L28 : Transformer working - Alternating current, Physics, Class 12 Video Lecture

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make sure that you have watched all the
videos to the part 27 before going ahead
with part 28 let us look at the working
of the transformer so what we do is
towards this primary coil we provide our
input voltage so this is the input so
the source voltage is applied at this
end and what happens when this when the
voltage so when the primary coil is
connected to the AC source alternating
alternating current is produced by the
source now because of this alternating
current and alternating flux will also
be produced in the coils now since the
flux is alternating that means the
magnetic flux is changing and due to the
change in the magnetic flux EMF will get
induced so there will be two EMF
induction that will take place here one
will be self induction and the other one
would be mutual induction both of these
will take place simultaneously so this
change in magnetic flux is taking place
in the primary coil so because of this
change in flux there will be self
induced EMF in the primary coil so self
induction will take place in the primary
coil and since there is a change in
magnetic flux with this primary coil
there will be a corresponding change in
magnetic flux with the secondary coil
which will give rise to an induced EMF
in the secondary coil so that means
mutual induction will take place in the
secondary coil so there will be a self
induced EMF in the primary coil which
can be written as EP the subscript P is
for the primary point so this is the
induced EMF in the primary coil which is
equal to minus NP D Phi by DT where D
Phi by DT represents the rate of change
of magnetic flux and n P represents the
number
phones in the primary file and n s
represents the number of turns in the
secondary point so how do I write
induced EMF is equal to minus D Phi by
DT remember your lesson on self
induction and mutual induction be
standing there that flux linkage with
any coil is defined as total number of
turns into the magnetic flux which is
equal to L I right this is how we define
and what is L I and from this we can say
that and D Phi by DT is equal to L di by
DT and what is L di by DT it is nothing
but self induced EMF so from this we can
write that into Z M F is equal to minus
n D Phi by DT where n represents the
number of turns on that coil now
similarly there will be mutual induction
which will be experienced by the
secondary coil so what would be the
mutual mutually induced EMF in the
secondary coil so EMF induced in the
secondary coil would be equal to minus n
s that is the number of turns in the
secondary coil into the rate of change
of magnetic flux right so these are the
EMF that will get induced on the primary
coils and the secondary coils in
respectively now let us assume that the
resistance of these windings is zero for
now we assume that the wires which are
used to bind and the primary n as well
as at the secondary end their
resistances are zero in that case the
voltage of the supply will be equal to
the voltage across the primary coil that
means if we assume direct resistance is
equal to zero in primary and secondary
points so if we assume their resistances
to be zero in that case the voltage of
the supply would be equal to the EMF
induced in the primary file that means
this should be equal to the source
voltage so therefore e P that is EMF
induced in the primary coil is equal to
VP that is voltage across the primary
file
similarly the EMF induced in the
secondary coil would be the voltage
across the secondary this that is the
secondary voltage so that is secondary
voltage so therefore we can write the
same expression in this way that this VP
is equal to minus NP D Phi by DT and V s
is equal to minus NS D Phi by DT so let
me number this equation as equation one
and two so if you divide equation one
with equation two what do you get you
get VP by V s is equal to NT divided by
n s that means the voltage that we will
receive at the secondary end because
primary voltage is the one which I am
supplying right that is basically the
supply voltage so we can say that the
voltage at the secondary end or
secondary voltage would be equal to NS
divided by NV into the primary voltage
so that means how much voltage I will
get at the output end that will depend
upon the ratio of the number of terms of
the secondary foil to the number of
terms of the primary coil right so it is
completely dependent on how many turns
we have at the output end at the input
end so in this case by looking at this
you can say that whenever number of
turns at the secondary coil would be
would the voltage which will receive at
the output end will be more so we will
talk about that in the next slide but as
of now we will s will have a quick
review of the working let us suppose we
connect the primary coil to an AC source
the AC source will supply AC current the
alternating current will give rise to
alternating flux because of this
alternating flux there will be a self
induced EMF in the primary coil and
there
a mutually induced EMF in the secondary
quite so he can write down self induced
EMFs in this way and the mutually
induced EMF in this way right now we
assume that the wires which are used for
winding purpose at the primary coil as
well as the secondary file their
resistances as zero in that case we can
say that the self induced EMF at primary
