 Let's summarize important topics of magnetic effects of electric current, which in short, I'm calling magnetism on the right hand side, you can see the index. So if you want to jump to any specific topic, feel free to do that. So let's begin with the mind map. I like to divide this entire chapter into three parts based on three scientists and three discoveries. The first one is by Urstad. He discovered that current carrying wires can deflect magnetic needles. In other words, they can produce magnetic fields. This is what laid the foundation for the magnetic effects due to electric current. And how can you remember in what direction the magnetic field is going to be? Well, you can use your right hand thumb rule or the right hand clasp rule to do that. We'll talk about its details a little later. Anyways, because we know that current carrying wires can produce magnetic field, we're going to look at three situations and look at the magnetic field produced by them. A straight current carrying wire, a loop of current carrying wire and lots of loops of current carrying wires, which are called solenoids. That brings us to the next section, ampere. He was saying, look, if a current carrying wire can push a magnetic needle and deflect it, maybe the opposite can also happen. Maybe a magnet can also push and deflect a current carrying wire. And that's exactly what he proved with his experiment. He passed current through a wire placed inside a magnetic field and he found that the wire deflected. There was a force experienced by the wire. And how do you remember the direction in which the force acts? Well, you can do that by using your left hand. This is where the Fleming's left hand rule comes into the picture. Again, more details later on. And this has huge numbers of applications because now we have a way to push current carrying wires by using magnets. And the robust application is found in motors. Where magnetic fields are used to turn current carrying coils. This is what happens in your fans or in washing machines, etc. That brings us to the last scientist, Michael Faraday. He started asking a big question. Now, if electric current can produce a magnetic field, is the opposite possible? Can the magnetic field generate an electric current? To answer this question, he played a lot with magnets and coils. And in a little while, we'll again talk in detail about what his discovery was. But anyways, a lot of experiments, a couple of experiments we need to study over here. And this is also where we're going to use our right hand to figure out the direction of the induced current. So the right hand rule will be used. The Fleming's right hand rule will be used to figure out the direction of the induced current. And what's the application of this? Well, we are generating electric current. So obviously this will be used as generators. This concept is used in generators where we are going to rotate something. And that rotating device is going to produce electricity for us. All right. So now we'll jump in and start recalling each and every bit of this, but not in the same order as your NCRT, mainly because to help you in your exams, it makes a lot of sense to recall all the hand rules together, to do all the numericals together. Similarly, we will look at generators and motors together so that we can differentiate between them we can compare and contrast. So like I said, if you want to jump to a specific topic, feel free to do that. So let's start by recalling the hand rules. Here's the first one. If you want to figure out the direction of force on a current carrying wire placed in a magnetic field, which hand rule are you going to use? Can you pause and try to recollect this? All right. Hopefully you're given it a thought. The answer is Fleming's left hand rule. You stretch your fingers in such a way that they are perpendicular to each other. And what does each of the finger represent? Well, the thumb represents the direction of the force. The forefinger gives the direction of the magnetic field, which I usually we do know it as B. And the middle finger gives the direction of the current. And the way I remember this is FBI. You keep hearing that in the Hollywood action movies, FBI. So that's how I remember that. All right. Next question. How do you figure out the direction of the magnetic field around a current carrying wire? Again, can you pause and think about which hand rule is going to help you that? The answer is your right hand thumb rule. Here you clasp the conductor in such a way that the thumb represents the direction of the electric current. Then your circle fingers will give you the direction of the magnetic field around that conductor. Lastly, what hand rule is going to help you find the direction of the induced or the generated current? Well, you know what to do. Pause and think about it. The answer is Fleming's right hand rule. The arrangement is very similar to the left hand rule. Even the fingers also represent the same thing. And that's the reason why I used to get so much confused with these two rules when to apply which one. So here's the big, big difference between the two. The major difference lies in what causes what. So if you're looking at this one over here, there's already a current in the wire that has produced my baby a battery, or maybe