Two concentric coils of 10 turns each are placed in the same plane. Their radii are 20 cm and 40 cm and carry 0.2 A and 0.3 A current respectively in opposite directions. The magnetic induction (in tesla) at the centre is

Options

A. `3/4 mu_(0)`
B. `5/4 mu_(0)`
C. `7/4 mu_(0)`
D. `9/4 mu_(0)`

Answer

b

Explanation

Two coils carry currents in opposite directions, hence net magnetic field at centre will be difference of the two fields. i.e.
`B_("net")=(mu_(0))/(4pi), 2piN[(i_(1))/(r_(1))-(i_(2))/(r_(2))]=(10mu_(0))/2[0.2/0.2-0.3/0.4]=5/4 mu_(0)`