Question

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

Answer 1

Answer: (B)

At a point 20 cm from the stronger pole

Answer 2

Suppose magnetic field is zero at point P. Which lies at a distance x from 10 unit pole. Hence at P

$\frac { \mu _ { 0 } } { 4 \pi } \cdot \frac { 10 } { x ^ { 2 } } = \frac { \mu _ { 0 } } { 4 \pi } \cdot \frac { 40 } { ( 30 - x ) ^ { 2 } } \Rightarrow x = 10 \mathrm {~cm}$

So from stronger pole distance is 20 cm.