A current passing through a circular coil of two turns produces a magnetic field B as its centr. The coil is then rewound so as to have four turns and the same current is passed through it. The magnetic field at its centre now is

Options

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

Answer

A

Explanation

According to questions
`2xx2pir_(1)=4xx2pir_(2)`
`r_(1)=2r_(2)`
and `B_(1)=(mu_(0)2piI)/(4pir_(1))`
`B_(2)=(mu_(0)2piI)/(4pir_(2))=(mu_(0)2piI)/(2pir_(1)//2)`
`B_(2)=2B_(1)`