Question

A straight wire of diameter 0.5 mm carrying a current of 1A is replaced by another wire of 1 mm diameter carrying the same current. The strength of magnetic field far away is :

• A. unchanged
• B. quarter of its earlier value
• C. half of the earlier value
• D. twice the earlier value

Answer 1

Answer: (A)

The strength of magnetic field at the observation point due to a straight current carrying wire is $B=\frac{{\mu }_{0}i}{2\pi r}$ where r= distance of point from the wire. It is obvious from the relation that magnetic field does not depend on diameter of the wire, so it remains unchanged. NOTE: Since $B\propto \frac{1}{r}$ . so B-r graph for an infinitely long straight wire is a rectangular hyperbola (as shown in figure).