SELF INDUCTANCE

•When an electric current is passed through an insulated conducting coil, it gives rise to a magnetic field in the coil so that the coil itself behaves like a magnet.

•The magnetic flux produced by the current in the coil is linked with the coil itself.

DEFINATION

As the strength of the current in thecoil is changed, the flux linked with the coilalso changes. Under such circumstances anemf is induced in the coil too. Such emf iscalled a self-induced emf and this phenomenonis known as self-induction.

Conducting coil

BatteryKey/Switch Rheostat

Wires

Direction of the Current

Current flows In anti-clock Wise direction

Current flows In clock Wise direction

If the number of turn in a coil is N and the flux linked with each turn is φ, then the total flux linked through the coil = Nφ. In this case, the total flux linked with the coil (which is called flux linkage) is directly proportional to the current I flowing through the coil.

N = LI

where the constant of proportionality L is called the self-inductance of a coil.

N = LI, L= N/I

The self inductance L is a measure of the flux linked with coil per unit current.

•The self-inductance L of a coil depends upon –

(1)The size and shape of the coil.

(2) The number of turns N.

(3) The magnetic property of the medium within the coil in which the flux exists.

NOTE:Self-inductance L does not depend on current I.

Diffrentiating equation N = LI with respect to time t,

N d/dt = L dI/dt

In the case of self-induction, Faraday’s law and Lenz’s law holds good. Hence self-induced emf in the coil is,

e = -N d/dt

Self-induced emf is also called “back emf”.

Form equation e = -L dI/dt

Self inductance L = -e/(dI/dt)

“The self-induced emf produced per unit rate of change of current in the circuits called self-inductance of the circuit.”

Unit of L =unit of emf(v)/Unit of rate of change of current (A/s)=Vs/A or Henry(H)

By:Karan Thawani

Group No. 2