A coil having N turns is wound tightly in the form of a spiral with inner and outer and radii a and b respectively . When a current I passes through the coil, the magnetic field at the center is

Options

A. `(qbB)/(m)`
B. `(q(b - a)B)/(m)`
C. `(qaB)/(m)`
D. `(q(b + a)B)/(m)`

Answer

B

Explanation

`R = (mv)/(Bq)`
If particle just enters `x gt b`,
`R ge (b - a)`
`(mv)/(Bq) ge (b - a)`
`v ge (Bq(b - a))/(m)`
`v_(min) = (q(b - a)B)/(m)`