by using some means there's already a current in the wire and that current is causing a force on the wire due to the current. There is a force acting on the wire. When that is happening, we use the left hand rule. Does that make sense? So if the force is acting on the wire due to the current placed in the magnetic field, that's when you use the left hand rule. On the other hand, literally if you are moving the wire, you or your friend or anyone is moving the wire, is pushing or pulling on that wire. And that is causing the current to be induced. See, there was no current to begin with, but because you moved, that is causing the current to be induced. Then we use the right hand generator rule. So over here, the force is causing the current to be induced. Over here, the current is causing the force. You see the big difference between them? So now the next question could be, how do we remember this? Well, what I did is I remembered right hand generator rule. So whenever the current is being generated, current is being induced, I'm going to use my right hand. And when it's not, I'm going to use my left hand whenever I have to calculate the force or something. But what if we forget whether it's right hand for generator or left hand for generator? How do we remember that? Well, one way I do that is I think of it as a right generator. Not the wrong generator, but the right generator. Okay, so that way, whenever I think about generating current or inducing current, I remember right generator, not the wrong one. And so I remember my right hand, not the left one. And of course, when do we use the right hand thumb rule, the right hand glass rule? This is when we are, we want, we are interested in the magnetic field that is produced by the electric current. Okay. That's when we go for this one. On the other hand, over here, these magnetic fields are not the ones created by this current. All right. This magnetic field may be created by some magnet. This is a separate current. All right. So when you're talking about magnetic field created by the current, that's when we go for the right hand thumb rule. You know what? This will make a lot of sense if we jump into the numericals. So let's do that. Here's the first one. It's given the below current carrying wire placed in the magnetic field, experiences a force out of the screen. Find the direction of the current. Okay. So great idea to pause the video and really think which hand rule you're going to use to figure out the direction of the current. Pause and try. All right. So it's given that the wire is experiencing a force out of the screen. So it's coming out of the screen towards you. So which hand rule should I use? I always start with, should I use my right hand generator rule? Is, is there current being generated over here? That's the big question. Let's look at the question again. The below current carrying wire placed in the magnetic field experiences a force. So, so there's already a current. Does that make sense? The current was already there. The current is causing the force. So there is no induction. Current is not being induced. There is no generation. So I'm not going to use my right hand generator rule. This means I have to use my left hand rule. Okay. So left hand rule is what needs to be used over here. And again, if you haven't tried earlier, now may be a good chance to use your left hand rule and see if you can figure out the direction of the current. All right. So the left hand rule needs to be aligned such that the thumb is pointing out F thumb is pointing out of the screen B magnetic field is to the left. So how does that look like? It looks like this thumb pointing out B that is the forefinger is towards the left. And so the current is pointing upwards and therefore the current will be upwards. That's the answer. All right. Let's go to the second one. A conducting wire is moved in a magnetic field as shown below find the direction of the induced current. So this time a wire is moved in the magnetic field and the magnetic field is given to be out of the screen. The magnetic field this time is coming out of the screen. You need to figure out the direction of the induced current gradated to pause and see which hand rule to use and orient your hand to figure this out. All right. So should I use my right hand generator rule or not? That's the question I ask. Well the current conducting wire is moved in a magnetic field as shown below find the direction of induced current. Oh, this means notice over here. Somebody is applying a force. Somebody is already pushing or pulling that wire and because of that a current is being used. So this is generation. This is generate. So I'm going to use my right hand generator rule because current is being generated generated over here. Does that make sense? So now I have to use my generator rule in such a way that the force is thumb is pointing upwards because that's the direction in which it's being moved. V stands for velocity which is useless over here. And the magnetic field that is the forefinger is pointing out. So thumb is up. The magnetic field is pointing out. It's going to look like this. And so you can see the finger, the middle finger, which uses the current is pointing towards the right, which means the current is going to be to the right. Okay, one