1.

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

Answer» Correct Answer - b
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)`


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