Question

How much electrical potential energy is stored in a capacitor that has 8.0 × 10-10 C of charge on each plate and a potential difference across the plates of 20.0 V?

A. 1.0 × 10-11 J
B. 8.0 × 10-9 J
C. 4.0 × 10-9 J
D. 2.0 × 10-11 J

Answer 1

THere is a standard relationship that gives this result where the capacity of the capacitor is used:
$E=1/2* \frac{Q^2}{C}$.
We know though that Q/c=V and thus we can use the relationship:
E=Q*V/2 where we have just substituted in. If we also take into account that Q=VC, we can also get that E=V^2*C/2.
We are given the charge and the potential, so the best expression to use is the middle one.
Substituting, we get that E=1/2*8*10^(-10)*20=8*10^(-9).

The answer is B