more, a little different one this time. We are given there is a magnetic field and there is an electron moving up now. Find the direction of the force on the electron. Can you pause and try? All right, hopefully you tried. Well, this time you might be wondering, certainly where does an electron come from? What's this? It's a different problem. But remember, electron is a charged particle. And when a charged particle is moving, there is current. So what this represents is basically current. They just twisted the question a little bit. So current is upwards. Or is it that there is another twist? Electron is a negatively charged particle, right? So current is not upwards. Current now will be in the opposite direction of the flow of electrons. So current is downwards. Ooh, very interesting problem. So again, which hand to use? Do I use my right hand generator rule? Is the current being generated over here? The answer is no. Current is not being generated. Current is already present. The electron is already moving and because of the current, I am asked what is the direction of the force. So this is not generation. There is no current being generated over here. Therefore, I'm going to use my left hand. So I have to use my left hand rule in such a way that forefinger B points towards the right. The thumb should point, sorry, the current that is the middle finger should point down. All right. So how do we orient that? It's going to look like this. And so you can see the thumb, which is the force, is acting out of the page or out of the screen. And so that's our answer out of the page or out of the screen. Okay. So that now brings us to the next topic, figuring out the field configurations due to an electric current. So here's the first one. Imagine I have a straight conducting wire carrying a current to the right. How will the magnetic field look like around this wire? Can you pause and think about the answer? Well, the answer is or start figure this out long time ago, the magnetic field will be in concentric circles like this. It'll be everywhere in concentric circles. I just shown at one point over here. And what is the magnetic field generated, the strength of that field? What does that depend on? Can you think about that? Well, it depends on two important things. One is the current. If there is no current, there is no magnetic field, right? So more current, more magnetic field. So that's one thing. It depends on the strength of the current. What else? Well, it also depends upon how far you are looking at the magnetic field. If you're very close, the magnetic field is very strong. And if you go farther and farther away, the magnetic field becomes weaker and weaker. And that's why when you draw these concentric circles, if you want to draw it the right way, as you go farther away, you should draw the concentric circles farther and farther. To represent that magnetic field is becoming weaker. But now here's another question. What is the direction of this magnetic field? Will it be this way or is it going to be this way? How do you figure that out? So which hand rule are we going to use? Left hand rule, right hand rule. Well, over here, notice we need to figure out the direction of the magnetic field. Not force or current, but the magnetic field due to the current, right? That's where we use the right hand clasp rule. So you clasp the conductor such that the thumb represents the direction of the current. And then the encircling fingers gives the direction of the magnetic field around it. So in this particular diagram, notice the encircling finger is downwards over here. So the magnetic field is going to go down like this and then go up this way. So this is wrong. That's not how it's supposed to be. The magnetic field is going to be like this. So right hand thumb rule. All right. The second case we're going to think about is what will be the magnetic field around a current carrying loop of wire? You think about this? Well, again, because you want to figure out the magnetic field, we have to clasp this wire. But since this is not a straight wire, what we can do is we can take tiny sections of it and keep clasping it. So I'm going to take two sections over here and let's clasp it with our right hand. It's going to look like this. So near those sections of the wire, we can say the magnetic fields are going to be that way given by the right hand clasp rule. And now we can kind of make a guess. We can say, look, as I go farther and farther away, the magnetic field has to go from here going this way to this way. So I can kind of guess the magnetic field must slowly become flat and then bend the other way around. And that's how I can guess the rest of the magnetic field should look somewhat like this. So this is how the magnetic field looks like for a current carrying loop. Let me make it short and keep it over there. The third and the final magnetic field we need to look at is for a solenoid. A solenoid is basically a very tightly coiled current carrying loop. It has a lot of loops, so it's very long. Okay, it has very tightly coiled. So if you pass a current through that, what will the magnetic field look like? It's going to look very similar to this one, except that it'll be having these lines over here in the middle will be very straight because of a lot of