Transformers Video Lecture | Physics Class 12 - NEET

when I was a kid there was this thing
you would say to another kid when you
greeted him you'd say something like
transformers more than meets the eye and
so today we are going to talk about
transformers and it's not that movie but
they're everywhere and they're taking
over also I will start by showing you
that this is the battery charger for the
very camera that you are seeing this
video through whoa this is kind of meta
right this is yeah okay cool
so it says right here input 120 volts to
100 100 volts to 240 volts huge range it
says 50 to 60 Hertz so something must be
changing up and down and the output is
4.2 volts DC and it's 500 milliamps so
that's a very small current and a very
small voltage output but a much bigger
voltage input so something must be
changing the voltage in here so let's
look at another one I've got this one
where's one oh yeah you might have a
transformer for your laptop inside a
block that looks like this this one says
that the input is again a huge range of
voltage inputs and a range of voltage
outputs between 22 and a half to 18
volts so the voltage is going down right
here it's called a step down transformer
when you do that got some other
transformers here let's see here's one
yeah there's a RadioShack transformer
and it's got this plug at the end right
here and it was designed to power some
small toy probably it says input 120
volts 80 milliamps and output nine volts
350 milliamps whoa okay so I get more
current out then I put in and I get less
voltage out than I put and that's an
interesting relationship maybe you're
thinking about how those are related but
I want to see what's inside of this
sucker inside of here we've got a oh
we've got this thing and this is a
transformer so we'll investigate how
that works we've also got some
capacitors and some diodes and we could
talk about how all that goes together at
a later time but yeah we don't need that
anymore and oh here's a nice one from a
Nintendo
Entertainment System ever heard of those
they're kind of old and it says input
120 volts and output nine volts all
right let's see what's inside of it
uh-oh inside of here we've got only a
transformer and a little resistor right
there that's how you power your Nintendo
Entertainment System
I mean it's how I used to power my
Nintendo Entertainment System here's one
that's already broken oh wow
now it's really broken yeah and you've
got a transformer right here and a
capacitor right here and some diodes
that's all you need to go from AC to DC
and to change your voltage it says it's
a hundred twenty volts and it goes to 12
volts like a factor of 10 that's pretty
cool in voltage what about this door
right here this is a lab transformer now
I want you to start noticing what sort
of things transformers have if you're
looking over here this guy has a it
looks like some kind of an iron core but
it's not just solid iron it's a whole
bunch of plates of iron and we can talk
about why that is actually that's a
little bit advanced I'm not going to
talk about that but lots and lots of
different plates of iron all glued
together and this has you'll see the
same thing if you look on the side view
you've got a whole bunch of plates of
iron glued together and I'm going to
unscrew this so you can peek inside of
it first of all you notice there's a top
thing and there's another that means
they're all different plates of iron all
stuck together and then this and that
are each coils and they're not
electrically interacting with each other
you've got a coil here and a coil here
it tells me how many turns I have I have
a 110 turns right there and I have 200
turns right there and the idea is you
can hook up electrically to these guys
and this coil will interact with that
coil by simply some magnetic flux that's
going through this because when this is
connected here you've got a magnetic
loop and so if you try to get a North
Pole here you'll kind of get well not
really a magnet you'll just get it like
a loop magnet and all of the magnetic
flux is contained right inside of there
I guess I should assemble that later
let's put it aside really at its most
basic all a transformer requires is a
coil and another coil that can talk to
each other like the magnetic field from
one coil interacts with the magnetic
field of the other coil so you could put
you can put them next to
each other but that's not perfect what
you really want is to have something
that's Ferris going between them
something to conduct the magnetic field
guide the magnetic field to make sure it
goes between them and then you really
want to loop this around so that the
field is only connecting those two so
you guarantee the flux through one is
equal to the flux through another so
I'll draw a schematic of that kind of a
thing let's see I'm going to start by
drawing the metal let's say that it's
green metal you've got this metal ring
it has to be ferrous metal so it is
magnetizable but you don't want it to be
a hard magnetic material you want it to
be a nice soft magnetic material so it
easily magnetizes and easily D
magnetizes and this thing has got to be
solid or plates glued together or
something like that then I'm going to
come in here with a wire and this wire
is part of my primary circuit Here I am
coming in here with a wire that's going
to go above and behind and above and
