question_answer

The adjacent figure shows the cross-section of a long rod with its length perpendicular to the plane of the paper. It carries a constant current flowing along its length. \[{{B}_{1}},{{B}_{2}},{{B}_{3}}\] and \[{{B}_{4}}\] respectively represent the magnetic fields due to the current in the rod at points 1, 2, 3 and 4 lying at different separations from the centre O, as shown in the figure. Which of the following shall hold true?

A)  \[{{B}_{1}}>{{B}_{2}}\ne 0\]                      

B)  \[{{B}_{2}}>{{B}_{3}}\ne 0\]

C)  \[{{B}_{1}}>{{B}_{2}}={{B}_{3}}=0\]       

D)  \[{{B}_{3}}>{{B}_{4}}_{3}\ne 0\]

Correct Answer: D

Solution :

                Magnetic field outside the long rod \[\because \]     \[B=\frac{{{\mu }_{0}}i}{2\pi r}\] \[\therefore \]  \[{{B}_{3}}>{{B}_{4}}\ne 0\]