Magnetism: The Motor Effect

The motor effect refers to the force that acts on a current-carrying conductor in a magnetic field. The direction of this force is always perpendicular to the plane containing the conductor and the magnetic field.
The size of the force on the conductor is directly proportional to the strength of the magnetic field, the current in the conductor and the length of conductor in the field.
The direction of the force can be found using Fleming’s left-hand rule. This states that if the left thumb, forefinger and second finger are all held at right angles to each other, with the forefinger pointing in the direction of the field, the second finger in the direction of the current, then the thumb will point in the direction of the force.
If current is flowing at right angles to a magnetic field, a force is exerted on the conductor. If the current is parallel to the magnetic field, no force is experienced by the conductor.
The force experienced by a current-carrying conductor in a magnetic field is used in electric motors to produce rotational movement.
An electric motor contains a coil of wire which is free to rotate in a magnetic field. Current in the coil creates numerous small motor effects that combine to spin the coil.
When the direction of the current reverses, the direction of the forces also reverses. This keeps the coil turning in the same direction.
In electric motors, most use a commutator, which is a device that reverses the direction of the current in a coil every half-turn. This ensures that the motor continues to spin in the same direction.
The speed of the electric motor can be controlled by varying the strength of the current or the magnetic field. The direction of the rotation can also be altered by reversing the current or the magnetic field.
The motor effect is used in a variety of applications, such as in loudspeakers to convert electrical signals into sound, and in hard drives to read and write data.