question_answer

A ring of radius 5 m is lying in the x-y plane and is carrying current of 1 A in anti-clockwise sense. If a uniform magnetic field \[\vec{B}=3\hat{i}+4\hat{j}\] is switched on, then the  co-ordinates of point about which the loop will lift up is

A) (3, 4)    

B) (4, 3)

C) (3, 0)    

D) (0, 3)     

Correct Answer: A

Solution :

[a] Magnetic moment \[\vec{M}=\pi {{r}^{2}}i\hat{i}\]and \[\vec{B}=3\hat{i}+4\hat{j}\] \[\therefore \vec{\tau }=\vec{M}\times \vec{B}=\pi {{r}^{2}}(3\hat{i}-4\hat{i})\] \[\vec{\tau }\]will be along the direction shown. Hence, the point about which the loop will be lift up will be (3, 4)