The distance of two points on the axis of a magnet from its centre are 10cm and 20cm, respectively. The ratio of magnetic intensity at these points is 12.5 : 1. The length of the magnet will be

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5cm

25cm

10cm

20cm

answer is C.

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Detailed Solution

B1B2=d1d2d22-l2d12-l22⇒12.51=1020400-l2100-l22⇒l = 5cmHence, length of magnet = 2l = 10cm

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