Electromagnetism: Faradays Law - Electromagnetic Induction Video Lecture - Class 12 | Best Video for Class 12

welcome to the fourth lesson in our
series on electromagnetism in our first
three lessons we explored Wu stairs
discovery that showed one link between
magnetism and electricity
he showed that when a current passes
through a conductor a magnetic field is
created at 90 degrees to the direction
of the current it was not long after
this discovery that ampere showed that
two current-carrying conductors placed
close to each other will either repel or
attract each other in just the same way
that two magnets will this showed that
because of the magnetic fields around
them there is a force between the two
current-carrying conductors michael
faraday realized that there is a link
between electricity magnetism and the
force which can be harnessed to do work
he applied this idea to design a simple
electric motor you will find electric
motors in almost all appliances we use
today in our homes and in industry the
invention of the electric motor was not
however Faraday's greatest achievement
he was really curious about the
relationship between electricity and
magnetism
he knew that electric current produced a
magnetic field and wondered if a magnet
could be used to produce an electric
current at the time Faraday was a
laboratory assistant and spent most of
his time doing experiments he was eager
to find a new way to generate
electricity because the chemical cells
he used in the lab were not always a
reliable source of electricity
he began his investigation by placing
magnets of different strengths near
different sized coils but each time he
set her panic
he was unable to detect a current in the
coil although at times he was
discouraged he never gave up the
breakthrough came one day when he moved
a magnet inside a coil and detect to the
current in the coil in today's lesson we
will focus on this famous discovery
Faraday made and analyzed the law he
used to explain his discovery by the end
of this lesson you should be able to
explain what happens when a bar magnet
is pushed into or pulled out of a coil
state Faraday's law and use this law to
solve problems
now before we set up an experiment
similar to the one Faraday did there is
one idea one to recap from the work you
have done on electric current do you
remember the term EMF it stands for the
electro-motive force scientists today
think of it as a measure of the total
electrical potential energy per unit
charge available in a battery or source
of electricity EMF is measured in volts
and in some cases is in fact equal to
potential difference a galvanometer can
be used to measure the EMF or potential
difference across the ends of a
conductor here is a metal conductor
connected to a light bulb in series a
galvanometer is connected in parallel to
the conductor and the light bulb the
reading on the galvanometer is zero this
means that there is no EMF across the
circuit and we can see that no current
passes through it since the light bulb
do not light up now when I connect a
battery of cells to the circuit the
needle of the galvanometer will move
let's see how
showing that there is a potential
difference across the circuit the pout
lights up showing that there is now a
current passing through the circuit in
this circuit the battery was the source
of the EMF it is important to remember
that without a potential difference or
EMF applied to the ends of a conductor
there will be no current in the
conductor knowing this we can now move
on to doing Faraday's experiment here is
a coil of insulated copper wire you can
see that there are many loops in this
coil and they are wound together very
closely the coil is connected to a
Center 0 galvanometer I'm going to push
the North Pole end of a strong magnet
into the coil observe what happens to
the needle of the galvanometer the
needle of the Center 0 galvanometer
moves to the right and then it comes
back to zero again and look when the
magnet is pulled out of the coil the
needle jumps in the other direction and
then it comes back to zero again and if
the magnet is pushed in and pulled out
and pushed in and pushed out we can make
the needle of the Center zero
galvanometer vibrate from side to side
what can you deduce from the fact that
the needle of the galvanometer moved the
moving needle tells us that an EMF has
been detected by the galvanometer across
the ends of the coil so current has
moved through the coil look carefully
again at the apparatus we have only a
magnet and a coil and a galvanometer
with some connecting leads there is no
battery here
so what is the source of the EMF well it
is the movement of the magnet in the
coil that created the EMF only when the
magnet moved was an EMF detected and
electricity generated michael faraday
used the term electromagnetic induction
for the process of generating
electricity by moving a magnet in a coil
the EMF produced by a moving magnet in a
coil is called the induced EMF and the
resulting current is called the induced
current let's investigate what happens
to the induced EMF when we change the
motion of the magnet for this experiment
i have swapped the galvanometer with a
voltage sensor so that we can record and
compare the size of the induced EMF for
each of our experiments the sensor is
linked to an explorer data logger and
the graph will be shown on the laptop
screen watch carefully what happens
first I'll push the North Pole of the
bar magnet into the coil slowly and then
stop look at the readings of EMF I have
for this time the voltage sensor
registered a small positive EMF while
the magnet was moving but then returned
to 0 volts when the magnets stopped
moving now I'm going to push the North
Pole of the bar magnet into the coil
very quickly watch what happens to the
EMF readings
once again the voltage sensor registered
a positive EMF in the coil but this time
the value was much greater what
conclusion can you make based on this
evidence I think it shows that when the
magnet moves faster in the coil the
induced EMF is greater faraday found
that there is another way of inducing an
EMF in a coil instead of moving the
magnet move the coil and keep the magnet
fixed in position watch what happens
