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Electric Charges And Fields

Question
CBSEENPH12037877

Two circular coils, one of radius r and the other of radius R are placed coaxially with their centres coinciding. For R >> r, obtain an expression for the mutual inductance of the arrangement. 

Solution
Given, two circular coils one of radius 'r' and the other of radius 'R' placed coaxially with their centers coinciding. 
Suppose a current I
2 flows through the outer circular coil. 
Magnetic field at the centre of the coil is 
                 
                   B2 = μ0I22R 

Magnetic field B2 may be considered constant over the cross-sectional area of the inner smaller coil. 

Hence,
                     ϕ = B.A. 

                    ϕ1 = πr2 B2     = μ0πr2I22R     = MI2 

                M = ϕ1I2 = μ0πr22R