question_answer

Two like magnetic poles of strength 10 and 40 SI units are separated by a distance 30 cm. The intensity of magnetic field is zero on the line  joining them

A) At a point 10 cm from the stronger pole

B) At a point 20 cm from the stronger pole

C) At the mid-point

D) At infinity                         

Correct Answer: B

Solution :

Suppose magnetic field is zero at point P. Which lies at a distance x from 10 unit pole. Hence at P,   \[\frac{{{\mu }_{0}}}{4\pi }\cdot \frac{10}{{{x}^{2}}}=\frac{{{\mu }_{0}}}{4\pi }\cdot \frac{40}{{{(30-x)}^{2}}}\,\,\,\,\Rightarrow \,\,\,x=10\,\,cm\] So from stronger pole distance is 20 cm.