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Video transcript

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 Erstad he discovered that current-carrying wires can deflect magnetic needles in other words they can introduce 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 gonna 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 deflected 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 but 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 coil this is what happens in your fans or in washing machines etc that brings us to the last scientist Michael Faraday he shot asking a big question now if electric current can produce a magnetic field is the opposite possible and the magnetic field generated electric current to answer this question he played a lot with magnets and coils and in a little while he will 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 will jump in and start recalling each and every bit of this but not in the same order as your NCERT 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 alright 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 that sum represents the direction of the force the forefinger gives the direction of the magnetic field which I usually we denote it as B and the middle finger uses the direction of the current and the way I remember this is FBI you keep hearing that in the Hollywood action movies svi 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 that thumb represents the direction of the electric current then your encircle fingers will give you the direction of the magnetic field around that conductor lastly what hand role is going to help you find the direction of the induced or the generated current well you know what I do pause and think about it the answer is Fleming's right hand rule the 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 there 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 is 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 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 who 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 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 like 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 classroom this is when we have 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 alright this mitotic 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 the force out of the screen find the direction of the current okay so great idea to pause the video and really think which handle 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 place in the magnetic field experiences a force so so there's already a current is 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 you 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 rules and see if you can figure out the direction of the current alright so the left hand rule needs to be aligned such that the sum is pointing out f thumb is pointing out of the screen be magnetic field is to the left so how does that look like it looks like this I'm pointing out B that is the fore finger 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 a 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 grid I need 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 asked well the current carried 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's already pushing or pulling that wire and because of that a current is being induced 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 used 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 used this over here and the magnetic field that is the four fingers pointing out so thumb is up the medic 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 on the electron can you pause and try alright hopefully you tried well this time you might be wondering certainly where is an electron come from what's this is 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'm 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've used my left hand rule in such a way that forefinger be points towards the right the thumb should point sorry the the current that is the middle finger should point down all right so how do we orient that it's gonna 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 Oersted figure this out long time ago the magnetic field will be in concentric circles like this it'll be everywhere in concept except because 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 field right so more current more magnetic fields 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 further 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 gonna be this way how do you figure that out so which hand drew are we going to use left hand drew the right hand rule but 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 sum represents the direction of the current and then the encircling fingers gives the direction of the magnetic field around it so in in this particular diagram notice the encircling finger is downwards over here so the magnetic field is gonna 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 gonna be like this so right hand thumb rule all right the second case we're gonna 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 gonna 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 hair 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 his 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 what you pass the current through that what's going what we'll the magnetics will look like it's gonna look very similar to this one except that it'll be having these lines over here in in the middle will be very straight because of a lot of coils so it's gonna 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 way we call 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 generated current the answer is no it can't magnetic fields cannot produce electrical and that's what we find but changing magnetic fields turns out can produce electric current all right and this is the whole idea of in electronic 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 Mira coil question is will there be an 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 minute is at rest so the magnetic field stays the same the strength says 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 go through 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 it's gonna 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 depends on how far I move the magnet the question is is the magnetic field changing answer is yes if thematic field is decreasing it is changing so because it is changing there will be an electric current so again when I move the weight 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 to be generally in one direction when I move it away the current will generally in the opposite direction okay another experiment the coil and the coil experiment this time I have one coil which is non ready 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 gonna 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 gonna 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'll 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 electronic 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 a current only induced then 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 gonna happen when I open the switch well again you'll find something very similar momentarily you will 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 will be no longer current induced in the secondary coil so in this experiment only when you close a 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 the 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 fan starts spinning if there is no power the fan stops 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 DC 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 windmills the wind is used to spin something spin the coil eventually and that causes electricity too that causes that generates the electricity now of course if you want details on there 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 rules 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 alright 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 you're not generating current over here we are we are putting the current from the battery or whatever and we are generating the spin right basically we are putting force right and to figure out and when that happens the current provides force or the current and the magnetic field provides force that's when you use your left hand flowing through 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 a reversal of current is required because without that this motor this coil wouldn't keep on turning if the mode 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 cycle and that keeps the coil spinning and so the community'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 committed 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 the 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 gonna 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 commutator z' 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 plate 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 to 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 our syllabus we won't talk about AC motors and that's why in our syllabus you only have DC motors but we have both DC generators and ez generators so remember for DC you need commutator 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 you use whole rings so now that brings us to the last topic domestic wirings 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 hole opposite to that that is gonna lead to a black wire it's called neutral it's call 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 in the neutral connectors 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 to the earth so it's not current to the main circuit it is connected 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 recover there 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 else first and it melts and it immediately breaks the brakes 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 low 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 it wrong and so this is 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 Lord 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 to many of these devices then what happens is each one will drop 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 ncert 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 NC R T which is wrong all right so that winds up our entire magnetism chapter if you had 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 way all the best for your exams