Change in potential energy of charge?

Question

The electric potential increases from 72 V to 686 V from the bottom plate to the top plate of a parallel plate capacitor. What is the magnitude of the change in potential energy of a -3 X 10^-6 C charge as it moves from the top plate to the bottom?

Answer 1

The formula here is U (potential energy) = - qV, where q = charge and V is potential difference (volts)

 

V = 686 - 72 = 614 volts

q = -3*10^-6   Coulomb

 

So: U =  - [-3*10^-6 * 614]  =  1.8 * 10^-3  Joules

 

The fact that the answer is + makes sense because we assume the top plate is +, so there is a drop in voltage (potential difference) going from top to bottom. A + charge would undergo a drop in PE as it goes from top to bottom, because it is attracted to the negative bottom plate. Therefore, a -charge would undergo an increase in PE from top to bottom because it is repelled by the  negative bottom plate.