A long solenoid carrying a current produces a magnetic field `B` along its axis. If the current is doubled and the number of turns per cm is halved, the new vlaue of the magnetic field is

Options

A. 2B
B. 4B
C. B/2
D. B

Answer

D

Explanation

`B=mu_(0)ni`
or `B prop ni`
`:. (B_(1))/(B_(2))=(n_(1)i_(1))/(n_(2)i_(2))`
Here, `n_(1)=n_(1), n_(2)=n/2 , i_(1)=I, i_(2)=2i, B_(1)=B`
Hence `B/(B_(2))=n/(n//2)xxi/(2i)=1`
or `B_(2)=B`