TPR FL2 #35 C/P section

[akimhaneul]

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I am confused about this question. Why is it that you use 1/2CV^2 in one situation and q^2/2C in the other?

Thanks!

 

[theonlytycrane]

Q = VC

PE (capacitor) = 1/2QV (but you can make substitutions using the equation above).

When the battery is connected, V is constant so we want to use PE = 1/2V^2C.

When the battery is disconnected, Q is constant so we want to use PE = 1/2Q^2/C.