## The Attempt at a Solution

First i can calculate the maximum kinetic energy that the photoelectrons have when knocked out of the metal.

\begin{align}

W_{k1}&=W - A_0\\

W_{k1}&= \tfrac{hc}{\lambda} - A_0\\

W_{k1}&= \tfrac{6.626\cdot 10^{-34}Js \cdot 2.99\cdot 10^{8}\tfrac{m}{s}}{200\cdot 10^9 m} - 4.7\cdot 1.602\cdot 10^{-19} J\\

W_{k1} &= 2.38\cdot10^{-19}J\\

W_{k1} &= 1.48eV\\

\end{align}

So now i know the kinetic energy of the photoelectrons knocked out of the metal, but where is my voltage ##U##? I know that in clasic photoeffect we need to provide enough voltage to lower the kinetic energy to ##0##, but here i really dont know where i can get the voltage from.

Can anyone tell me how to finish this problem ?