ELECTROMAGNETIC INDUCTION
- Consider a conductor moving inside a magnetic field or a magnetic field changing around a fixed conductor. This was first studied by English experimental physicist Michael Faraday (1831). He discovered how a moving magnet can be used to generate electric currents. This effect can be observed by the following activity.
Experiment:
- Take a coil of wire AB having a large number of turns.
- Connect the ends of the coil to a galvanometer.
- Take a strong bar magnet and move its north pole towards the end B of the coil.
- The needle of the galvanometer shows a momentary deflection to the right. It indicates the presence of a current in the coil AB. The deflection becomes zero the moment the motion of the magnet stops.
- Now withdraw the north pole of the magnet away from the coil. So the galvanometer is deflected toward the left, showing that the current is set up in the opposite direction.
- Place the magnet stationary near the coil. Keep its north pole towards the end B of the coil. The galvanometer needle deflects toward the right when the coil is moved towards the magnet. Similarly, the needle moves to the left when the coil is moved away.
- When the coil is stationary with respect to the magnet, the deflection of the galvanometer drops to zero.
- If the south pole of the magnet is moved towards the end B, the deflections in the galvanometer are opposite to the previous case. When both the coil and magnet are stationary, there is no deflection in the galvanometer.
- Thus, this activity shows that the motion of a magnet with respect to a coil produces an induced potential difference, which sets up an induced electric current in the circuit.
- Galvanometer: An instrument to detect the presence of current in a circuit. The pointer remains at zero for zero current. It deflects to the left or right depending on the direction of the current.
Michael Faraday (1791–1867): Faraday had no formal education. He developed his interest in science by reading books in a book-binding shop where he worked. He made notes of Humphrey Davy’s lectures and sent them to Davy. Soon he became an assistant in Davy’s laboratory at the Royal Institute. Faraday discovered electromagnetic induction and the laws of electrolysis. He turned down honorary degrees conferred by several universities.
Experiment using current-carrying coil:
- Take two coils of copper wire having many turns (say 50 and 100 turns). Insert them over a non-conducting cylindrical roll.
- Connect coil-1 (larger number of turns) in series with a battery and a plug key. Connect coil-2 with a galvanometer.
- Plug in the key. The galvanometer needle instantly jumps to one side and just as quickly returns to zero. It indicates a momentary current in coil-2.
- Disconnect coil-1 from the battery. The needle momentarily moves to the opposite side. It means that the current flows in the opposite direction in coil-2.
- As soon as the current in coil-1 reaches either a steady value or zero, the galvanometer in coil-2 shows no deflection.
- We conclude that a potential difference is induced in coil-2 whenever the electric current through coil-1 is changing (starting or stopping). Coil-1 is called the primary coil and coil-2 is called the secondary coil. As the current in the first coil changes, the magnetic field associated with it also changes. Thus, the magnetic field lines around the secondary coil also change. Hence, the change in magnetic field lines associated with the secondary coil is the cause of the induced electric current in it. This process, by which a changing magnetic field in a conductor induces a current in another conductor, is called electromagnetic induction.
- In a coil, current can be induced either by moving it in a magnetic field or by changing the magnetic field around it. It is convenient to move the coil in a magnetic field.
- The induced current is highest when the direction of motion of the coil is at right angles to the magnetic field.
- The direction of the induced current can be found by Fleming’s right-hand rule. Stretch the thumb, forefinger, and middle finger of the right hand perpendicular to each other.
- Forefinger indicates the direction of the magnetic field.
- Thumb shows the direction of motion of the conductor.
- Middle finger shows the direction of the induced current.