coils. So it's going to look somewhat like this. Pretty similar to this. But if you look at this, this is going to resemble something that you may have already seen, maybe familiar with, that's a bar magnet. And so a solenoid produces a magnetic field very similar to a bar magnet. And that's why a solenoid can be used as a replacement for bar magnet. This is why we call the solenoid as an electromagnet. And so now we know electric current can produce a magnetic field that brings us to the very next question which Faraday asked, is the opposite possible? Can a magnetic field generate an electric current? We already know the answer to that, so no climax over here. But that brings us to electromagnetic induction. So here is the big question that Faraday was asking. Can magnetic fields produce electric current? What do you think? Well, when it comes to physics, we need to be very, very careful about wordings and everything. In physics, if somebody asks you, can magnetic field generate electric current? The answer is no, it can't. Magnetic fields cannot produce electric current. That's what we find. But changing magnetic fields, turns out, can produce electric current. All right, and this is the whole idea of being electromagnetic induction. If you want electric current, you need to have a changing magnetic field. So let's test this by a couple of examples. Here's the first one. It's an experiment. Let's say I take a magnet and I keep it at rest near a coil. Question is, will there be a current induced in this coil? What do you think? Well, to answer this question, we need to ask, is the magnetic field changing around the coil? The answer is no, because the magnetic field is at rest. So the magnetic field stays the same. The strength stays the same. So it's not changing. That's important. Since it's not changing, there will be no electric current generated over here. Okay, let's consider the second case. Suppose I have the same arrangement, but this time I take that magnet and I move it towards the coil. See, I move it towards the coil. Now what's going to happen? Do you think there will be a current induced? Well, again, let's see. When I move that magnet towards the coil, the magnetic field is increasing. It's becoming stronger associated with the coil, right? And so there is a changing magnetic field. Electric current will be induced. So if I move the coil, the magnet towards the coil, yes, there will be a current induced. Okay, here's a question for you. What happens if I move the magnet away from the coil? Do you think there will be a current over here? Again, let's see. When I move the magnet away from the coil, what happens to the magnetic field over here? Magnetic field decreases. It might even become zero. I don't know. It depends on how far I move the magnet. The question is, is the magnetic field changing? The answer is yes. If the magnetic field is decreasing, it is changing. So because it is changing, there will be an electric current. So again, when I move the magnet away, there will be an electric current. However, the difference will be this time, since the magnetic field is decreasing, the current will be generated in the opposite direction. So when I move the magnet towards, current will be generated in one direction. And I move it away. The current will be generated in the opposite direction. Okay, another experiment, the coil and the coil experiment. This time I have one coil, which is not connected to any battery, but another coil or another solenoid, which is connected to a battery. I'm going to close the switch, and I want you to tell me what's going to happen. When I close the switch, a current will fast through this. And as we saw before, a magnetic field will be generated over here. So what will you see in the second coil? Can you think about this? I'm going to tell you the answer directly. When I close the switch, momentarily there will be a current and then the current disappears. That's what you'll find. Only momentarily there will be a current and the current disappears. That's what you'll find. Why is that? Well, let's look at it in a little bit more detail. When I close that switch, a current starts running through the coil. The current starts increasing. It was zero before, but now the current starts increasing. The battery starts pushing the charges. As the current increases, a magnetic field starts getting generated and the magnetic field starts increasing. That means the magnetic field is changing. And it's during this time, a current gets generated, electromagnetic induction. As long as the magnetic field is increasing because the current is rising, growing, as long as that's happening, there is induction. But very quickly, the current will reach its maximum value. After a point, the current will become steady. The current will reach a maximum value. Once that happens, the magnetic field won't grow anymore. And as a result, there is no longer a changing magnetic field. And so there'll be no more current induced in the coil. And that's why the current only induced when that magnetic field was growing, when that current was growing for that moment, for that fraction of a second, and then it disappears. What do you think is going to happen when I open the switch? Well, again, you'll find something very similar. Momentarily, you'll