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Sources of e.m.f. – internal resistance.
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Sources of e.m.f. - internal resistance. A source of e.m.f. always has some resistance to electric current within it, called its internal resistance. The internal resistance of a source of e.m.f. has two effects: 1. It results in a voltage across the terminals of the source dropping as a current is drawn from it. 2. It results in the source being less than 100% efficient as energy is dissipated in the internal resistance as current flows through it. The voltage quoted on the label of a source of e.m.f. such as battery is the voltage measured when no current is being drawn from it, often called the open-circuit voltage. The internal resistance of a source of e.m.f. may be thought of as a resistance r in series with the nominal e.m.f. E. When a current I is drawn from the source a p.d. ?V appears across the internal resistance r, so that ?V=Ir. The...
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