Magnetics 2 Question 12

12. A coil having N turns is wound tightly in the form of a spiral with inner and outer radii a and b respectively. When a current I passes through the coil, the magnetic field at the centre is

(2001, 2M)

(a) μ0NIb

(b) 2μ0NIa

(c) μ0NI2(ba)lnba

(d) μ0IN2(ba)lnba

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Answer:

Correct Answer: 12. (c)

Solution:

  1. Consider an element of thickness dr at a distance r from the centre. The number of turns in this element,

dN=Nbadr

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

dB=μ0(dN)I2r=μ0I2NbadrrB=r=ar=bdB=μ0NI2(ba)lnba

NOTE The idea of this question is taken from question number 3.245 of IE Irodov.



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