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                   [JIPMER 1999]

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                    \[\frac{{{\mu }_{0}}}{4\pi }.\frac{10}{{{x}^{2}}}=\frac{{{\mu }_{0}}}{4\pi }.\frac{40}{{{\left( 30-x \right)}^{2}}}\Rightarrow x=10cm\]            So from stronger pole distance is 20 cm.