Learn It: Magnetic Effect of Electric Current

We say that a magnetic field exists in any region of space where a magnetic force is experienced. The direction of a magnetic field at any point is the direction of the force on a north pole at the point.

The magnetic effect of an electric current was discovered by Hans Christian Oersted in 1820. To demonstrate the effect, a plotting compass is placed close to a long straight wire which is carrying current. A deflection is noted. By reversing the direction of the current, the compass needle will be deflected in the opposite sense.

Experiment: To demonstrate the magnetic effect of an electric current

Apparatus: Accumulator or power supply unit, rheostat, ammeter, switch, connecting wires, stiff board, plotting compass, 20 cm of straight copper wire. The copper wire may be obtained from a waste piece of electrical cable.

unit 4 compass
  1. Set up the apparatus as shown, with the board suitably supported horizontally and the wire passing vertically through its centre.
  2. Switch on and adjust the rheostat to give an ammeter reading of 4 A.
  3. Place the compass at any point on the board and mark the position of the compass’s head and tail, having tapped it gently to ensure it is not sticking.
  4. Move the compass and mark the new position of the head when the tail is directly over the previous position of the head.
  5. Proceed in this fashion until a complete loop has been traversed. Join the dots using a pencil.
  6. With the compass placed at different distances from the wire, complete other loops.

Result: It will be seen that the magnetic lines of flux form circles, which are concentric with the wire.

The magnetic effect of an electric current has many applications, e.g. in electromagnets, electromagnetic relays, loudspeakers, electric motors, ammeters, voltmeters, etc.

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