When a current-carrying coil is placed in a magnetic field it rotates due to the motor effect and the area swept out by the coil changes. This means that the magnetic flux linkage changes and an e.m.f. is induced in the coil. According to Lenz's law the induced e.m.f. acts in the opposite direction to the change that caused it. This means it acts to oppose the potential difference across the coil and is called the back e.m.f. The induced e.m.f. is proportional to the speed of rotation so at low speeds the back e.m.f. is small and the current is high. At high speeds the back e.m.f. is high and the current is low. Power is wasted due to the circuit resistance. The total power supplied IV is distributed between Iε (electrical power transferred to mechanical power) and I2R. (power wasted).