MHT CET 2021 Physics Solved Paper

(b) The coefficient of mutual induction is given by
M=‌

µ0N1N2A

I

‌‌‌⋅⋅⋅⋅⋅⋅⋅(i)
where, µ0 is the permeability of free space, N1 is the number of turns in primary coil, N2 is the number of turns in secondary coil, A is the common area of cross-section and l is the length of coils.
Thus, for toroid the Eq. (i) is given as
M=‌

µ0N⋅N⋅πR2

2πr

=‌

µ0N2R2

2r

[

∵N1=N2=N
A=πR2	l=2πr

]
where, R is the major radius and r is the minor radius.