Question
Sun July 03, 2011 By: Vipul Kaushik

A long cylindrical tube of inner and outer radii a and b carries a current i distributed uniformly over its cross-seceion.Find the magnitude of the magnetic filed at a point(a)just inside the tube(b) just outside the tube.

Expert Reply
Mon July 18, 2011
Suppose we need to find the magnetic field at a point distant x from the centre.
 
We draw a closed loop of radius x with centre on the axis of the rod such that the above point lies on this closed path.
 
Current enclosed by this loop.
According to Ampere's law,
 
                                      
 
(a) At a point just inside the tube:
 
          x = a
 
          B =
 
 
(b) At a point just outside the tube:
 
          x = b

           B = 
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