PPT No. 28
* Faraday’s Experiments
* Faraday’s Law
* Electromagnetic Induction
Before Faraday’s Experiments
Oersted and Sturgeon had discovered that
an electric current creates a magnetic field
Thus, an important scientific fact about
the relationship between electricity and magnetism
became evident i.e.
Electricity & Magnetism –
The two branches of physics
cannot be considered as separate and unrelated.
Magnetic Field => Electric Current
A possibility of the inverse effectwas investigated independently by the
English physicist Michael Faraday (1791-1867) and
American physicist Joseph Henry.
After performing a series of experiments in 1831
Faraday was the first to present results about
the effect of time varying magnetic fields.
His historical experiments were crucial to learn about
the connection between Electricity and Magnetism.
Categories of Faraday’s Experiments
Faraday’s experiments are grouped under 4 Main categories:
Expt 1: Relative motion between a Magnet and a Coil
Expt 2: Relative motion between two Coils
Expt 3: Two stationary Coils and a Tap key
Expt 4: Cutting of magnetic flux by a conductor
Expt 1: Relative Motion between a Magnet and a Coil
A coil of wire is wound on a paper cylinder and
connected to a sensitive galvanometer.
A bar magnet is moved back and forth inside the coil.
His observations were as follows
* When North pole of a magnet is pushed towards a coil,
the galvanometer shows a deflection.
•When the North pole of a magnet is pulled out,
the deflection is in the opposite direction.
Expt 1: Relative Motion between a Magnet and a Coil
•When the South pole of the bar magnet is
pushed towards or pulled away from the coil the deflections
in the galvanometer are opposite to that
observed with the North pole for similar movement.
•When the bar magnet is held fixed and
the coil is moved towards or away from the magnet,
similar results are obtained.
•If the magnet is stopped/ held stationary,
there is no deflection in the galvanometer.
•When the magnet is moved faster towards the coil,
the deflection is larger.
Expt 1: Relative Motion between a Magnet and a Coil
The letters N and S drawn inside the coil show
the direction of induced current flowing in the coil.
Expt 1: Relative Motion between a Magnet and a Coil
The Conclusion of Faraday’s I experiment
The relative motion between the Coil and the Magnet
causes a change in the magnetic flux
threading through the coil and
an electric current flows in the coil.
The magnitude of the current depends on
the magnitude of their relative velocity and
also on the strength of the magnet.
Experiment 2: Relative Motion between Two Coils
Two coils A and B are in the vicinity of each other.
Coil A connected to a battery carries steady electric current.
When coil B is moved towards A, a galvanometer in
the circuit of B coil shows a deflection.
When the B coil is moved away, the galvanometer shows
a deflection again, but in the opposite direction.
The deflection lasts as long as the B coil is in motion.
Experiment 2: Relative Motion between Two Coils
Experiment 2: Relative Motion between Two Coils
When the B coil is held fixed and
the A coil is moved,
the same effects are observed.
Thus, magnetic flux surrounding
a current carrying coil
changes due to a relative motion
between it and another coil
and causes the flow of electric current.
Experiment 3: Two Stationary Coils and a Tap Key
Two coils of wire A and B are wound around
opposite sides of a ring of soft iron.
By closing the switch a current is passed through coil A.
The compass needle in (or a pointer in the Galvanometer
connected to) the B coil deflects momentarily and
returns immediately to its original position.
By opening the switch,
the compass again deflects momentarily,
but in the opposite direction.
Experiment 3: Two Stationary Coils and a Tap Key
Experiment 3: Two Stationary Coils and a Tap Key
Thus even if the coils are not moved physically (as in expt1)
but magnetic flux is changed, only by making
the switch on (causing current to build up in coil A), or
switch off (current is reduced to zero in a coil A),
magnetic flux produced by the current carrying coil changes,
some of this magnetic flux linking (crossing)
another nearby coil B also changes
and transient current flows in this coil B,
as shown by the kick of the pointer in galvanometer.
