# Electromagnetic Induction(E.M.I.)

Electromagnetic Induction is the phenomenon of generating an e.m.f. by changing the number of magnetic lines of force associated with a circuit. The e.m.f. so generated is called induced e.m.f. and the corresponding current is called induced current. The number of magnetic lines of force crossing surface normally is called magnetic flux liked with the surface. It is represented by ɸ=vector B. vector A= BA cosѳ

Where B is strength of magnetic field, A is area of the surface and ѳ is the angle which normal to the surface area makes with the direction of magnetic field. Thus component of magnetic field normal to the area is significant. If a coil has N turns, then ɸ= BA cosѳ The S.I. unit of magnetic flux is weber which is the amount of magnetic flux over an area of 1 m^{2} held normal to a uniform magnetic field of one tesla. The cgs unit of ɸ is Maxwell.

1 weber = 18^{8} maxwell

When plane of coil is perpendicular to magnetic field, ѳ = 0˚,

ɸ = NBA cos0˚ = NBA (maximum)

When plane of coil is parallel to the magnetic field, ѳ = 90˚,

ɸ = NBA cos90˚ = NBA (minimum)

### Faraday’s phenomenon of E.M.I.

Faraday discovered the phenomena of E.M.I. and established the following two laws of E.M.I.

1) First law- Whenever the amount of magnetic flux linked with a circuit changes, an e.m.f. is induced in the circuit. This induced e.m.f. last so long as the change in magnetic flux continues.

2) Second law- The magnitude of e.m.f. induced in a circuit is directly proportional to the rate of change of magnetic flux linked with the circuit.

### Eddy currents

Eddy currents are the currents induced in the body of a conductor when the amount of magnetic flux liked with the conductor changes. These currents are also called Foucault currents. The magnitude of eddy currents is given by i= -e/R where R is resistance of the conductor. The direction of eddy currents is given by Lenz’s law or Flemming’s right handrule. Some of the important applications of eddy currents are : Electromagnetic damping, induction furnace, electromagnetic brakes, induction motor, speedometer and in diathermy i.e. deep heat treatment of part of human body. Some of the undesirable effect of eddy currents is that they oppose the relative motion, involve loss of energy in the form of heat and reduce the life of electrical devices. To minimize eddy currents, we use laminated cores.

### Self induction

It is the property of a coil by virtue of which the coil opposes any change in the strength of current flowing through it by including an e.m.f.

### Mutual Induction

It is the property of two coils by virtue of which each coil opposes any change in the strength of current flowing through the other by developing an induced e.m.f.

### Reactance

It is the opposition offered by a coil or a condenser or both to the flow of alternating current. Reactance arises on account of induction effects. The alternating voltage and alternating current do not remain the same phase through the reactance. There are two types of reactance

a) Inductive and

b) capacitative reactance

### Impedance Triangle

It is a right angle triangle whose base represents ohmic resistance R, perpendicular represents reactance ( X_{l} – X_{c}) and hypotenuse represents impedance Z of the LCR circuit.