The maximum power transfer theorem is a principle in electrical engineering that states that to obtain maximum external power from a power source with internal resistance, the resistance of the load must equal the resistance of the source as viewed from its output terminals. The theorem results in maximum power transfer from the power source to the load, and not maximum efficiency of useful power out of total power consumed. The theorem can be extended to alternating current circuits that include reactance, and states that maximum power transfer occurs when the load impedance is equal to the complex conjugate of the source impedance. The theorem is not limited to electrical circuits and can be applied to other physical interactions such as mechanical collisions, the sharing of charge between two capacitors, and liquid flow between two cylinders. The maximum power transfer theorem is useful in system design, particularly in radio transmitter design, where the antenna or transmission line impedance is matched to the final power amplifier impedance for maximum radio frequency power output.