Basics of Electrical Engineering
• Electromagnetic Induction
• Lenz’s Law
• Ampere Rule
Ms. Nishkam Dhiman
Assistant Professor -EEE Deptt.
Chitkara Institute of Engg. & Technology
• 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
Direction of Induced Emf(Generator
• 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
• 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
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.
• 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'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.
• 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.