PhysicsChapter 11: Electricity

EMF and Energy per Unit Charge

Electromotive force (emf) is the energy supplied by a source per unit charge passing through it. Despite its name, emf is a voltage, not a force.

Essential equations

Key equation

ε=WQ

ε: emf (V)
W: energy supplied by the source (J)
Q: charge passing through the source (C)

Key equation

W=ε Q

Key equation

P=ε I

Key equation

1\ V=1\ J\,C^{-1}

EMF measures energy supplied per coulomb.
Potential difference measures energy transferred per coulomb between two points.
Open-circuit terminal voltage is approximately equal to emf.
For a discharging real cell, V=ε−Ir.

Quick solved example

A 9.0 V battery moves 20 C through itself:

Key equation

W=ε Q=(9.0)(20)=180\ J

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