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Problem 1: At what distance from a long straight wire carrying a current of 10 A is the magnetic field equal to the earth’s magnetic field of 5 * 10-5 T.


Given data           I = 10 A                 B = 5 * 10-5 T

Required              r =?


When current passes through a conducting wire, a magnetic field is produced around the wire. The value of the magnetic field at any point distant r from the wire is given by Ampere’s Circuital Law.

Where μ0 is called the permeability of free space and its value is 4π * 10-7 wb/A.m.

The value of r (distance from the wire) can be calculated from the above equation.

Put the values in the above equation from the given data



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