What is EMF and internal resistance?

Also known as: electromotive force · internal resistance

Definition 12.18 High School Physics · Chapter 12 — Electric Circuits and Power

A battery converts chemical into electrical energy. Its electromotive force (EMF) $\mathcal{E}$ , in volts, is the energy it hands each coulomb crossing it; its internal resistance $r$ accounts for the Joule effect inside its own chemistry. Model: an ideal source $\mathcal{E}$ in series with $r$ (curly $\mathcal{E}$ : $E$ is energy).

Examples

Example 12.20 (A battery under load)

A battery ( $\mathcal{E} = 9.0\,\mathrm{V}$ , $r = 1.0\,\Omega$ ) feeds a bulb of resistance $R = 8.0\,\Omega$ . The loop obeys $\mathcal{E} = rI + RI$ , so $I = \mathcal{E}/(R + r) = 1.0\,\mathrm{A}$ and $U = 9.0 - 1.0 \times 1.0 = 8.0\,\mathrm{V}$ : the bulb receives $8.0\,\mathrm{W}$ while the battery wastes $1.0\,\mathrm{W}$ warming itself.

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Examples

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