question_answer

The magnetic field due to a straight conductor of uniform cross-section of radius a and carrying a steady current is represented by

A)   

B)  

C)

D)

Correct Answer: A

Solution :

The magnetic field at a point outside-the straight conductor is given by \[B=\frac{{{\mu }_{0}}i}{2\pi r}\] It means \[B\propto \frac{1}{r}\,\,\,\,(if\,\,r>a)\] The magnetic field at a point inside the conductor is \[B=\frac{{{\mu }_{0}}i\times r}{2\pi {{a}^{2}}}\] or \[\operatorname{B}\,\,\propto \,\,r\,\,\,\,\,(if\,\,r\,\,<\,\,a)\]