behind above and behind above and behind
how many should we put over here what do
you want I don't know I can't hear you
and behind and then we'll leave at that
point all right so that was what I'm
going to call the number of turns on the
primary side and we'll connect something
over to the primary side soon I'm going
to put fewer turns over here on the
secondary side I'm going to come around
here and just to we're going to go
behind and then we'll go above and
behind will go above and behind and then
we'll leave so this is the number of
turns on the secondary side and I'm not
doing them in any particular ratio it
looks like I got a factor of two to one
we got six over here and three over here
and this thing right here is called the
core of the transformer and that's that
ferrous metal so I'm going to have some
voltage over here I'm going to have a
resistor right which is probably my load
so that's our goal is to deliver some
power to the load in the transformer so
that might be your Nintendo
Entertainment System or your laptop
something and this would be the voltage
input from the primary but across the
load I'm going to be able to measure a
voltage and that voltage I would then
call the voltage of the secondary
secondary voltage of the secondary and
over here I'm going to be actually
providing a voltage this is the symbol
for an AC voltage generator the reason I
have to use AC is because well if I want
the flux to change in this core that's
easy all I have to do is turn on even
just a battery right here and that would
change the flux which would call it a
voltage over here but gradually you see
this loop would get used to the new flux
and then there wouldn't be any voltage
over here but what I need to do is I
need to change the voltage through here
continuously let's see if I continuously
change this voltage then I'm going to
get a flux first this way and then that
way and then this way and then that way
and this way and that way and this way
and that way and similarly over here
because the flux of the whole loop will
be going counterclockwise or clockwise
and clockwise
its flux sloshing back and forth instead
of it's really domain changing direction
instead of electron sloshing back and
forth as are present in this circuit so
in the circuit electrically you've got
electrons moving here you've got
magnetic domains shifting orientation
and as a result inside here this loop
gets continually pissed off because you
keep trying to change the flux through
it and it doesn't like it so it induces
a voltage that gives you a well then a
current through your load so that's the
idea now let's do some equations I'm
saying that the voltage of the primary
this is the voltage of the primary I'm
not going to call it the voltage of the
source because then we could confuse it
with the voltage on the secondary side
I'm going to use just primary and
secondary the primary voltage will be
well I guess it's negative and primary I
have to label which number turns I'm
talking about right times well it's just
the change in flux in the primary
circuit divided by time and the voltage
on the secondary side of the circuit
will just be n times the well n
secondary times the change in
flux in the secondary circuit / time and
I might oh wait oh wait
but the flux
well the flux is always equal through
here and through there so the flux
through this loop is equal to the flux
through that loop and so the change in
flux will be equal as a function of time
as well so there we go now I want to
actually find the ratio of the primary
voltage to the secondary voltage if I
say that primary voltage divided by
secondary voltage I'm going to plug all
this stuff in I get negative n times
primary Delta v primary over delta T all
that divided by a negative and secondary
divided by mmm-hmm
Phi secondary over delta T but these two
are equal so they can cancel out delta T
in the denominator of the numerator and
TT in the denominator of the denominator
also cancel out the minus signs cancel
out giving me a plus sign and I find
this beautiful equation that the voltage
ratio depends on the ratio of the number
of turns so the side that has more turns
will have a greater voltage assuming
that the flux through each side is the
same yes and that makes sense because
this is more sensitive to changes in
flux and this is less sensitive to
changes in flux so if I put a 2 volt
supply over here then I'm going to get
out only one volt ah or I could make
this side this the primary and make this
side the secondary I could put one volt
in over here and get out two volts so it
depends on the ratio of the number of
wires wrapping around pretty handy now
this is a way for me to change voltage
what if I used let me just sketch one
out for you I don't want this to get
confusing I'm going to give you a green
core
let me give you green core and we'll go
around here just for time two three four
all right and then on the other side I'm
going to go around 40 times 1 2 3 4 5 6
7 8 9 10 11 12 13 14 15 16 17 18 19 20 1
2 a 2 2 or 3 - 4 - 5 - 8 6 4 saturate -
a 9 30 31 32 33 34 35 36 SFA then I
appoint great over here I'm going to
have the voltage of the secondary side I
can measure across this resistor which
is going to be my load and I've got the
voltage of my source put out by an AC
generator supposed to be a picture of a
wave because that's what that voltage
