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.
(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:
- (a)Change the polarity of the coil.
- (b)Decrease the strength of the magnetic field around the coil.
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(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)
- By reversing the direction of the current.
- By decreasing the current.