Question

The effect due to uniform magnetic field on a freely suspended magnetic needle will be?

• A. both torque and net force are present
• B. torque is present but no net force
• C. both torque and net force an absent
• D. net force is present but no torque

Answer 1

Answer: (B)

torque is present but no net force

It happens only if its potential energy will be zero.And it happens only if both needle and magnetic field will be far to each other
$\mu =-\overrightarrow{m}\cdot \overrightarrow{B}=0$
$mBcos\theta =0$
$\therefore \theta =90$
$\tau =\overrightarrow{m}\times \overrightarrow{B}$
$=mBsin\theta$
$=mBsin\theta$
$=mB$
$\therefore$ The torque is non-zero,but force is zero as line of force is same but direction is opposite.