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Magnetic field due to current

An electric generator is a machine that generates electricity by rotating its rotor in a magnetic field. Thus, it converts mechanical energy into electrical energy.


The basic principle on which electric generator works is electromagnetic induction. When rotor rotates in a magnetic field, current is induced in the rotor.


A generator consists of a rectangular coil MNST of insulated copper wire placed between two strong magnetic poles.The two ends of the coil MNST are connected with brushes A and B of rings C and D respectively. The inner sides of the rings are insulated. They are attached with an axle X, which can be rotated mechanically. Brushes A and B are connected with a galvanometer that can measure the flow of current in coil MNST.


When the axle is rotated, lengths MN and ST move up and down respectively. Since lengths MN and ST are moving in a magnetic field, a current gets induced in these lengths caused by an electromagnetic induction. The direction of the induced current in both the lengths is given by Fleming’s right hand rule.

Since length MN is moving upwards in the magnetic field that acts from left to right, the direction of the induced current will be from M to N. Similarly, the direction of the induced current in length ST will be from S to T. Hence, an induced current will set up in the coil in the direction MNST, which produces deflection in the galvanometer.

After half-rotation, length MN starts moving down, whereas length ST starts moving up. The direction of the induced current in the coil gets reversed i.e., the induced current will now flow from T to M via S and N i.e., TSNM. Therefore, we can conclude that after each half-rotation, the direction of the induced current is reversed. This current is called an alternating current (AC). An AC reverses its direction after equal time intervals. A machine with this arrangement is called an AC generator.

To get a current that flows in one direction only, a split ring is used.

In this arrangement, brush A always remains in contact with the length moving up, whereas brush B always remains in contact with the length moving down. Here, split rings C and D act as a commutator. In this case, the direction of the current induced in the coil will be from M to T via N and S for the first half-rotation, and from T to M via S and N for the second half-rotation of coil MNST. Hence, we get a unidirectional current called direct current (DC). A machine with such an arrangement is called a DC generator.

Do You Know:

Electricity supplied to our homes, school buildings, company buildings etc. is AC, which reverses its direction after every second with a frequency of 50 Hz. Most power stations in the world generate AC than DC. This is because AC can be transmitted over very long distances without much loss of energy.

Amar watches his father fix an electric bell in their new house. He wonders how a bell produces such a loud sound when its switch is pressed on.

In this section, we will learn about the construction and working of an electric bell.

An electric bell works on the principle of electromagnetism.

Let us show you the working of an electric bell.

Let us perform a small experiment.

Hans Christian Oersted (1777-1851) was on…

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