Question

describe briefly any one way of inducing e.m.f.

Answer 1

Let us consider a coil of wire connected to a galvanometer, which we can use to measure current in the coil. There is no battery or power supply, so no current should flow. Now a magnet is brought close to the coil.
We notice two things:

• If the magnet is held stationary near, or even inside, the coil, no current will flow through the coil.
• If the magnet is moved, the galvanometer needle will deflect, showing that current is flowing through the coil.

When the magnet is moved one way (say, into the coil), the needle deflects one way; when the magnet is moved the other way (say, out of the coil), the needle deflects the other way. The direction of the current depends on how the magnet is moved. It seems like a constant magnetic field does nothing to the coil, while a changing field causes a current to flow.
To confirm this, the magnet can be replaced with a second coil, and a current can be set up in this coil by connecting it to a battery. The second coil acts just like a bar magnet. When this coil is placed next to the first one, which is still connected to the galvanometer, nothing happens when a steady current passes through the second coil. When the current in the second coil is switched on or off, or changed in any way, however, the galvanometer responds, indicating that a current is flowing in the first coil.
We notice one more thing. If we squeeze the first coil, changing its area, while it's sitting near a stationary magnet, the galvanometer needle moves, indicating that current is flowing through the coil.
We can conclude from all these observations that a changing magnetic field will produce a voltage in a coil, causing a current to flow. To be completely accurate, if the magnetic flux through a coil is changed, a voltage will be produced. This voltage is known as the induced emf.