The direction of this current for switch-on condition
is opposite to that due to switch-off condition.
Experiment 4: Cutting of Flux by a Conductor
When part of a current carrying conductor
Is moved in a magnetic field of a magnet
and it cuts magnetic flux then
an electric current flows in the circuit.
The direction of current
when conductor is moved upward is opposite to
the direction of current
when conductor is moved downward
as seen from the opposite deflections
in the Galvanometer
Experiment 4: Cutting of Flux by a Conductor
Faraday’s Law
Faraday summarized the observations of
his experiments and stated a Law as follows
Whenever the magnetic flux due to
a source of magnetic field changes in time,
it induces an electric field,
causes an Electromotive Force (emf)
to be "induced" in the coil and
an induced current flows in the closed circuit.
Faraday’s Law
The change in the magnetic flux could be produced by
•changing the magnitude of
magnetic field strength linking a wire,
•changing the direction of the magnetic field
•(moving a magnet toward or away from the coil or
making current on/ off in a current carrying coil),
• changing the position, shape or orientation of the circuit
(e.g. moving the coil into or out of the magnetic field,
•rotating the coil relative to the magnet, etc.).
Faraday’s Law
A summary of Faraday’s observations is given by Faraday's
law of magnetic induction or the “Flux Rule” as follows
(i) Whenever the magnetic flux linked with a circuit changes,
an Electromotive Force is induced in the circuit,
which lasts as long as the change in
magnetic flux associated with the circuit continues.
(ii) The magnitude of the induced emf is proportional to
the rate at which the magnetic flux
linked with the circuit changes.
The phenomenon is called as electromagnetic induction
Faraday’s Law
Mathematically, if the magnetic flux ΦB through a circuit
changes by an amount dФB in a time interval dt then
the emf E generated in the circuit is given by
SI units are selected to have
the constant of proportionality in this law to be unity.
Induced emf is one volt when magnetic flux changes
at the rate of one Weber per second.
Faraday’s Law
Faraday’s law gives the relationship between
electromotive force or "voltage" &
changing magnetic flux.
It gives a summary of the ways in which
voltage can be generated by
a changing magnetic field.
This effect was inverse to that found by Oersted that
an electric current flowing in a wire
produces a magnetic field.
Electromagnetic Induction
The Phenomenon
The generation of an electromotive force and
an electric current by a changing magnetic field
is called as the “electromagnetic induction”.
This phenomenon was discovered by Michael Faraday
and Joseph Henry almost at the same time.
However, Faraday published it first.
Two Mechanisms of Electromagnetic Induction
It is seen from Faraday’s experiments that
electromagnetic induction occurs by
two apparently quite different mechanisms as follows:
(1).The existence of a changing magnetic field
in some region of space:
A changing magnetic field creates
an induced electric field –
It can produce a charge separation
in a conducting matter and
a measurable potential difference.
Two Mechanisms of Electromagnetic Induction
(2). The movement of a conductor
through a region where
there is a magnetic field:
Charged particles within a moving conductor
experience magnetic forces
which produce a charge separation
which results in a potential difference
Electromagnetic Induction
Two Mechanisms but Single Mathematical Law
It turns out that both of the effects called as
electromagnetic induction
can be described by
the same mathematical law
given by Faraday's lawInduced emf is proportional to
the rate of change of magnetic flux.
Electromagnetic Induction
An Example of Electromagnetic Induction
Electrical generators are of two types.
Working of both can be explained on the basis of
Faraday’s law of electromagnetic induction
however based on two different mechanisms.
The main parts of Electrical generator are
a coil & a magnet.
Two Mechanisms of Electrical Generators
When a magnet is rotated
around a stationary conductor,
the changing magnetic field induces
an electric field and an emf is induced.
If the coil is rotated,
keeping the magnet stationary
then motional emf is produced.
Thus they convert mechanical energy
into electrical energy.