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Current time:0:00Total duration:13:29

Video transcript

how do power stations provide electricity to thousands of houses around the city they don't use giant batteries they do that by spinning a giant turbine like this and to spin city 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 does turning something create electricity well the technology is based on electromagnetic induction discovered by Michael 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 wire 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 us 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 that then the middle finger will give us the 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 have 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 not moving downwards so the thumb will be pointing down the fore finger 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 would 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 look at that later and then the current comes back like this and if 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 a right hand rule when the curve 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 its 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 coil comes in this position perpendicular to the magnetic field the current direction keeps changing now of course in the animation I'm stopping and pausing the animation when my coil is perpendicular to the magnetic field so that we can see the current flipping its direction but of course in reality we the the coil will be pushed continuously there will be no stopping there will be no jerking motion like we are 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 wire start twisting and tangling and turning and whatnot 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 are 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 its 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 have some great advantages over unidirectional currents or DC and it's for that reason the current that we get at our houses the electricity of active guitar 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 we have 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 bulb again as the coil comes to now this position finishing another half a rotation again notice 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 commutator z' so split rings act like commutator z' 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 other 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