Lenz’s law:-
Lenz’s law is named after the German scientist H. F. E. Lenz in 1834. Lenz’s law
obeys Newton’s third law of motion (i.e to every action there is always
an equal and opposite reaction) and the conservation of energy (i.e
energy may neither be created nor destroyed and therefore the sum of all
the energies in the system is a constant)
Lenz law is based on Faraday’s law of induction so before understanding Lenz’s law one should know what Faraday’s law of induction
is "When a changing magnetic field is linked with a coil, an emf is
induced in it. This change in magnetic field may be caused by changing
the magnetic field strength by moving a magnet toward or away from the
coil or moving the coil into or out of the magnetic field as desired".
Heinrich Friedrich Emil Lenz
LENZ’S LAW
Lenz law states that when an emf is generated by a change in magnetic
flux according to Faraday’s Law, the polarity of the induced emf is
such that it produces a current whose magnetic field opposes the change
which produces it.
The negative sign is used in Faraday’s law of electromagnetic induction, indicates that the induced emf ( ε ) and the change in magnetic flux ( δΦ
B ) have opposite signs.
Where
ε = Induced emf
δΦ
B = change in magnetic flux
N = No of turns in coil
Reason for opposing, cause of induced current in Lenz’s law?
• As stated above Lenz law obeys the law of conservation of energy
and if the direction of the magnetic field that creates the current and
the magnetic field of the current in a conductor are in same direction,
then these two magnetic field would add up and produce the current of
twice the magnitude and this would in turn creates more magnetic field,
which cause more current and this process continues on and on and thus
leads to violation of the law of conservation of energy.
• If the induced current creates a magnetic field which is equal and
opposite to the direction of magnetic field that creates it, then only
it can resist the change in the magnetic field in the area which is in
accordance to the Newton’s third law of motion
EXPLANATION OF LENZ’S LAW
For understanding Lenz’s law consider two cases :
CASE-I When a magnet is moving towards the coil.
When the north pole of the magnet is approaching towards the coil, the magnetic flux linking the coil increases. According to Faraday’s law of electromagnetic induction,
when there is change in flux, an emf and hence current is induced in
the coil and this current will creates its own magnetic field . Now
according to Lenz law, this 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. Once we know the magnetic
polarity of the coil side, we can easily determine the direction of the
induced current by applying right hand rule. In this case the current
flows in anticlockwise direction.
CASE-II When a magnet is moving away from the coil.
When the north pole of the magnet is moving away from the coil, the magnetic flux linking the coil decreases. According to Faraday’s law of electromagnetic induction,
an emf and hence current is induced in the coil and this current will
creates its own magnetic field . Now 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. Once we know the magnetic polarity of the coil
side, we can easily determine the direction of the induced current by
applying right hand rule. In this case the current flows in clockwise
direction.
NOTE : For finding the directions of magnetic field or
electric current use Right hand thumb rule i.e if the fingers of the
right hand are placed around the wire so that the thumb points in the
direction of current flow, then the curling of fingers will show the
direction of the magnetic field produced by the wire.
Right hand thumb rule
The Lenz law can be summarized as under:
• If the magnetic flux Ф linking a coil increases, the direction of
current in the coil will be such that it oppose the increase in flux and
hence the induced current will produce its flux in a direction as
shown below (using right hand thumb rule).
• If magnetic flux Ф linking a coil is decreasing, the flux produced
by the current in the coil is such that it aid the main flux and hence
the direction of current is as shown below
APPLICATION OF LENZ’S LAW
• Lenz law can be used to understand the concept of stored magnetic
energy in an inductor. When a source of emf is connected across an
inductor, a current starts flowing through it. The back emf will oppose
this increase in current through the inductor. In order to establish the
flow of current, the external source of emf has to do some work to
overcome this opposition. This work done by the emf is stored in the
inductor and it can be recovered after removing the external source of
emf from the circuit
• This law indicates that the induced emf and the change in flux
have opposite signs which provide a physical interpretation of the
choice of sign in Faraday’s law of induction.
• Lenz’s law is also applied to electric generators. When an
electric current is induced in a generator, the direction of this
induced current is such that it opposes its cause i.e rotation of
generator (as in accordance to Lenz’s law) and hence the generator
requires more mechanical energy. It also provides back emf in case of
electric motors.
• Lenz’s law is also used in electromagnetic braking and induction cook tops.
Faraday’s law of electromagnetic induction
In 1831, Michael Faraday, an English physicist gave one of most basic law of electromagnetism called
Faraday’s law of electromagnetic induction. This law explains the working principle of most of electrical motors, generators, electrical transformers
and inductors. This law shows the relationship between electric circuit
and magnetic field. Faraday performs an experiment with a magnet and
coil. During this experiment he found how emf is induced in the coil
when flux linked with it changes. He has also done experiments in
electrochemistry and electrolysis.
