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 A current-carrying circular loop of radius R is placed in the x-y plane with the centre at the origin. Half of the loop with x >0 is now bent so that it now lies in the y-z plane.
(a) The magnitude of the magnetic moment now diminishes.
(b) The magnetic moment does not change.
(c) The magnitude of B at (0,0, z), z>>R increases.
(d) The magnitude of B at (0,0,z), z>>R is unchanged.

Answers (1)

The answer is option (a) According to the right-hand thumb rule, the direction of the magnetic moment of the circular loop is perpendicular to the loop. So, in order to compare their magnetic moments, we need to analyse them in a vertical field.

As shown in the above figure-(a), the circular loop placed in the x-y axis has its magnetic moment across z-axis . And if the loop is half bent with x > 0, it lies in the y-z plane as shown below.

The magnitude of the magnetic moment of each semicircular loop of radius R lies in the x-y plane and the y-z plane is

 M_{1}= M_{2}= I \frac{\pi R^{2}}{2}

 and the direction of the magnetic moments are along z-direction and x-direction respectively. Their resultant is

M_{net}=\sqrt{M_{1}^{2}+M_{2}^{2}}=\sqrt{2I}\frac{\pi R^{2}}{2}=\frac{M}{\sqrt{2}}

So Mnet < M or M diminishes.

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