1. By Squadron Leader Zahid Mir CS&IT Department , Superior University
PHY-AP -20 Self & Mutual Induction By
Squadron Leader Zahid Mir
CS&IT Department , Superior University

2. Self Induction
The phenomena in which a changing current in coil induces an emf in itself is called self induction

3. As we know that an emf is induced in a coil whenever the magnetic flux linked with the coil changes.
Let a current i is flowing in the coil then,
φB α i
if the coil has ‘N’ number of turns;
φB α Ni
φB =KNi
Where KN = L = Inductance
φB =Li
Differentiating both sides w.r.t.time
dφB =L di
dt dt

4. According to Faraday’s law of electromagnetic induction;
dφB / dt = -(emf)ind
(emf)ind = -L(di/dt)
The negative sign indicates that the induced emf will oppose the cause i.e change in current.
Inductance
“L” is called the coefficient of self inductance or simply inductance. From above equation;
L = (emf)ind / (di/dt)
Self inductance is defined as the ratio of self induced emf to the rate of change of current in the coil.
Henry The unit of self inductance is henry. The self inductance of a coil is one henry if current changes at a rate of one ampere per second through the coil which causes an induced emf of one volt in it.

5. Mutual Induction
The phenomena in which a changing current in one coil induces an emf in another coil is called mutual induction

6. Let two coils are placed close to each other.
N2Ф21 = Net flux passing through coil 2 due to magnetic field of coil 1.
N2Ф21 α I1
N2Ф21 = MI1
Where M = N2 Ф21 I1
is the mutual inductance of the two coils.

7. According to Faraday’s Law;
(emf)2 = -N2 dφ21/dt
(emf)2 = -N2 d{(M/N2)I1}/dt
(emf)2 = - M dI1/dt
M = (emf)2
dI1/dt
Mutual Inductance
The mutual inductance of the pair of coils will be one henry if the rate of change of current in the one coil will be one ampere per second, causes an induced emf of one volt in the other coil.