With the help of a diagram of an experimental set-up describe an activity to demonstrate that the magnetic field around a straight current-carrying conductor decreases with increase in distance from the conductor .

Explanation

Activity to demonstrate that the magnetic field around a straight current-carrying conductor decreases with increase in distance from 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 cardboard 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] Keep the rheostat in a fixed position and note down the value of current using ammeter.
5] Close the plug key and keep the copper wire vertical.
6] Keeping the current fixed, move the compass needle towards or away from the copper wire and observe the changes .
Inference : The deflection in the compass decreases if we move it away from the wire. Hence , B is inversely proportional to the distance from the wire r,i.e., B `prop` 1/r .