Describe an activity to obtain magnetic field lines around current-carrying straight conductor.

Explanation

Activity to plot the magnetic field lines produced by a current carrying wire (straight conductor) :
Figure : Circuit to study magnetic field lines around a straight current carrying wire .
1] Connect a plug key, a 12 V battery, a variable resistance (or a rheostat) , an ammeter (0-5 A) in series as shown in the figure .
2] Insert a thick copper wire through the centre of a rectangular cardboard. Ensure that the carbdoard remains in fixed position and does not slide up or down.
3] Connect the two ends of the copper wire between points X and Y in the circuit .
4] Sprinkle some iron filings on the carbdoard uniformly using salt sprinkler.
5] Keep the rheostat in a fixed position and note down the value of current using ammeter.
6] Close the plug key and keep the copper wire vertical.
7] Gently tap the cardboard a few times and observe the pattern formed by the iron filings.
8] Move a compass needle on one of the lines in the pattern and note down the direction of north pole of the needle.
9] Reverse the connection of the battery so that the current flows in the opposite direction and observe the change in the deflection of compass needle.
10] Keeping the compass needle at a fixed position, change the current using rheostat and observe the changes in deflection of the needle.
Figure : Pattern formed by iron filings show the magnetic field lines around a straight current carrying wire.
Inference : The iron filings arrange themselves in the form of concentric circles around the copper wire which represent the magnetic lines of force. The direction of the north pole of the compass needle gives the direction of the magnetic field. We observe that this direction is reversed if the direction of current in the wire is reversed.