coil is equal to the voltage across the
primary coil similarly we can see that
the EMF induced at the secondary coil is
equal to the voltage across the
secondary coil so we replace them with
primary voltage and secondary voltage
and we find that the secondary voltage
we received is equal to the ratio of the
number of terms till all of the
secondary coil to the primary coil is
equal in two multiplied by the voltage
at the primary end now one important
thing to be noted here is that the power
at the input end remains the same at the
power at the output end so if I say that
if I am giving us some voltage at the
input end and I am getting a higher
voltage at the output end that means
even though the voltage is increasing
but the net power has to remain the same
so how is the net power remain the same
if your voltage is increasing we know
that power is equal to voltage times
correct right I am saying that I want
power to be constant but I want voltage
to increase so if voltage is increasing
then current has to decrease so if
voltage is becoming double then current
has to half itself so that the net power
remains the same so this is also a very
important specification of a transformer
that the power at the input end and the
power at the output end remains the same
so when the power when the transformer
increases the voltage it decreases the
current and when the transformer will
decreases the voltage it increases the
current so that it keeps the total power
at the input end as the same as the
total power at the output end so when we
see so when we assume that error no
energy loss
see right now we are just talking about
an ideal transformer we are not
considering any loss due to resistance
we are not considering any hysteresis
loss we are not considering any flux
leakage we are not considering any loss
as such we are considering that this is
the ideal transformer right so here we
say that the power input should be equal
to the power output that implies that I
P V P that is the product of voltage the
primary voltage and the primary current
should be equal to the product of the
secondary voltage and the secondary
current so now we will talk about the
types of transformer there are two types
of transform and effect better based on
its wording one is called
a step-up transformer and the second one
is a step-down transformer when we talk
of a step-up transformer that means it
takes the voltage up so that means it
amplifies the voltage increases the
voltage if I give 10 volts as primary
voltage the secondary voltage I get is
say 20 volts 30 volts or 40 volts that
means when the secondary voltage or when
the output which we get is higher than
the input then we say that it is a
step-up transformer that means it is
taking the voltage up that is why it is
called step up so when will a transform
and act as a step-up transformer we saw
the previous slide that the voltage at
the secondary n is given by number of
terms of secondary coil divided by
number of terms of primary coil into the
primary voltage so if I want that if the
transformer should be a step up
transformer step-up transformer means
secondary voltage should be greater than
primary voltage so when is when is that
possible that is possible only when
number of secondary turns is greater
than the number of primary turns as you
can see here here if you see the number
of turns around the secondary coil is
larger when compared
you that on the primary coils so in this
case the transformer will act as a
step-up transformer and in such a
transformer the secondary current will
always be less than the primary current
so that the power at the input end that
is power input is equal to power output
right so what is the output of a step-up
transformer the output of a step-up
transformer is a high voltage and low
current right now let us talk about the
step-down transformer in step-down
transformer it is just the other way
round in this case we want that the
secondary voltage should be less than
the primary voltage so for secondary
voltage to be less than primary voltage
what do we want that is possible only
when the number of turns on the
secondary coil is less than the number
of turns on the primary coil as you can
see here in this diagram so for a
step-down transformer the secondary
current is very high when compared to
the primary current because the voltage
is less so the current has to be more so
that the input power is equal to the
output power right so in this case what
is the net output of a step-down
transformer we get low voltage and high
current so these different kinds of
transformers have different applications
for example the step-down transformer is
used in for welding purposes because in
these transformers in the output we get
low voltages but very high current and
extremely high current are used for many
welding applications so step-down
transformer finds its use in those cases
similarly when we talk about step-up
transformer we have the power supplies
which comes to our homes their
transformers play a very important role
we will discuss about that in detail in
the next slide but then we will see that
we really need something which can amp
find the voltage but it should not
increase the current so transformer it
becomes the best choice there where it
increases the voltage but at the same
time it breezes the current so we will
see that in the next slide thank you
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