- [Instructor] How do
power stations provide electricity to thousands
of houses around a city? They don't use giant batteries. They do that by spinning
a giant turbine like this, and to spin such a turbine, some power stations use very hot steam that blows over the turbines and spins it. Or maybe we can use the
energy of the falling water, or another example could be we fit these inside giant windmills and let the wind do the work for us. Whichever way you choose, all
we do is spin a giant turbine, but how do does turning
something create electricity? Well, the technology is based on electromagnetic induction. Discovered by Micheal Faraday
more than 200 years ago. The basic idea is that if you take a wire and move it up or down
inside a magnetic field, it induces an electric current. So, all we have to do is attach a coil of wire to these giant turbines, and place them inside a magnetic field. As the turbine rotates,
the coil starts rotating, and the wires start moving up and down inside the magnetic field, that produces the electric current, and this can now be
used to light up things. These devices are called
electric generators, they convert spinning or mechanical energy into electrical energy. So, let's look at them in detail now. So, let's start by figuring out, in what direction a current gets induced or generated in this coil,
once it starts rotating. So, let's say in our example,
the coil rotates clockwise, somewhat like this. Now, how do we figure out the direction of the induced current? Well, we have already learned something called the Fleming's
Right Hand Generator Rule which says you stretch the fingers of your right hand like this as that they are
perpendicular to each other, then the thumb represents the direction of the motion of the wire, that is the direction in
which you're pushing the wire, the forefinger gives the
direction of the magnetic field, then our middle finger will give us the direction of the current. So, all we have to do
is align our right hand, to make sure the thumb
and the forefinger point in the direction of the
motion and the magnetic field, then the middle finger will give us a direction of the current. So, let's use our right
hand rule on the pink wire that is going upwards as you can see, and on the blue wire
that is moving downwards. So, it'll be great idea if you can first see whether you can try it yourself. So, go ahead and use your right hand and see if you can
figure out the direction of the current in the
pink and the blue wire. Alright, if you've done it, let's first start with the pink wire, because the pink wire is going up, the thumb should be pointing upwards. The magnetic field is to
the right so the forefinger should be pointing to the right, and so if we align our fingers that way, it will look like this. Since the middle finger
is pointing inwards, this means the current in the
pink wire must be inwards. Similarly, for the blue wire, the blue wire is moving downwards, so the thumb will be pointing down, the forefinger will still
be pointing to the right, and so if we arrange our right hand over here, it will look like this. The middle finger is
pointing out of the screen, that means the current in the blue wire will be coming out of the screen. So, what we have seen is if a wire is moving upwards over here, then the current will be into the screen, and if the wire is moving downwards, then the current will
be out of the screen. Remember this, this will be important. So now, let's get rid of the hands and put arrow marks to
indicate the current, and now we can guess what direction the current will be in
the rest of the wires. Since the current has to flow
from the pink to the blue, we can say that the current
has to move here like this, and then the current has to
move here this way, it goes out, it goes to that external circuit maybe where there is a bulb,
we'll look at that later, then the current comes back like this and it flows in this way, and the current continues to flow like this as the coil keeps rotating, because you can see the pink wire is still going up and the blue wire is still going down until
we come to this point, because now the pink wire
starts moving downwards, and now the blue wire starts moving upwards, can you see that? Again, if I go back and come back again, notice the pink wire is coming down and the blue wire is going up, this means now the
current in the pink wire should be out of the screen, because we already saw
using our right hand rule. When the wire is going down,
the current must be out, and in the blue wire which is going up, the current must now be inwards. In other words, the current
will now change its direction. This is the important thing. So, the current changes it's direction and it continues to flow this way until again the pink wire comes on the left side, because now again, the pink wire starts going up, the blue wire starts going down, and as a result the current in the pink wire will be inwards, the current in the blue
wire will now be outwards, the current will again reverse. So, every time our coil
is in this position, which is perpendicular
to the magnetic field, we will see that the
current direction will flip, and so if you look at
the entire animation now, it looks somewhat like this. Every time the coils
comes in this position, perpendicular to the magnetic field, the current direction keeps
changing now of course, in the animation, I'm stopping, I'm pausing the animation when my coil is perpendicular to the magnetic field. So, that we can see the current
