EMF and Internal Resistance

Energy in Circuits

In circuits there are two fundamental types of component: energy givers and energy takers.

Electromotive Force (emf), ε

Energy givers provide an electromotive force, they force electrons around the circuit which transfer energy.

The size of the emf can be calculate using:

This is similar to the equation we use to find voltage/potential difference and means the energy given to each unit of charge. We can think of this as the energy given to each electron.

The emf of a supply is the p.d. across its terminals when no current flows

EMF is measured in Joules per Coulomb, JC-1 or Volts, V


Energy takers have a potential difference across them, transferring energy from the circuit to the component.

emf = energy giver                               p.d. = energy taker

Energy is conserved in a circuit so energy in = energy out, or:

The total of the emfs = The total of the potential differences around the whole circuit

Internal Resistance, r

The chemicals inside a cell offer a resistance to the flow of current, this is the internal resistance on the cell.

Internal Resistance is measured in Ohms, Ω