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A circular wire loop of radius 3 cm carries a current. The magnetic field produced by it at a point on its axis 4 cm from the centre is 54 μT. What is the magnetic field at the centre of the loop?

125 μT
150 μT
250 μT
75 μT

Correct answer: 250 μT

Solution

B_centre = B_axis*(R^2+x^2)^(3/2)/R^3 = 54*(3^2+4^2)^(3/2)/3^3 = 54*125/27 = 250 microT.

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