2. Force on a Current-Carrying Conductor in a Magnetic Field

Fleming’s Left Hand Rule

According to this rule, stretch the thumb, forefinger and middle finger of your left hand such that they are mutually perpendicular. If the first finger points in the direction of magnetic field and the second finger in the direction of current, then the thumb will point in the direction of motion or the force acting on the conductor.

Magnetism in Medicine

• Two main organs in the human body where the magnetic field produced is significant, are the heart and the brain.
• The magnetic field inside the body forms the basis of obtaining the images of different body parts through a technique called Magnetic Resonance Imaging (MRI).
• Analysis of these images helps in medical diagnosis.

Electric Motor: An electric motor is a rotating device that converts electrical energy to mechanical energy.

Principle of electric motor: An electric motor works on the principle that when a current carrying rectangular coil is placed perpendicular to the magnetic field it experiences a force which rotates it continuously.

Construction of electric motor:
→ It consists of a rectangular coil ABCD of insulated copper wire.
→ The coil is placed perpendicularly between the two poles of a magnetic field.
→ The ends of the coil are connected to the two halves P and Q of a split ring. Split rings act as a commutator which reverses the flow of current in the circuit.
→ The inner sides of split ring are attached to an axle which is free to rotate.
→ The external edges of the split ring touch two conducting stationary brushes X and Y.
→ These brushes are attached to the battery to complete the circuit.

Working of electric motor:

→ Current in the coil ABCD enters from the source battery through conducting brush X and flows back to the battery through brush Y.
→ Current flows through coil from A to B and then from C to D. The direction of magnetic field is from North to South.
→ By Fleming's left hand rule, the force acting on arm AB pushes it downwards while the force acting on arm CD pushes it upwards.
→ Thus the coil and the axle O rotate anti-clockwise.
→ After half rotation, Q touches brush X and P touches brush Y. Therefore the current in the coil gets reversed along the path DCBA.
→ Now, the current in CD flows from D to C and in AB from B to A.
→ So, CD moves downwards and AB moves upwards.
→ Thus, the coil and the axle keep rotating until the battery is switched off.

Commutator: A device that reverses the direction of flow of current through a circuit is called a commutator.
Significance of commutator: In electric motors, the split ring acts as a commutator. The split ring is used to reverse flow of current to make coil rotate in a single direction. Otherwise, in absence of split the coil would have rotated half in clockwise direction and half in anticlockwise direction.

The commercial motors use
(i) An electromagnet in place of permanent magnet
(ii) Large number of turns of the conducting wire in the current carrying coil and
(iii) A soft iron core on which the coil is wound.
The soft iron core, on which the coil is wound, is called an armature. This enhances the power of the motor.