Basics of Electrical Engineering

• Electromagnetic Induction• Lenz’s Law• Ampere Rule

By

Ms. Nishkam DhimanAssistant Professor -EEE Deptt.

Chitkara Institute of Engg. & Technology

Electromagnetic Induction

• The phenomenon by which an emf is induced in a circuit(and hence current flows when the circuit is closed) when magnetic flux linking with it changes is called electromagnetic induction.

Direction of Induced Emf(Generator Action)

• This rule states "Hold out the right hand with the first finger, second finger and thumb at right angles to each other. If forefinger represents the direction of the line of force, the thumb points in the direction of motion or applied force, then second finger points in the direction of the induced current.

Direction of Force developed(Motor Action)

• It is found that whenever a current carrying conductor is placed inside a magnetic field, a force acts on the conductor, in a direction, perpendicular both to the direction of the electric current and the magnetic field. 

• Hold out your left hand with forefinger, second finger and thumb at right angle to one another. If the fore finger represents the direction of the field and the second finger that of the current, then thumb gives the direction of the force.

Statically Induced Emf

• STATICALLY INDUCED EMF :

The emf induced in a coil due to

change of flux linked with it (change

of flux is by the increase or decrease

in current) is called statically induced

emf. Transformer is an example

of statically induced emf.

Here the windings are stationary,

magnetic field is moving around

the conductor and produces the emf.

1. Self induced emf 2. Mutually induced emf

DYNAMICALLY INDUCED EMF

• DYNAMICALLY INDUCED EMF: The emf induced in a coil due to relative motion of the conductor and the magnetic field is called dynamically induced emf.

• Example: dc generator works on the principle of dynamically induced emf in the conductors which are housed in a revolving armature lying within magnetic field.

Lenz’s Law

• CASE-I When a magnet is moving towards the coil.• Flux is increasing• Current will be produced in such a direction so as to oppose

the increase in flux

The magnetic field created will oppose its own cause or we can say opposes the increase in flux through the coil and this is possible only if approaching coil side attains north polarity, as we know similar poles repel each other.

• CASE-II When a magnet is moving away from the coil• Flux linking with coil decreases• an emf and hence current is induced in the coil and this current

will creates its own magnetic field• according to Lenz's law, this magnetic field created will

oppose its own cause or we can say opposes the decrease in flux through the coil and this is possible only if approaching coil side attains south polarity, as we know dissimilar poles attract each other

Ampere Rule• AMPERE'S RULE: "Ampere's Rule this is the rule that applies

to the direction in which the magnetic field is when a current is associated with it"The direction in which the magnetic field current is associated and this is the rule. Grasping a conductor with your right hand and making sure that you thumb and current are pointing in the same direction, will make your fingers curl in the direction of the current field around the conductor.

Eddy Currents

• In a changing magnetic field, voltages will be induced not only in coils, but in any conductor in the field. As a result loops of current called eddy currents will be set up in the conductor. Often they are a nuisance; for example the cores of transformers and motors have to be made of insulated layer (laminations) of iron to prevent eddy currents in the core, and consequent waste of energy.

Eddy Currents