The ratio of magnetic fields due to a bar magnet at the two axial points P 1 and P 2 which are separated from each other by 10 cm is 25 : 2 . P 1 Point is situated at 10 cm from the centre of the magnet. (Points P 1 and P 2 are on the same side of magnet and distance of P 1 and P 2 from the centre of magnet is much greater than the distance of two ends of the magnet from the centre). Magnetic length of the bar magnet is

Question

The ratio of magnetic fields due to a bar magnet at the two axial points P 1 and P 2 which are separated from each other by 10 cm is 25 : 2 . P 1 Point is situated at 10 cm from the centre of the magnet. (Points P 1 and P 2 are on the same side of magnet and distance of P 1 and P 2 from the centre of magnet is much greater than the distance of two ends of the magnet from the centre). Magnetic length of the bar magnet is, 5 cm, 10 cm, 15 cm, 20 cm

MARKS App · Accepted Answer

B 1 B 2 = 25 2 μ 0 4 π M d 1 ( d 1 2 − l 2 ) 2 μ 0 4 π M d 2 ( d 2 2 − l 2 ) 2 = 25 2 d 1 d 2 × ( d 2 2 − l 2 ) 2 ( d 1 2 − l 2 ) 2 = 25 2 d 1 = 10 c m , d 2 = 20 c m 10 20 × ( 20 2 − l 2 10 2 − l 2 ) 2 = 25 2 400 − l 2 = 5 ( 100 − l 2 ) 4 l 2 = 100 ⇒ l 2 = 25 l = 5 c m

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