CBSE Class 12 Physics 2018 Delhi Set 1 Paper

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Question : 14
Total: 26
(a) An iron ring of relative permeability µr has windings of insulated copper wire of n turns per metre. When the current in the windings is I, find the expression for the magnetic field in the ring.
(b) The susceptibility of a magnetic material is 0.9853 . Identify the type of magnetic material. Draw the modification of the field pattern on keeping a piece of this material in a uniform magnetic field.
Solution:  
(a) Expression for Ampere's circuital law
Derivation of magnetic field inside the ring
(b) Identification of the material
Drawing the modification of the field pattern
(a)From Ampere's circuital law, we have,
∮
→
B
→
dl
=µ0µrI‌endosed ‌
......(i)
For the field inside the ring, we can write
‌∮
→
B
→
dl
=∮
→
B
→
dl
=B.2πr‌‌(r=‌ radius of the ring ‌)

‌‌ Also, ‌I‌enclosed ‌=(2πrn)I‌‌‌ using equation (i) ‌
‌∴‌‌B.2πr=µ0µr⋅(n.2πr)I
‌∴‌‌B=µ0µrnI
(b) The material is paramagnetic. The field pattern gets modified as shown in the figure below.
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