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Say we have a balloon, negatively charged, the voltage on it is 500 V. Can I measure the charge on it or in other words, the number of excess electrons?

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The voltage $V$ is actually the potential on the surface so you could use this formula to calculate the charge on it $V$=$\frac{q}{(4\pi\epsilon_0)R}$ where $R$ is the radius of the balloon and the value of $\frac{1}{4\pi\epsilon_0}$ is $9*10^9$ –  Akash Mar 17 '13 at 5:01
    
what if i was use a charged metal rod or something –  luigy Mar 20 '13 at 6:23
    
the same applies here also –  Akash Mar 20 '13 at 10:10
    
what would the R represent if i was using a charged metal plate? –  luigy Mar 21 '13 at 9:09
    
actually the same formula is not applicable in this case but the same concept is applicable. If you know how to calculate potential on the surface of the plate you can calculate the charge on it but I don,t know the formula I will tell you after some time –  Akash Mar 21 '13 at 10:52

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We can calculate charge on baloon by assuming it a perfect sphere with uniform charge distribution.$$V=\frac{q}{4\pi\epsilon_0\times r}$$

where $r$ is radius of ballon.

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