the voltage sensor registered an induced
EMF when the coil moved over the magnet
this shows exactly what Michael Faraday
discovered we can say that whenever you
move a magnetic field relative to a coil
an EMF will be induced in the coil it
does not matter if the coil moves or if
the magnet moves
what matters is relative motion between
the coil and the magnet from his
experiments Faraday developed a theory
to explain why an EMF is induced in a
coil when there is relative motion
between a coil and a magnet his
explanation has become known as
Faraday's law of electromagnetic
induction have a careful look at the
statement of his law the magnitude of
the induced EMF in a conductor is
directly proportional to the rate of
change of magnetic flux through the
conductor for a given coil we can write
a mathematical expression for Faraday's
law as follows e is directly
proportional to Delta Phi divided by
delta T let's look at what all these
letters and symbols stand for e stands
for the magnitude or size of the induced
EMF and Delta Phi over delta T is the
rate of change in magnetic flux remember
that the magnetic flux Phi is equal to
the product of the magnetic field B and
the cross-sectional area a through which
the field passes so why did Faraday link
the size of the induced EMF to the rate
of change of magnetic flux to understand
this let's think back to Faraday's or
regional experiment when he placed a
stationary magnet
inside a coil the magnetic field lines
from the bar magnet cut across the area
of the coil so there is a magnetic flux
cutting through the coil but notice that
there is no induced EMF in the coil
there is no reading on the galvanometer
watch what happens when a magnet moves
relative to the coil when the magnet
enters the coil the field lines only cut
through a small area of the coil meaning
that there is a small magnetic flux
cutting through the coil but as the
magnet moves further into the coil the
area increases which means that the
magnetic flux has increased so there has
been a change in magnetic flux this
change in magnetic flux occurred during
a certain time interval while the magnet
moved a change in magnetic flux per unit
time is called the rate of change of
magnetic flux if we move the same magnet
faster in the coil the change in
magnetic flux will be the same as when
the magnet moves slowly but the time
taken for this change will be less and
so the rate of change will be greater
remember when the magnet moved slowly in
the coil the induced EMF was smaller
than when the same magnet moved quickly
in the coil so that's why Faraday
concluded that the rate of change in
magnetic flux is directly proportional
to the magnitude of the EMF induced in
the coil we can expand this expression
as follows now looking at this
expression of Faraday's law can you
suggest any other factors that will
affect the induced EMF can you see that
the magnetic flux is dependent on the
strength of the magnetic field B if we
make the magnet stronger we will
increase the magnetic field
because a strong a magnet will have more
field lines it will cut through a larger
area on the conductor than a weak magnet
the result of using a stronger magnet
with more field lines is a greater
change in magnetic flux when it moves
relative to the coil also a greater EMF
should be induced in the coil than when
a weaker magnet moves let's test this
idea by measuring the EMF induced by a
strong and a weak magnet moving into the
same coil at the same speed we will do
this by dropping the two magnets through
the coil from the same height your job
is to watch the reading on the voltage
sensor to see if there is a difference
in the EMF
can you see that the stronger magnet
induced a bigger EMF in the coil this
experiment confirms the relationship
between B the magnetic field and the
magnitude of the induced EMF as shown in
Faraday's law which can be written like
this
Faraday found that another way to
increase the induced EMF is to increase
the number of loops or turns in the coil
let's test this experimentally here is
another coil of the same length as the
previous one but this one has less turns
it has 240 turns and when I drop the
magnet through this coil you will be
able to see that I get a maximum reading
on the voltage sensor that is smaller
than the first one
what conclusions can we draw from this
experiment we have confirmed that the
number of turns in a coil is related to
the size of the EMF induced in the coil
in this experiment we have moved the
same magnet through the two different
coils in the same amount of time so the
rate of change of magnetic flux passing
through each coil is the same in both
coils in this experiment the rate of
change of magnetic flux is a constant we
can therefore state that the size of the
induced EMF is directly proportional to
the number of turns in these experiments
have confirmed different types of
relationships for induced EMF in a coil
now we can combine the two relationships
we have found and write an equation for
Faraday's law as follows e is equal to n
times Delta Phi divided by delta T e
stands for the magnitude of the induced
EMF n the number of turns in the coil
and Delta Phi over delta T is the rate
of the change in magnetic flux remember
that the magnetic flux Phi is equal to
the product of the magnetic field B and
the cross-sectional area a through which
the field passes this equation can be
used to calculate the magnitude of the
EMF induced in a coil I want you to use
the equation to solve the problem in
your task for today
here are two different situations a
magnet with a magnetic field of 0 comma
1 Tesla takes 0 comma 1 seconds to move
through a coil with 120 turns a magnet
with a magnetic field of 1 comma 1 Tesla
takes 1 comma 5 seconds to move through
a coil with 200 turns the coils are both
the same shape and length the
cross-section area through which the
magnetic field passes is 1 meter square
in both coils calculate the magnitude of
the induced EMF in each coil to find
which is greater in our next lesson we
will show how the relative motion of the
magnetic field is related to the current
induced in the coil see you then
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