Magnetic field due to currentHard
Question
A coil having N turns is wound tightly in the form of a sprial with inner and outer radii a and b respectively. When a current I passes through the coil, the magnetic field at centre is
Options
A.

B.

C.

D.

Solution
Consider an elements of thickness dr at a distance r from the centre. The number of turns in this elements,
dN =
Magnetic dield due to this element at the centre of the coil will be

∴
=
dN =

Magnetic dield due to this element at the centre of the coil will be

∴
=
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