Question

The electric potential increases from 96 V to 700 V from the bottom plate to the top plate of a parallel-plate capacitor.
(a) What is the magnitude of the change in potential energy of a -3x10^-4 C charge that is moved from the bottom plate to the top plate?
(b) Does the potential energy increase or decrease in this process?

Answer 1

what do you think?!?!

Answer 2

I thought you were supposed to subtract the electric potentials from each other and then multiply it by the potential energy. And I did that and got -.1812, but the answer is +.1812. I know you have explained it before but I am still having an extremely hard time understanding why the signs switch

Answer 3

Electrostatic Potential Energy is the negative of the work you must do to move a charge from one position to another. So if a positive charge moves from the top plate to the bottom plate due to the electric field, you do no work, you do negative work. So the potential energy will be positive.

Similarly, a negative charge moves (due to the electric field) from the bottom to the top, you do negative work again, so potential energy will be positive.



Looking at the definition of Electrostatic Potential Energy, if q is positive and \(\triangle V\) is also positive, then U will be positive. In going from the bottom plate to the top \(\triangle V=96-700=-604\) (which is negative, of course) and since q is also negative U will be positive:
$$
U=q\triangle V
$$
Hope this helps!

Answer 4

(LOL, Looks like the first part of my note got chopped off )

Electrostatic Potential Energy is the negative of the work that you must do to move a charge from one point to another.

For example, if a positive charge is moved from the top plate to the bottom, the electric field does the work, so you do negative work. Then the potential energy will be positive.

Similarly, if you have a negative charge and it moves from the bottom plate to the top, the electric field does the work, you do negative work. So potential energy will be positive.

That's it! Simple huh?

Answer 5

@GVSU2016 not to worry. the trick as i Said before is to draw it. draw the field lines between the plates. you should know which plate is +ve from the voltages. then we can logically work through how a change of either persuasion would behave in that field.