The diagram below shows a cardboard on which iron filings are kept. A wire bent in the form of a loop is seen passing through the cardboard. When current flows through it the iron filings arrange themselves as shown below with the direction of magnetic field.

(a) State the polarities of the battery at A and B.

(b) State the effect on the magnetic field if an iron rod is held along the axis of the coil.

(c) State one way to :

1. Change the polarity of the coil.
2. Decrease the strength of the magnetic field around the coil.

(a) Polarity of A is positive and B is negative.

Reason — Since at the left side of the loop, the lines of force are anti-clockwise, so, by right hand thumb rule we find that the current is going out of terminal A hence, it is positive. Similarly, at the right side of the loop, the lines of force are clockwise, so, terminal B is negative.

(b) The strength of the magnetic field increases when an iron rod is held along the axis of the coil.

(c)

1. By reversing the direction of the current.
2. By decreasing the current.