Concepts of Physics

IIT JEE Physics (1978-2018: 41 Years) Topic-wise Complete Solutions

Magnetic Effect of Current


A current carrying wire produce a magnetic field in its vicinity. This field can be detected by a small magnetic compass places close to it.


Magnetic compass, battery, wire, small plastic box



Place a magnetic compass on a plastic or wooden block, away from all magnetic material. When the compass needle comes to rest, fix a wire over the compass, parallel to the needle. Connect the wire to a battery through a switch, as shown in figure. Close the switch to pass a current through the wire. The compass needle will get deflected, and comes to rest at right angles to its original position. If the direction of the current is from south to north, the north pole of the needle will come to rest pointing west.

Now, hold the compass above the wire. The needle will get deflected in the opposite direction. The direction of deflection will also change if you reverse the direction of the current in the wire by interchanging the battery connections. If you switch off the current in the wire, the needle will go back to its original position.

Make sure that you pass a current through the wire only for short period of time (say, 5 seconds). Allowing current to pass through the wire for a long time will heat the wire considerable and also drain the battery rapidly.