find a current. Why? Because again, when I open the switch, the magnetic field starts reducing. And when that happens, again, there will be a current generated in the opposite direction, of course. But as long as the magnetic field is reducing, there will be a current generated. But once the magnetic field has completely reduced to zero, the current is already gone, no longer change happens. And then there'll be no longer current induced in the secondary coil. So in this experiment, only when you close the switch or when you open it momentarily, there will be a current induced. And hopefully now understand why, because of this electromagnetic induction. With this, we can now jump to motors and generators. So if you look at the setup, they look very similar to each other. Their setups are very similar. But there's a huge difference between motors and generators. Can you first recall what a motor does and what a generator does? What's the difference between them? Major difference? All right. So in motors, you supply electricity, maybe using a battery, let's say. And that electricity produces the spin. So the current is producing the spin. A quick example would be in your fans. When you switch on the current, the fans start spinning. If there is no power, the fans stop spinning. The current provides spinning. That's what motor is. On the other hand, the generators are exact opposite. Here we spin that coil and that generates the current that produces the current. So it's exactly the opposite. A quick example could be generators that we use in our houses, maybe diesel generators where the diesel is your petrol or diesel is used to spin that. And then because of the spin, we get electricity to our house or maybe in wind mills, the wind is used to spin something to spin the coil eventually. And that causes electricity to that causes that generates the electricity. Now, of course, if you want details on their working, we have dedicated videos for each one of them. Feel free to go back and check them out. Having said that, let me highlight a couple of important details. First one is which hand rule are we going to use for motors and generators to figure out the direction of the force and the direction of the current? Can you pause and think about that? All right. As you have seen before, I remember the right hand generator rule. That itself reminds me, Hey, I have to use the right hand Fleming's rule for my generator. And why is that? Because we use our right hands whenever we are generating current, whenever electromagnetic induction is happening. And that's what happens over here, right? On the other hand, literally, we use left hand for motor. Why? Because we're not generating current over here. We are pretty we are putting the current from the battery or whatever. And we're generating the spin, right? Basically, we're putting force, right? And to figure out when that happens, the current provides force or the current and the magnetic field provides force. That's when you use your left hand Fleming's rule. Another important thing I want to mention is the role of these split rings, right? This is called a commutator. What's his job? Well, because we have a split ring, what happens is every half a cycle, the contact reverses. And as a result, the current starts reversing in this coil. Okay. And that reversal of current is required because without that, this motor, this coil wouldn't keep on turning. If the more, if the coil needs to keep on turning, the current needs to keep on reversing. So you see the battery doesn't reverse the current. And so we reverse the contacts every half a cycle. That reverses the current in the coil every half a cycle. And that keeps the coil spinning. And so the commutator's job is to keep reversing the contact. So as to keep reversing the current over here. On the other hand, why do we need to commit it over here? Well, just like what we saw here, if you want this thing to keep on spinning, the current needs to keep on reversing every half a cycle, right? Every half a rotation. So over here, it's the exact opposite. If you keep spinning the coil, then the current that we get, the current that we get over here will keep reversing automatically. So if we didn't use this, there's a current that you, that might go through the bulb will keep on reversing. Now, if you don't want that, if you don't want the current to keep reversing over here, again, we use a commutator. So what's going to happen is as the current reverses, this will also keep reversing the contact. So current reverses, the contact reverses. And as a result, you will see that the current that we eventually get will not reverse. Okay. So long story short, commutators will ensure that the current in the external circuit over here, they will be in one direction, but the current in the coil will keep on reversing. And so since the current in the external circuit, the circuit over here, this external circuit, since that does not reverse, we call these devices DC. DC stands for direct current, which means the current does not reverse in the external circuit. On the other hand, what happens if you don't use a commutator over here? What if you didn't use split rings over here? Well, like I said before, if you don't use split rings, but let's say if you use whole