are connected to slip rings C₁ and C₂, which rotate along with the coil. Initially C₁ is in contact with B₁ and C₂ is in contact with B₂. After half rotation, the brushes come into contact with the other slip rings in such a way that the direction of current through the coil is reversed. This happens every half rotation. Thus the direction of rotation of the coil remains the same. This is the principle used in “electric motor."

    In electric motors, electrical energy is converted into mechanical energy.

    We have learned that a current carrying coil rotates when it is kept in a uniform magnetic field.

• What happens when a coil without current is made to rotate in magnetic field?
• How is current produced?
Do you know?

• The relation between direction of induced current, magnetic field and force can also be explained by Flemings left hand rule. Stretch the left hand thumb, middle finger and forefinger in such a way that they are mutually perpendicular to each other. The forefinger indicates the direction of magnetic field, the middle finger indicates the direction of current and thumb indicates direction of force. By using the Flemings left hand rule we can explain the working of electric motor

Electromagnetic induction

Activity 9


    Take a wooden base as shown in figure 13(a). Fix a soft iron cylinder on the wooden base vertically. Wind copper wire around the soft iron as shown in figure 13(a). Now take a metal ring which is slightly greater in radius than the radius of the soft iron cylinder and insert it through the soft iron cylinder on the wooden base. Connect the ends of the coil to an AC source and switch on the current.

• What do you notice?

    You notice that the metal ring is levitated on the coil. Switch off the current, the ring will jump into the air very dramatically. Remove the AC supply and connect a DC supply. Observe what happens.