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An electric motor takes electrical energy and converts it into mechanical energy, often to produce motion (such as in a car). A generator, on the other hand, takes mechanical energy and converts it back into electrical energy, typically to generate electricity (such as a wind turbine). The duality comes from the fact that electric motors and generators share a very similar design, and are often built with the same components, such as a coil of wire and a magnetic field. In both devices, a magnetic field is used to induce a current in a wire, which then produces a force in accordance with Faraday's Law of Electromagnetic Induction. In a motor, this force is what drives the device's mechanical output, whereas in a generator, the force is what produces the electrical output.

In an electric motor, a current-carrying coil is placed within the magnetic field produced by permanent magnets or electromagnets. According to Fleming's left-hand rule, the current through the coil interacts with the magnetic field, generating a force that moves the coil, causing the rotor to rotate and produce mechanical work. The input is electrical energy, and the output is mechanical energy. In a generator, the opposite process occurs, and the rotor is mechanically rotated within the magnetic field. This time, the motion of the coil within the magnetic field induces an electromotive force (EMF) according to Faraday's Law of Electromagnetic Induction. The induced current in the coil is the generator's output, and the input is mechanical energy.

Due to their similar design and operating principles, a motor can often be used as a generator, and vice versa, with the primary difference being in how the device is driven. For example, if a motor is mechanically driven, it will act as a generator and produce electrical energy. Conversely, if a generator is provided with electrical power, it can act as a motor and generate mechanical energy. This duality has practical applications, such as in regenerative braking systems for electric vehicles. In this case, the vehicle's motor is used as a generator during deceleration, turning mechanical energy from the vehicle's motion back into electrical energy that can be stored in the battery.

The motor-generator duality is based on the fact that electric motors and generators share the same design and operating principles, and can be used interchangeably for different purposes. Both devices involve a coil of wire inside a magnetic field, and the primary difference is in how they are powered and what type of energy they output. This duality is used in practical applications like regenerative braking systems for electric vehicles to convert excess mechanical energy back into electrical energy for storage and reuse.