Michael Faraday
Faraday’s Experiment
RELATIONSHIP BETWEEN INDUCED EMF AND FLUX
Faraday’s law
In this experiment Faraday takes a magnet and a coil and connects a
galvanometer across the coil. At starting the magnet is at rest so there
is no deflection in the galvanometer i.e needle of galvanometer is at
centre or zero position. When the magnet is moved toward the coil, the
needle of galvanometer deflects in one direction. When the magnet is
held stationary at that position, the needle of galvanometer returns
back to zero position. Now when the magnet is moved away from the coil ,
there is some deflection in the needle but in opposite direction and
again when the magnet become stationary at that point with respect to
coil , the needle of galvanometer return back to zero position.
Similarly if magnet is held stationary and the coil is moved away and
towards the magnet, the galvanometer shows deflection in similar manner.
It is also seen that the faster the change in the magnetic field, the
greater will be the induced emf or voltage in the coil.
Position of magnet |
Deflection in galvanometer |
Magnet at rest |
No deflection in galvanometer |
Magnet moves towards the coil |
Deflection in galvanometer in one direction |
Magnet is held stationary at same position (near the coil) |
No deflection in galvanometer |
Magnet moves away from the coil |
Deflection in galvanometer but in opposite direction |
Magnet is held stationary at same position (away from the coil) |
No deflection in galvanometer |
CONCLUSION: From this experiment Faraday concluded
that whenever there is relative motion between conductor and a magnetic
field, the flux linkage with a coil changes and this change in flux
induces a voltage across a coil.
Michael Faraday formulated two laws on the basis of above experiments. These laws are called
Faraday’s laws of electromagnetic induction.
Faraday’s Laws
Faraday’s First Law
Any change in the magnetic field of a coil of wire will cause an emf
to be induced in the coil. This emf induced is called induced emf and if
the conductor circuit is closed, the current will also circulate
through the circuit and this current is called induced current.
Method to change magnetic field:
1. by moving a magnet toward or away from the coil
2. by moving the coil into or out of the magnetic field.
3. by changing area of a coil placed in the magnetic field
4. by rotating the coil relative to the magnet.
Faraday’s Second Law
It states that the magnitude of emf induced in the coil is equal to
the rate of change of flux linkages with the coil. The flux linkages of
the coil is the product of number of turns in the coil and flux
associated with the coil.
Faraday Law Formula
Faraday’s law
Consider a magnet approaching towards a coil. Here we consider two instants at time T
1 and time T
2.
Flux linkage with the coil at time, T
1 = NΦ
1 Wb
Flux linkage with the coil at time, T
2 = NΦ
2 wb
Change in flux linkage = N(Φ
2 – Φ
1)
Let this change in flux linkage be, Φ = Φ
2 – Φ
1
So, the Change in flux linkage = NΦ
Now the rate of change of flux linkage = NΦ / t
Take derivative on right hand side we will get
The rate of change of flux linkage = NdΦ/dt
But according to Faraday’s law of electromagnetic induction the rate of change of flux linkage is equal to induced emf.
Considering Lenz’s Law
Where flux Φ in Wb = B.A
B = magnetic field strength
A = area of the coil
HOW TO INCREASE EMF INDUCED IN A COIL
• By increasing the number of turns in the coil i.e N- From the
formulae derived above it is easily seen that if number of turns of coil
is increased, the induced emf is also increased.
• By increasing magnetic field strength i.e B surrounding the coil-
Mathematically if magnetic field increases, flux increases and if flux
increases emf induced will also increased Theoretically if the coil is
passed through a stronger magnetic field, there will be more lines of
force for coil to cut and hence there will be more emf induced.
• By increasing the speed of the relative motion between the coil and
the magnet. – If the relative speed between the coil and magnet is
increased from its previous value, the coil will cut the lines of flux
at a faster rate so more induced emf would be produced.
Applications of Faraday Law
Faraday law is one of the most basic and important law of
electromagnetism . This law finds its application in most of electrical
machines, industries and medical field etc.
• Electrical Transformers
It is a static ac device which is used to either step up or step down
voltage or current. It is used in generating station, transmission and
distribution system. The transformer works on Faraday’s law.
• Electrical Generators
The basic working principle of electrical generator is Faraday’s law of
mutual induction .Electric generator is used to convert mechanical
energy into electrical energy.
• Induction Cookers
The Induction cooker, is a most fastest way of cooking. It also works on
principle of mutual induction. When current flows through the coil of
copper wire placed below a cooking container, it produces a changing
magnetic field. This alternating or changing magnetic field induces an
emf and hence the current in the conductive container and we know that
flow of current always produces heat in it.
• Electromagnetic Flow Meters
It is used to measure velocity of blood and certain fluids. When a
magnetic field is applied to electrically insulating pipe in which
conducting fluids are flowing then according to Faraday’s law an
electromotive force is induced in it. This induced emf is proportional
to velocity of fluid flowing .
• Form the bases of Electromagnetic Theory
Faraday’s idea of lines of force is used in well known Maxwell’s
equations. According to Faraday’s law change in magnetic field gives
rise to change in electric field and the converse of this is used in
Maxwell’s equations.
• Musical Instruments
It is also used in musical instruments like electric guitar, electric violin etc.