1.

The electric current in a circular coil of two turns produced a magnetic induction of 0.2 T at its centre. The coil is unwound and then rewound into a circular coil of four turns. If same current flows in the coil, the magnetic induction at the centre of the coil now isA. 0.2TB. 0.4TC. 0.6TD. 0.8T

Answer» Correct Answer - D
When there are two turns in the coil, then
`l=2xx2pi r_(1) or r_(1)=(l)/(4pi)`
`"then "B_(1)=(mu_(0)N_(1)I)/(2r_(1))=(mu_(0)2xxI)/(2xx(l//4pi))=(mu_(0)4piI)/(l)`
When there are four turns in the coil, then
`l=4xx pir_(2)" or "r_(2)=(l)/(8pi)`
`"Then "B_(2)=(mu_(0)N_(2)I)/(2r_(2))=(mu_(0)xx4xxI)/(2xx(l//8pi))=(mu_(0)16pil)/(l)`
`(B_(1))/(B_(2))=(4)/(16)=(1)/(4) or B_(2)=4B_(1)=4xx0.2T=0.8T`


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