# Negative potential energy and capacitors

wrobel
potential energy is defined up to an additional constant by the way

anuttarasammyak
Gold Member
Say parallel opposite charges plates whose distance d are:

1. infinite
2. finite ##d_0##
3. 0 and all electrons and positive ions on the plate reunite after releasing energy with short circuit.

Then energy of the states are
$$E_3<E_2<E_1$$

If we take ##E_1=0## as you do for gravity, ##E_2## is negative.

If we take ##E_3=0## as usually we do in electric circuit studies, ##E_2## is positive.
Even in gravity study, we have learned that a object of mass m in height h has potential energy ##mgh >0##.

We can take energy out of a capacitor by:

1. Letting the positive charges move further apart. That decreases the positive energy of one plate.
2. Letting the negative charges move further apart. That decreases the positive energy of one plate.
3. Letting the opposite charges get closer together. That increases the negative energy of the capacitor.
4. Draining the capacitor, which involves all those three things above.

In the link in post #1, there it is mentioned something about zero being a special number among numbers, and gravitational potential at infinity being a special gravitational potential among gravitational potentials. Well, I agree with that part of the answer. (Actually only zero being an important number was mentioned. Well, it's not a very good answer IMO)

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