A practical electrical power source which is a linear electric circuit may, according to
Thévenin's theorem, be represented as an
ideal voltage source in series with an
impedance. This impedance is termed the
internal resistance of the source. When the power source delivers
current, the measured voltage output is lower than the no-
load voltage; the difference is the voltage drop (the
product of current and resistance) caused by the internal resistance. The concept of internal resistance applies to all kinds of electrical sources and is useful for analyzing many types of electrical circuits.