A charge Q is uniformly distributed over the surface of non-condcting disc of radius R. The disc rotates about an axis perpendicular to its plane and passing through its centre with an angular velocity ω. As a result of this rotation a magnetic field of induction B is obtained at the centre of the disc. if we keep both the amount of charge placed on the disc and its angular velocity to be constant and vary the radius of the disc then the variation of the magnetic induction at the centre of the disc will be represented by the figure:
Text Solution
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Consider a elementary ring of radius 
and thickness dr of a disc as shown in figure.
Charge on the ring,
Current due to rotation of charge on ring is
Magnetic field at the center due to the ring element
Magnetic field at the center due to the whole disc
Since, 
and 
are constants
Hence variation of 
with 
should be a rectangular hyperbola as represented in option .
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