question_answer

A long straight wire is carrying a current of \[I=2A\]. A bar magnet of magnetic dipole moment M is placed as shown in figure, gently. Then, the bar magnet  

A)  Experiences a net force only.

B)  Experiences a torque only.

C)  May undergo translatory motion as well as rotational motion.

D)  remains at absolute rest.

Correct Answer: C

Solution :

The magnetic field due to wire At N-pole \[{{B}_{1}}=\frac{{{\mu }_{0}}i}{2\pi a}\otimes \] At S- pole \[{{B}_{2}}=\frac{{{\mu }_{0}}i}{2\pi b}\otimes \] Force exerted at N-pole. \[{{F}_{1}}=n{{B}_{1}}\] Similarly at S-pole, \[{{F}_{2}}=m{{B}_{2}}\] Since, \[{{B}_{1}}>{{B}_{2}}\] \[\therefore \]  \[{{F}_{2}}<{{F}_{1}}\] \[\Rightarrow \]               System has net force. Similarly, net torque exists on system.