# Capacitor Stored Energy - Joules/Watts/Amps

Discussion in 'General Electronics Chat' started by Mad Professor, Feb 23, 2013.

Apr 15, 2009
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Good day all.

How do you work out the stored energy of a capacitor, In Joules/Watts/Amps?

Lets just say I have a large audio capacitor, 3.5 Farad 10%, (ESR): 0.00195 Ohms @ 120 Hz, that is charged to 20volts.

What would be the stored energy in this capacitor?

And what is the right term when talking about stored energy in capacitor's.

I have read it's Joules, Watts, Watt-Seconds, Amps, Etc.

Best Regards.

Aug 27, 2009
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3. ### Raymond-L New Member

Feb 23, 2013
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The formula for stored energy in a cap is 1/2(CV^2) as is probably shown in nsaspook's links.

As for terminology I would use Joules for stored energy, Watts is Joules / Seconds and Amps is flowing current, I believe the above formula gives an answer in Joules.

4. ### GopherT AAC Fanatic!

Nov 23, 2012
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Note that C is capacitance in the previous post, not Coulombs and not Current

5. ### crutschow Expert

Mar 14, 2008
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Joule is energy.
Watt is power (energy per unit time)
Current is electron flow and is not a measure of energy unless volts are included.

6. ### WBahn Moderator

Mar 31, 2012
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Fundamentally, in the SI system of units, energy has units of kgm^2/s^2. We create named units, usually named in honor of someone, for combinations of basic units. So the unit of energy is also Nm since a 1N (newton) is 1kgm/s^2, the unit of force. In the case of energy we use the joule and define it to be 1Nm.

This continues on into the electrical realm where is all starts with the definition of electric field as a force per unit charge (N/C) (newton/coulomb) and voltage as the work done in moving a charge from one place to another in an electric field, giving it units of Nm/C, or J/C (joules/coulomb).

Power is the rate at which energy is exchanged and has units of watts, so 1W=1J/s (joule/second). But this also means that 1W for a period of 1s is equivalent to 1J, so 1J=1Ws (watt-second).

In the basic formula for energy stored in a capacitor, you have

U=(1/2)CV^2

which can be written as

U=(1/2)(CV)V

Since Q=CV is the amount of charge stored on the capacitor, we then have

U=(1/2)QV which would have units of volt-columbs.

Since a coulomb is the amount of charge associated with an ampere of current over a duration of one second, we have 1C = 1As (amp-second). So the unit of energy can also be expressed as volt-amp-seconds.

So, as you can see, we can use a bunch of different units to express the quantity of energy, just as we can use many different units to express length or volume.

Of the ones you mentioned, joules and watt-seconds are correct and watts and amps are wrong.