Mutual Inductance - Practice Questions & MCQ

Whenever the current passing through a coil or circuit changes, the magnetic flux linked with a neighboring coil or circuit will also change. Hence an emf will be induced in the neighboring coil or circuit. This phenomenon is called ‘mutual induction’.

or The phenomenon of producing an induced emf in a coil due to the change in current in the other coil is known as mutual induction.

Coefficient of mutual induction (M)-

If two coils (P-primary coil or coil 1, S-Secondary Coil or coil 2) are arranged as shown in the below figure. 

If we change the current through the coil P (i.e   )  then flux passing through Coil S (i.e ) will change.

I.e   

where 

 = mutual induction of Coil 2 w.r. t Coil 1

 Number of turns in the primary coil

 Number of turns in the secondary coil

 current through the primary coil or coil 1

Similarly, if we exchange the position of Coil 1 and Coil 2

then 

If we change the current through the coil S (i.e   )  then flux passing through Coil P (i.e ) will change.

I.e   

where 

 = mutual induction of Coil 1 w.r. t Coil 2

 Number of turns in the primary coil

 Number of turns in the secondary coil

 current through the coil 2 or Coil S

• As 

    I.e coefficient of mutual induction of two coils is numerically equal to the magnetic flux linked with one coil when unit current flows through the neighboring coil.

• Using Faraday Second Law of Induction emf we get 

I.e  The coefficient of mutual induction of two coils is numerically equal to the emf induced in one coil when the rate of change of current through the other coil is unity.

Units and dimensional formula of ‘M’-

S.I. Unit - Henry (H)

And     

And  its dimensional formula is 

Dependence of mutual inductance

• Number of turns (N₁, N₂) of both coils
• Coefficient of self inductances (L₁, L₂) of both the coils

         and the relation between  is given as

      where K =coeffecient of coupling.

         If L=0 then M = 0

         If K = 0 i.e case of No coupling then M = 0.

• Distance(d) between two coils (i.e As d increases then  M decreases)
•  The magnetic permeability of medium between the coils ()

Consider two long co-axial solenoids of the same length ..Let A₁ and A₂ be the area of cross-section of the solenoids with A₁ being greater than A₂ as shown in the below figure.

The turn density of these solenoids are n₁ and n₂ respectively are given as 

Let i₁ be the current flowing through solenoid 1, then the magnetic field produced inside it is given as

As the field lines of    are passing through the area A₂

So the magnetic flux linked with each turn of solenoid 2 due to solenoid 1 and is given by

The total flux linkage of solenoid 2 with total turns N₂ is

And Using   we get 

Where  M₂₁  is the mutual inductance of the solenoid 2 with respect to solenoid 1.

Similarly M₁₂ =mutual inductance of solenoid 1 with respect to solenoid 2 is given as 

Hence 

So, In general, the mutual inductance between two long co-axial solenoids is given by

If a dielectric medium of permeability   is present inside the solenoids, then

Consider two circular coils one of radius 'r₁' and the other of radius' r₂'placed coaxially with their centers coinciding as shown in the below figure.

Since  so we can assume coil 2 is at the center of coil 1.

If 

Suppose a current   flows through the outer circular coil.
Then Magnetic field at the center of the coil 1 is given as

So the total flux passing through coil 2 will be given as 

And using 

we get 

 Where M=mutual inductance between two concentric coils