10.1.5—The effects of the internal resistance of a source of e.m.f. on the terminal

A real source can be modelled with internal resistance r. When current I flows, terminal voltage V=ε−Ir, so some source energy is dissipated internally.

The lost voltage Ir grows with current. At open circuit I=0, terminal voltage approaches ε.

For ε=6.0 V, r=0.50 Ω and I=2.0 A, terminal voltage is 5.0 V.

Internal resistance is not a separate external component that must be drawn outside the source; it represents loss inside the source model.