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An electric current passes through a long straight wire. At a distance 5 cm from the wire the magnetic field is `B`. The field at 20 cm from the wire would beA. B/2B. B/4C. B/8D. B

Answer» Correct Answer - B
`B=(mu_0I)/(2pi a) therefore B prop (1)/(a)`
`(B_2)/(B_1)=(a_1)/(a_2) therefore B_2=(B)/(4)`


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