# What is the formula for caculating energy in a capacitor.

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Consider the capacitor with the capacitance C, the charge on the plates *q*, and the potential difference between the plates *U.*

Then the energy stored in the capacitor is `E=(qU)/2` .

Since by definition the capacitance is `C=q/U` , the formula for energy can be rewritten in terms of *C* and *U*:

`q =CU` , so `E=((CU)U)/2=(CU^2)/2`

Alternatively, it can be written in terms of *q *and *C*:

`U = q/C` , so `E=q/2 (q/C) = q^2/(2C)`

Therefore,

`E=(qU)/2 = (CU^2)/2 = q^2/(2C)`