1.

An extremely long straight wire, with radius of cross-section a, carries current I. Then ratio of magnetic fields at distances a/2 and 2a from its axis would be ______ .

Answer»

`1/4`
4
`1/2`
1

Solution :For first POINT, `B_(i)=(mu_(0)I)/(2pia^(2))(r)`
`thereforeB_(i)=(mu_(0)I)/(2pia^(2))(a/2)=(mu_(0)I)/(4pia)""...(1)`
For second point, `B_(0)=(mu_(0)I)/(2pia^(2))`
`thereforeB_(0)=(mu_(0)I)/(2pi(2A))=(mu_(0)I)/(4pia)""...(2)`
From equation (1) and (2),
`B_(i)/B_(0)=1`


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