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-   -   Affect on stored energy if charge is doubled.. (http://www.physicsforums.com/showthread.php?t=181271)

 t_n_p Aug22-07 03:50 AM

Affect on stored energy if charge is doubled..

The problem statement, all variables and given/known data

A parallel plate capacitor is air filled and assumed to be ideal. It is connected to a battery so it has constant potential difference. By what factor does stored energy change if the charge is doubled..

The attempt at a solution

Using the formula U = 0.5QV..

If charge is doubled, then U is doubled correct?

I say the answer is stored energy is doubled, but the answer at the back reckons quadrupled. Who is right?

 BlackWyvern Aug22-07 04:20 AM

E = Q^2 / 2C is another formula. That would yield a 4 times greater energy being stored.

Also, if E = (0.5)QV, then if Q = 2Q

Q = CV
C = Q/V

C = 2Q/2V, as C cannot change, so V must change in order to keep it constant.

E = (0.5).2Q.2V

I think.

 t_n_p Aug22-07 04:58 AM

well I'm given two formulas, U = (0.5)QV and U = 1/2CV^2

The second formula is not relevant as charge is not a variable, hence my above reasoning...

 vinayakbhat82 May8-09 06:00 AM

Re: Affect on stored energy if charge is doubled..

In case the charge doubles, the voltage would also get doubled. Hence stored energy would get quadrupled. All the formulas would yield the same result.

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