Magnetic lines of force can be drawn in any magnetic field. We have seen that in the magnetic field of a current carrying wire, the lines of force are in the form of concentric circles (fig. 17.1). In the case of a bar magnet the lines start to flow from the North Pole and after passing around the magnet end at the south ole. Then they pass from South Pole to the North Pole through the magnet, thus making a closed loop (fig. 17.12).
If a coil is placed in the magnetic field of a bar magnet, some of the lines of force will pass through its face. If the coil is far away from the magnet, only a few lines of force will also pass through it. On the other hand, if the coil is close to the magnet, a large number of lines of force will pass through it (fig. 17.13). the number of magnetic lines of force passing through any surface is known as magnetic flux through that surface.
If the magnetic flux through a coil or a solenoid is changing, an emf is induced in it. Let us demonstrate this through an experiment.
Take a solenoid and connect a galvanometer with its two ends (fig. 17.14).
- If the north pole of a bar magnet is quickly moved towards the end A of the solenoid, the needle of galvanometer gets deflected which shows that an emf has been generated in the solenoid due to which a current flows through the galvanometer, (fig. 17. 14-a). This is known as induced current.
Now stop the motion of the magnet, you will see that as soon as the magnet stops moving, the deflection in the galvanometer gets zero. This means that induced current the amount of deflection in the galvanometer depends upon the speed of the magnet. Higher the speed, the larger is the value of induced current in the circuit.
- If the north pole of the magnet is moved away from the end in opposite direction (fig. 17. 14-b).
- When the south pole of the magnet is brought close to current flows through the galvanometer but the direction of this current is opposite to the of the current induced in case of north pole.
- Similar results are obtained if the magnet is kept stationary and the solenoid is moved towards or away from the magnet.
From this experiment we conclude that an emf is induced in the coil when there is a relative motion between the coil and the magnet. The magnitude of this induced emf depends upon the speed of relative motion. This phenomenon is known as electromagnetic induction. Micheal faraday discovered it in 1831, according to faraday when a magnet is moved towards or away from a coil, the magnetic flux associated with the coil changes due to which an emf is induced in the coil.
The value of the induced emf is directly proportional to the rate of change of flux. This is known as faraday’s law of electromagnetic induction.
Category: 9th 10th