<P> Fleming's left - hand rule is used for electric motors, while Fleming's right - hand rule is used for electric generators . </P> <P> Different hands need to be used for motors and generators because of the differences between cause and effect . </P> <P> In an electric motor, the electric current and magnetic field exist (which are the causes), and they lead to the force that creates the motion (which is the effect), and so the left hand rule is used . In an electric generator, the motion and magnetic field exist (causes), and they lead to the creation of the electric current (effect), and so the right hand rule is used . </P> <P> To illustrate why, consider that many types of electric motors can also be used as electric generators . A vehicle powered by such a motor can be accelerated up to high speed by connecting the motor to a fully charged battery . If the motor is then disconnected from the fully charged battery, and connected instead to a completely flat battery, the vehicle will decelerate . The motor will act as a generator and convert the vehicle's kinetic energy back to electrical energy, which is then stored in the battery . Since neither the direction of motion nor the direction of the magnetic field (inside the motor / generator) has changed, the direction of the electric current in the motor / generator has reversed . This follows from the second law of thermodynamics (the generator current must oppose the motor current, and the stronger current outweighs the other to allow the energy to flow from the more energetic source to the less energetic source). </P>

Left hand rule for the magnetic field around a conductor