Q.

A wire carrying current I has the shape as shown in adjoining figure.
Linear parts of the wire are very long and parallel to X-axis while semicircular portion of radius R is lying in Y-Z plane. Magnetic field at point O is           [2015]

1 B→=-μ04πIR(πi^+2k^)  
2 B→=μ04πIR(πi^-2k^)  
3 B→=μ04πIR(πi^+2k^)  
4 B→=-μ04πIR(πi^-2k^)  

Ans.

(1)

Given situation is shown in the figure.

Parallel wires 1 and 3 are semi-infinite, so magnetic field at O due to them

            B→1=B→3=-μ0I4πRk^

Magnetic field at O due to semi-circular arc in YZ-plane is given by B→2=-μ0I4Ri^

Net magnetic field at point O is given by 

            B→=B→1+B→2+B→3

=-μ0I4πRk^-μ0I4Ri^-μ0I4πRk^=-μ0I4πR(πi^+2k^)