flipping it's direction, but of course in reality, the coil will be pushed continuously, there'll be no stopping, there'll be no jerking motion like we're seeing over here. Now, the next question we might have is how do we connect this coil to an external circuit like say to a bulb? Well, we could connect it directly right. Well, let's see what happens if we connect the circuit directly. Current will flow, no problem, but as the coil starts rotating, notice the wires start
twisting and tangling and turning and what not. So, that's going to be a problem. So, to avoid that, we will not
connect the wires directly, instead we will use an arrangement involving brushes and slip rings. It looks somewhat like this. So, basically we have two metallic rings, the pink one and a blue one, and what you may not make out from my diagram over here, is that each ring is
connected to one wire only. So, the pink ring is only
connected to the left wire, and the blue ring is only
connected to the right wire, and these wires are
connected to carbon brushes, which is also conducting, and they're just touching these rings so that there is a metallic contact, but they're not stuck to it. So, right now, there is a contact, the circuit is complete, and as a result, the current comes out of the
blue ring as you can see, moves this way, and current
flows into the pink ring, and goes like this, and when
the coil starts rotating, as you can see, the rings
rotate along with the coil, but since the brushes are
not stuck to the rings, the rings just slip through the brushes, that's why they're called as slip rings. As a result, the brushes will not rotate, that solves the problem of
wires twisting and tangling, and all the while, an electric
contact is maintained. Now, once our coil comes in this position, we have seen that the current reverses. So now, the current will flow out of the pink ring, goes like this, through the bulb and now
enters into the blue ring. So, for every half a rotation, the current through
the bulb also reverses. Let's say when the current
is flowing this way, the bulb glows blue, and let's say when the current reverses, and flows like this, the
bulb will glow yellow, and so now if you look
at the entire animation, it looks somewhat like this, and so we have successfully
built our generator, and what's interesting to see is that the current from that generator is continuously changing it's direction. Such a current is called
alternating current or AC, and this might sound a little weird, but it turns out that when you want to transmit electricity
over a long distance, like from the power
station to your houses, then alternating currents or AC has some great advantages
over unidirectional currents or DC, and it's for that reason, the current that we get at our houses, the electricity what we get
at our houses are all AC, and these generators are
called AC generators. Finally, what if we want
to build a DC generator? Where we don't want the
current direction to change, we want it to remain the same throughout. Let's say in this direction,
how do we do that? Well, to build that, first
let's get rid of these rings. All right, now to make sure that the current direction remains the same, what we will need is that these brushes to continuously keep changing contacts between these wires for
every half a rotation. Let's see why. So, if I want the
current to flow this way, right now in this position, I can connect this brush to
this wire, so the pink side, so that the current flows
like this and comes out, but as the coil rotates, notice once it comes to this position, you've seen that the
current starts flipping, current reverses and so now, to maintain the current
in the same direction, we would now require this brush to come in contact with this wire, that is the blue side,
right, and then again, once we come to this
position, again it flips, the current flips and
again we would want now this brush to come in
contact with this side. And so, as a result can you see that for every half a rotation, we would want the
contacts to keep changing. But how do we make sure
that happens automatically? We can do that by attaching split rings. Split rings as the name suggests, is a ring that is split in between, giving two half rings
with some gap in between. Now, let's see how this
arrangement automatically changes contact for every half a rotation. So, right now, this brush is in contact with the pink side but as the coil rotates and comes to this position, the current reverses and now notice the brush is in contact
with the blue side, making sure the current still flows in the same direction through the build. Again, as the coil comes
to now this position, finishing another half a rotation, again notice it just changed contact, it is now in contact with the pink ring connected to the pink side, and this way we have now
built our DC generator where the current only
flows in one direction through any external circuit, and this arrangement which helps us automatically change contacts,
we call them as commutators. So, split rings act like commutators. So, to summarize what we learned, if you take a coil and
spin in a magnetic field, then due to electromagnetic induction, a current gets generated in that coil. Now, the direction of the current depends on whether the wire is going
up or where it's going down, and as a result for every half a rotation, we see that the direction of
the current keeps changing, and so this generator is
called an AC generator, because it generates
an alternating current, a current whose direction
keeps continuously changing. On the hand, if we use split rings, then it acts like a commutator, it keeps changing the contacts for every half a rotation
and make sure that the current does not change the direction in the external circuit. We call this a DC generator, and so this is how we
can generate electricity just by rotating a coil in
between a couple of magnets.
