Question

Two concentric circular coils of ten turns each are situated in the same plane. Their radii are `20` and `40 cm` and they carry respectively `0.2` and `0.3` ampere current in opposite direction. The magnetic field in `Wb//m^(3)` at the centre is
A. `mu_0//80`
B. `7mu_0//80`
C. `(5//4)mu_0`
D. zero

Answer 1

Correct Answer - C
`B=(mu_0nI)/(2pia)because Bprop(n I)/(a)`
`B_1-B_2=(mu_0n)/(2)[(I_1)/(a_1)-(I_2)/(a_2)]`
`=(10xx4pixx10^-7)/(2xx10^-2)[(0.2)/(20)-(0.3)/(40)]`
`B_1-B_2=(4pixx10xx2xx10^-7)/(2xx10^-2xx800)=1.57xx10^-6=(5mu_0)/(4)T` .