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The Difference Between Electromotive Force and Potential Difference

When charge passes through a power supply such as a battery, it gains electrical energy. The power supply is said to exert an electromotive force or emf – in spite of it's name, the emf is not a force. The electromotive force measures in volts the electrical energy gained by each coulomb of charge that passes through the power supply. The energy gained by the charge comes from chemical reactions inside the battery.

When charge passes through a resistor, it's electrical energy is converted to heat energy in the resistor. The resistor has a potential difference across it. The potential difference measures in volts the potential energy lost by each coulomb of charge that passes through it.

Note that voltage is not the same as either emf or potential difference, though often all three terms are used interchangeably.

We may think of potential difference as height, amd charge as mg then we can write the above as

Continuing this analogy, the difference between emf and potential difference is the energy per unit mass lost by overcoming air resistance.