Question

A wire of length l carries a steady current. It is bent first to form a circular plane loop of one turn. The magnetic field at the centre of the loop is B. The same length is now bent more sharply to give a double loop of smaller radius. The magnetic field at the centre caused by the same is
A. B
B. 2B
C. 3B
D. 4B

Answer 1

Correct Answer - D
`B_1=B=(mu_0 n_1 I)/(2r_1)=B`
When same length is bent into 2 turns radius of coil is halved
`B_2=(mu_0 n_2 I)/(2 r_2)=4(mu_0 I)/(2 r) = 4B`
`(B_2)/(B_1)=(4B)/(B)=(4)/(1)`
`B_2=4B_1=4B`.