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A few theoretical questions on Capacitors!

Hello I have a few problems understanding this topic, Can anyone clarify the following:

When you charge a capacitor when does it actually stop charging when it reaches the potential difference equal to the power supply?

Once charged and disconnected for later use, where is that charged stored in a parallel plate capacitor..in the dielectric material?

Also when charging a capacitor what does 1 Capacitance actually mean the amount of charge that needs to raise the capacitor to 1V greater..because at the end of the day the charge around the circuit is constant so it means that voltage must be lost when charging the capacitor or else it would be the same as the rate of charge flow in the circuit?

Thanks a lot

Reply 1

Eddie K

I am assuming you mean a capacitor on it's own with, say, a battery connected to it.

1. The charge is limited by the potential you apply across the capacitor, yes. A lone capacitor will reach the potential of the supply, while a capacitor connected with another component like a resistor will reach a potential defined by the characteristics of the two components.

2. There will be a small charge that remains on the plates of the capacitor after discharge. There is a very small leakage charge that may exist in the dielectric, but for good capacitors this may be ignored.

3. Think of capacitance simply as the storage capability of a capacitor. A capacitance of 1 Farad means that for every 1 Volt applied across the capacitor, 1 Coloumb of charge will be stored. I don't understand what you mean by your second point, it is a bit muddled. Please clarify.

Reply 2

Neo1

In my second point, what I mean is, once the capacitor is fully charged with electrons full of energy..where is this stored in the parallel plate capacitor? In which plate?

3) So basically it's how many columbs you need to raise the capacitor to a voltage of 1...Does this mean it loses energy at some point since really shouldnt the energy stored in the capacitor equal to the amount of charge with a voltage given by the power supply..where does the other energy go to if more charge is needed to raise the capacitor by a voltage of 1 ?

Reply 3

Eddie K

There are different types of energy in this case, the thermal energy of the electrons, and the energy associated with the electric field. The former is stored in the electrons and the latter is stored across the electric field, i.e, in the gap separating the plates, since this is the region in which there exists a difference in electrical potential. Some energy is lost in heating, and a small amount in hysteresis, which is due to continual loading of the capacitor over time.

Reply 4

Neo1

Oh right...What I'm also not understanding is:

If you have a circuit with two capacitors connected in series being supplied a voltage...and one capacitor has a value less than the other one...the smaller capacitor would have the highest voltage across it right..but which would have the most energy...it is common to think that the one with the most voltage but since 1/2 = CV² the one with the greater value of capacitance would have greater energy?

Thanks a lot

Reply 5

sufiankane

think of 0.5QV= energy on a capacitor. in series the charge stored on each plate is different however the voltage across the smaller capacitor is larger, so the smaller one has more energy.

Reply 6

Neo1

sufiankane

think of 0.5QV= energy on a capacitor. in series the charge stored on each plate is different however the voltage across the smaller capacitor is larger, so the smaller one has more energy.

Isnt the charge stored in each capacitor the same in a series circuit??

I understand that the capacitor with the smallest value will have the greatest voltage across it for the same charge received as the other. So 1/2QV is greatest right? But if you use the formula 1/2CV² the capacitor with the smallest value would have less energy according to that formula?

Thanks a lot