looks like and this is the voltage of
the primary circuit now I could argue
that the voltage of the secondary
circuit is the voltage of the primary
circuit times what do we have we have
this ratio over here let me see if I can
make some sense out of this thing I need
to make some sense out of that basically
I'm solving it for voltage secondary so
I think it's going to be the number of
what this is the number of turns primary
and this is the number of turns
secondary enough and when I'm solving it
for voltage secondary it's going to be
the number of turns in the secondary
circuit divided by the number of turns
in the primary circuit which in this
case well I think that I'd plugged in
that this is voltage primary divided by
or multiplied by 40 divided by 4 which
is 10 times in this case the voltage of
the primary circuit so I've increased
the voltage that's awesome and in fact
you see these transformers all over the
place at a power generation station
they've got big transformers and the
wires if I'm thinking about telephone
poles like this and I've got another
telephone pole over here these wires are
incredibly high voltage so if you see if
you see a balloon and you want to go up
and get it don't just don't go up there
because it's actually really really
dangerous there very high voltage
voltage is high but before it goes up to
somebody's house like let's say your mom
and dad live over here in a tiny little
house with just one door and no windows
then there's going to be a well all
three of these will join the
little panel and usually there'll be a
little transformer right here there'll
be a transformer maybe here and it looks
like a cylinder it's sometimes gray in
the United States hanging off the pole
and it feeds this wire over here and
those guys go over to your house it's
usually three wires all twisted together
all right that probably depends on where
you live but you're going to get some
some energy delivered to their house
because this transformer is lowering it
down to only 120 volts in the United
States and these guys out here might be
a thousand volts or two thousand volts
and in fact the ones that go from city
to city the really big power lines might
be hundreds of thousands of volts so
we'll talk about why you'd want to do
that in just a moment but first agree
that I can raise the voltage which means
more pushing right that's awesome more
pushing but it comes at the expense of
what greater output voltage is possible
but who's buying right why can that
happen it seems like you're getting more
energy and you know the voltage is
energy per charge so I'm thinking maybe
you don't have as many charges let's see
if we can justify that I want to argue
that power in must be equal to power out
and in fact that's sort of even assuming
that efficiency is equal to 1 that would
mean that efficiency well you know
efficiency if you work out you win right
and if I divide both of these by time I
would find power out over power in if
you pout then you pin that's sort of a
wrestling phrase but this is input power
so it's going to be power in time I'm
sorry current in times voltage in and
output power has to be current out times
voltage out and in the language of a
transformer we're talking about current
of the primary times voltage of the
primary must be equal to current of the
secondary times voltage
of the secondary circuit so if I
rearrange these for voltage Oh what if I
rearrange these for voltage primary over
voltage secondary
that out then I'm going to get current
secondary divided by voltage whoa how
that happens sorry
current Wow let's just back the heck up
this is current secondary divided by
current primary so I gain I gain voltage
based on the number of turns remember
this was voltage primary over voltage
secondary this is the number of turns
primary divided by the number of turns
secondary so this is all a true
relationship and it says that voltage
might increase but you pay for it
because the current decreases now here's
a beautiful thing you know that
resistive heating is what we call it
Joule heating Joule Heaton's like I
squared times R and if you've got some
power lines up here and those power
lines have a fixed resistance maybe you
decided you're Ameren and you decided
the best way to the best way to build
your power line they have a fixed
resistance here do you want a high
voltage up here and therefore a small
current or do you want a high current up
here and therefore a small voltage
here's the problem a really high voltage
would be very inconvenient you couldn't
lay your lines on the ground because if
some kid touches them that kid would die
and that's not OK
so if you're going to have the bother of
having extremely high voltages like the
voltage of many cross country supply
lines is like 750 down volts this is
extremely dangerous so it's got to be
way up high in the sky or varied well
beneath the ground and varying it is
even more expensive than just hanging up
in the sky so the advantage of this
though is that since your supply voltage
is so high your supply current
can be very very small yet still supply
the same power that's necessary the
higher the voltage goes the lower the
current goes and if the lower the
current goes then the less energy you
spend heating the countryside is that
the point of electrical distribution
systems to heat the countryside no