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


 

Authored by Ian C. Purdie VK2TIP

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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
C = Capacitance in Farads
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
C = Capacitance in Farads
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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the author Ian C. Purdie, VK2TIP of www.electronics-tutorials.com asserts the moral right to be identified as the author of this web site and all contents herein. Copyright © 2000, all rights reserved. See copying and links. These electronic tutorials are provided for individual private use and the author assumes no liability whatsoever for the application, use, misuse, of any of these projects or electronics tutorials that may result in the direct or indirect damage or loss that comes from these projects or tutorials. All materials are provided for free private and public use.
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Updated 15th May, 2000

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