Left Hand Rule

Force on a current carrying conductor in a magnetic field

In previous section, you have read that a current carrying conductor exerts a force when a magnet is placed in its vicinity. Similarly, a magnet exerts equal and opposite force on the current carrying conductor. This was suggested by Andre Marie Ampere, a French Physicist and considered as founder of science of electromagnetism.

The direction of force over the conductor depends on the direction of current. The direction of force over the conductor gets reversed with the change in direction of flow of electric current. It is observed that the magnitude of force is highest when the direction of current is at right angles to the magnetic field.

Fleming's Left Hand Rule

If direction of electric current is perpendicular to the magnetic field, the direction of force is also perpendicular to both of them. The Fleming's Left Hand Rule states that if the left hand is stretched in a way that the index finger, the middle finger and the thumb are in mutually perpendicular directions, then the index finger shows the direction of magnetic field and middle finger shows the direction electric current. Finally, the thumb shows the direction of motion or force acting on the conductor. The directions of electric current, magnetic field and force are similar to three mutually perpendicular axes, i.e. x, y and z axes. In this figure, you can see that the index finger is showing the direction of magnetic field, middle finger is showing the direction of electric current and thumb is showing the direction of motion because of interaction between magnetic field and electric current.

Electric Motor:

Electric motor is a device which converts electrical energy into mechanical energy. Electric motor works on the basis of rule suggested by Marie Ampere and Fleming’s Left Hand Rule.

In an electric motor, a rectangular coil is suspended between the two poles of a magnetic field. The electric supply to the coil is connected with a commutator. Commutator is a device which reverses the direction of flow of electric current through a circuit.

Working of Electric Motor

When electric current is supplied to the coil of electric motor, it gets deflected because of magnetic field. As it reaches the half way, the split ring which acts as commutator reverses the direction of flow of electric current. Reversal of direction of current reverses the direction of forces acting on the coil. The change in direction of force pushes the coil; and it moves another half turn. Thus, the coil completes one rotation around the axle. Continuation of this process keeps the motor in rotation.

In commercial motor, electromagnet instead of permanent magnet, and armature is used. Armature is a soft iron core with large number of conducting wire turns over it. Large number of turns of conducting wire enhances the magnetic field produced by armature.

Electric motor is used in those electrical devices which work on rotary motion of at least one of its parts. For example, the blades of ceiling fan rotate because of electric motor. Some other examples of electrical appliances with electric motor are, mixer grinder, vacuum cleaner, hard disc drive, hair dryer, CD player, etc.