Electromotive force is the Energy (in joules) given to each coulomb of charge in passing through the source.

ie. A 12 V cell gives 12 joules to each coulomb of charge that passes through it.

(This is of course, not taking into account any Internal Resistance that the said cell may have!)

If anyone's confused by the above :

When you lift a heavy object off the ground and hold it, the object has the energy to move toward the ground and is trying pretty hard. That's called potential energy.

But gravity isn't the only force that pulls things together. One other place they attract is in electricity, where opposites attract.

If you have a bunch of negatively charged electrons near a positively charged material, they will have potential energy because they're trying to move toward the positive material.

Voltage is the amount of potential energy per electron1. So, if each electron is screaming to get to the positive material, you have high voltage. If it's not trying so hard, you have less voltage.

The unit they use for voltage is volt. Volt = joule (the unit for energy) / coulomb (the unit for measuring how negatively or positively charged something is).


To "prove" this to yourself with common formulas :

  1. I = q / t (amperes = coulombs / second)
  2. P = E / t (watts = joules / second)
  3. P = I * V (watts = volts * coulombs)
Plug current and power from the first two equations into the third equation and solve for voltage. You end up with :
  • V = E / q (volts = joules / coulomb)

For you strange people out there, it comes to a (kg * m2) / (s3 * A).


1I realize it's not technically the energy per electron. I also realize that it's much easier to teach it that way and it's not likely to get in the way once someone learns that it's a Joule per Coulomb.

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