rings so that the contact does not reverse. See over here, this, this is always in contact with this ring. This is always in contact with this ring. There is no reversal of the contact. Then, like I'm like we mentioned before, the current over here will keep on reversing. And such currents are also useful to us. We call them alternating current. So an alternating current is where in the external circuit, the current will keep on reversing continuously. Now lastly, you may be wondering, we can have an AC generator, right? So can we have similarly an AC motor where in the external circuit of the motor, we have alternating current? The answer is yes, you can have AC motor, but you can't simply put whole rings over here and expect to get that. Turns out AC motors, their setup is a little complicated. And therefore in our syllabus, we won't talk about AC motors. And that's why in our syllabus, we only have DC motors, but we have both DC generators and AC generators. So remember, for DC, you need commutators, something to keep reversing the contact. And for that, you need split rings. But for AC, AC generator, you don't need any split rings. You don't need reversal. So use whole rings. So now that brings us to the last topic, domestic wire rings. So you are familiar with this socket, your household socket, which has three holes, right? Can you recall what are the different wires that these holes are connected to? And what are the colors of those wires? All right. One of the hole is connected to a red wire, which is called live wire or a hot wire. It's called so because it is at a very high voltage. And so extremely dangerous to touch that. The whole opposite to that, that is connected to a black wire and it's called neutral. It's called so because it's maintained at a very low voltage, usually at the same voltage as the ground. But again, need not be exactly the same. So again, not a good idea to touch that as well. And the third wire, the third hole is connected to a green wire, which is called the earth wire. Now, ideally it shouldn't be carrying any current because the live and the neutral connect this complete the circuit sorry. However, sometimes if there are falls in the devices and let's say the devices have metallic bodies, then there could be some leakage of current. And if you touch that, the current can flow through your body to ensure that does not happen. We make a earth wire, basically any leakage of current goes through this and goes all the way to the ground or goes to the earth. So it's not going to the main circuit. It is going to the earth. Now, because current can cause heating, that can be bad because the insulation can melt and expose the wire. So to ensure that the current does not exceed the maximum value that the wire can handle, a device is connected over here. What device is that? Can you recall what that is? That device to ensure the current does not exceed a maximum value is called a fuse. A fuse basically contains a wire of a particular material and it's built in such a way that if the current exceeds a particular value, then it's this fuse wire that melts first and it melts and it immediately breaks the contact, breaks the circuit, ensuring no current flowing. So the next question could be what are the different situations in which current might tend to exceed the maximum value in our circuit? The two very common situations are short circuit and overloading. Can you recall what each one of those are? Okay, so short circuit basically means a lower resistance path is created. This can happen when, say, the live wire and the neutral wire, they lose their insulation and come in contact with each other. In that case, what happens is, you see, there is absolutely no resistance over here and the current can just flow from here to here. Very high voltage, very low resistance, meaning very high current gets drawn. And so this is a situation where a high current can get drawn and so, again, the fuse is going to help us over here protect the circuit. So another situation is overloading. What does that do? Well, overloading basically means too many devices are connected. So you can think of load as device. So what happens if you connect too many devices over here? Well, in a household circuit, remember that devices are connected in parallel so that each device gets the same voltage, but they will have, they will draw, each one will draw a different current. Now, if you connect too many of these devices, then what happens is each one will draw, each one will draw their own current. And as a result, the total current drawn from the main live wire, that can exceed, again, the maximum value. And again, in that case, the fuse is going to protect us. Now, one last point before we wind up is if you look at NCRT, it defines overloading. It says overloading can occur when the live wire and the neutral wire come into direct contact. We just saw that's short circuiting, not overloading. So that part is wrong. Okay. At least as of December 2019, this is the sentence in the NCRT, which is wrong. All right. So that winds up our entire magnetism chapter. If you have difficulty in recalling any of these concepts, no worries at all. You can go back and watch videos and practice those specific concepts on our Khan website. All the best for your exams.