When a capacitor is charged up, then disconnected from the charging source, it has a difference in electrons between the plates and the dielectric molecules are under stress of electrostatic lines of force

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# ENERGY STORED IN A CAPACITOR

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### What energy is stored in a capacitor?

When a capacitor is charged up, then disconnected from the charging source, it has a difference in electrons between the plates and the dielectric molecules are under stress of electrostatic lines of force. If the charged capacitor were connected across a light bulb for example the excess electrons would flow from the negative plate through the light filament to the positive plate of the capacitor, the light bulb might glow for an instant.

The amount of energy stored in a capacitor (expressed as watt-seconds or joules) can be computed from:

En = CE2 / 2

where:
En = Energy in watt-seconds
E = Volts

### Quantity of charge in a capacitor

The charge in a capacitor is the number of electrons on the two plates. This involves the difference in the quantity of electrons and the unit of quantity is the coulomb.

Q = CE

where:
Q = Coulombs
E = Volts

As an example if we have a 4,700 uF capacitor across a 12V supply the electron difference between + and - plates would be:

Q = CE = 0.0047 X 12 = 0.0564 Coulomb

We learnt in the topic current that:

"The quantity of electrons used in determining an ampere is called "coulomb" which one ampere is one coulomb per second. A coulomb is 6,280,000,000,000,000,000 or 6.28 X 10 18 electrons. This is the unit of measuring electrical quantity or charge".

Therefore 0.0564 Coulomb = 0.0564 X 6.28 X 10 18 = 3.542X 10 17electrons

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