Question

Describe two experiments to illustrate the production of electric current by electromagnetic induction. State the laws which determine the direction and magnitude of the induced current.

Answer 1

Experiment 1
(i) Take a coil of wire AB having a large number of turns.
(ii ) Connect the ends of the coil to a galvanometer as shown in figure.
(iii ) Take a strong bar magnet and move i ts north pole towards the end B of the coil.
(iv) There is a momentary deflection in the needle of the galvanometer, say to the right. This indicates the presence of a current in the coil AB. The deflection becomes zero the moment the motion of the magnet stops.
(v) Now withdraw the north pole of the magnet away from the coil. Now the galvanometer is deflected toward the left, showing that th e current is now set up in the direction opposi te to the first.
(vi) Place the magnet stationary at a point near to the coil, keeping i ts north pole towards the end B of the coil. We see that the galvanometer needle deflects toward the right when the coil is moved towards the north pole of the magnet. Similarly the needle moves toward left when the coil is moved away.
(v) When the coil is kept stationary with respect to the magnet, the deflection of the galvanometer drops to zero.

Experiment 2
(i) Take two different coils of copper wire having large number of turns (say 50 and 100 turns respectively). Insert them over a non -conducting cylindrical roll, as shown in figure.
(ii ) Connect the coil -1, having larger number of turns, in series with a battery and a plug key. Also connect th e other coil -2 with a galvanometer as shown .
(iii ) Plug in the key. Observe the galvanometer. We will observe that the needle of the galvanometer instantly jumps to one side and just as quickly returns to zero, indicating a momentary current in coil -2.
(iv) Disconnect coil -1 from the battery. We will observe that the needle
momentarily moves, but to the opposi te side. It means that now the current flows in the opposi te direction in coil -2.

From these observations, we conclude that a potential difference is induced in the coil -2 whenever the electric current through the coil - 1 is changing (starting or stopping). This process, by which a changing magnetic field in a conductor induces a current in another conductor, is called electromagnetic induction.

To determine the direction of induced current, we use Fleming's left hand rule:

Stretch the thumb, forefinger and middle finger of right hand so that they are perpendicular to each other. If the forefinger indicates the direction of the magnetic field and the thumb shows the direction of motion of conductor, then the middle finger will show the direction of induced current.

Magnitude of induced current can be measured wi th the help of a galvanometer.