Electromagnetic induction is the phenomenon of inducing an e.m.f. into a circuit due to a changing magnetic field.
Faradays’ experiments
i. Iron Ring Experiment: two coils of insulated copper wire are wound round a soft iron ring. The ends of coil A are connected to a battery and a switch. When the switch is closed, the current flowing in coil A sets up a magnetic field in the soft iron ring. A compass is placed below the wire which is parallel to coil B. The compass needle only deflects when the switch is turned on and off. There is no deflection of the needle when there is a continuous supply of current to coil A. Therefore, there is only a current present in coil B when the current in A is switched on and off. The current in B is said to be induced. The induced current only arises when there is a change in the magnetic field linking the ring to coil B. The iron ring experiment led to the construction of transformers. ii. Solenoid Experiment: when a permanent magnet is placed into solenoid which is connected to a galvanometer, then the needle of the galvanometer deflects into one direction, and when the magnet is removed, the needle moves into the opposite direction. Only relative movement between the magnet and the solenoid induces an electromotive force into the circuit. The magnitude of the induced e.m.f. depends on: the number of turns in the solenoid; strength of the magnet; speed with which the magnet is placed into the solenoid (or withdrawn).
The Laws of Magnetism
Faradays Law of Induction: the e.m.f generated in a conduction is proportional to the rate of change of magnetic flux linking the circuit.
Lenz’s Law: the direction of the induced e.m.f. and therefore the induced current in a circuit is always as to oppose the change in magnetic flux opposing it.
Experiment:
Demonstrating Faraday’s and Lenz’s Laws.
Apparatus: 2.0 m insulated wire with bare ends, sensitive galvanometer, bar magnet
Procedure:
1) Coil the wire to make a solenoid approximately 5 cm in diameter. (The coil can be flat like in the diagram rather than long.) Alternatively, use a ready-made solenoid. 2) Connect the ends of the coil to the terminals of the meter. 3) Bring one pole of the bar magnet towards and into the centre of the coil. Observe the deflection on the meter. 4) Now investigate how the deflection on the meter changes in different circumstances: • Use the opposite pole of the bar magnet. • Move the bar magnet out of the coil. • Hold the bar magnet stationary at different distances from the coil.
Factors affecting magnitude of induced e.m.f
Induced e.m.f. increased when
1. the magnet moves at a faster speed in and out of the coil 2. a stronger magnet is used 3. the number of turns in the coil is increased
Making a magnet by electrical method – most efficient method
a steel bar to be magnetised is placed inside a solenoid
direct current is passed through the solenoid, it becomes a magnet
– polarity of the magnet determined by
viewing from one end of the solenoid o if current flows in an anticlockwise direction, that end will be the North pole
viewing from one end of the solenoid o if current flows in a clockwise direction, that end will be the South pole