Lesson 1 - What is an Inductor? Learn the Physics of Inductors & How They Work - Basic Electronics

hello and welcome to the section of the circuit analysis tutor in this section we're going to talk about the circuit element called the inductor so we'll talk about what an inductor is why it's important how does it compare to what we've learned in the past and then in the next several sections we will have some basic circuits that will begin to analyze that will have inductors in them and you'll get some experience with trying to understand and predict what's going to happen when you have inductors in circuits after that we're going to talk about a capacitor the circuit element that we called capacitors and we'll do the same kind of thing we'll talk about why it's important why it's useful some basic circuits with capacitors and then as we march down the course meaning this set of lessons and the lessons that follow in volumes to come capacitors and inductors will be in almost every single circuit there is not a single circuit that you could open up in your house you know television phone video camera anything there's not a single circuit that you could open up in your house and look in there and not find a capacitor there's very few that you could open up and not find an inductor they're absolutely central critical circuit elements to shape and cause a circuit to do what we want it to do alright so before we get there let's kind of back the truck up a little bit do a little bit of a synopsis verbally of what we've covered to this point in the circuit classes that we've done so far because ultimately it's taught this way for a reason okay so what we did is we first started talking about what is voltage what is current what is resistance those are fundamental things that affect all circuit elements they're also going to apply to capacitors and inductors so the concept of what current is the concept of what voltages things like that they're going to apply to what we're doing here right then we started doing basic resistive circuits using Ohm's law to calculate the current through the circuit all right so we did that kind of thing and then we did Kirchhoff's laws you know car cuffs current laws and Kerr calls voltage laws where we looked at different circuit meshes and circuit configurations and figuring out that at a node the current coming in the current leaving a node is guided algebraically balanced going and walking around loops in circuits or Karkov voltage laws you have to sum those voltages up to all of those concepts apply to circuits with inductors and capacitors so that's why we learn this bedrock first because if you kind of make it too complicated with inductors and capacitors first then it gonna gets again smushy so we do a lot of analysis with resistors first so that you can get the concept of the of the analysis down and then we introduce slightly more complicated elements like this are you finishing up the discussion we did Thevenin and Norton equivalent circuits which is a nice way to simplify circuits and we did a lot of solving the circuits solving the current solving the voltage everywhere by using simultaneous equations we've used mesh currents and we've used node voltages so if you if you think back we've learned several techniques for analyzing circuits up to this point right and what I'm trying to tell you is once we get through the basic idea of inductors and capacitors what we're going to end up doing is going to more complicated type of circuits that have resistors inductors and capacitors and we'll have to analyze those circuits and kind of the punchline is all of these techniques that we've learned before the Thevenin equivalent the Norton equivalents the node voltages the mesh currents all of those relations that we are at least the basic flavor of those techniques they're basically going to be applicable to circuits that have different elements like inductors and capacitors so that's why we do it that way that's why we spend so much time with resistors all right so before we can walk we need to crawl I need to explain what an inductor is to you so what we're going to do is write that so here we have something called inductor right the symbol for an inductor in a circuit is very simple so it goes like this and you kind of see like a coil of wire like a little spring like that so here's the two terminals you know like a resistor goes up down up down up down well an inductor is like a little coil of wire in there all right let me write in a different color the units of inductance or the units of inductors as you might see written in a circuit diagram is called the Henry and the symbol is H that's why I put it in brackets like that all right so if you think back to resistors the symbol for resistor was that up and down line the units of resistance was the ohm which is Omega right so here the symbol of inductance is a coil of wire we got two sides of it and you've got a coil here and the units is a Henry now just to kind of show you kind of what you might actually see if you open up you know computer or a power supply and look on the board you're probably going to see a few places where it looks like there's actually coiled of wire on the board those are inductors right and the purpose of an inductor we'll get to a little bit later but that's what they're basically doing now if you look at different inductors see the way I've drawn it here it looks just like there's a coil of wire with like air inside that's pot that's that's very much like inductors are actually constructed they literally are coils of wire that are wound very closely together but not touching because you know you know you want to have an insulation between each coil of the wire you don't want the wires touching so I have to be insulated wires but they're very tightly coiled wires now some inductors do have air inside so you can look through it and actually just see right down the barrel there but some inductors have metal like maybe iron inside of the coil so you'll see the coil and then you'll see like a piece of iron inside right the reason that they build them that way is because what we're about to talk about now an inductor basically is a circuit element that stores energy in a magnetic field right so that bears repeating so it's it's so incredibly important that you just have to you have to internalize this and understand it so that whenever you look at circuits you know what's going on a inductor the purpose of it is basically the store energy in a magnetic field and you might say well that sounds complicated how could that possibly happen well what's going on here is you have a wire and you have it coiled so if I send current electric current down this wire what's going to happen is the current is going to go and then it's going to start going through the coil right because it's one continuous piece of of wire that's coiled up on it like it like a spring so the current is going to go round and round and round and maybe maybe this coil I have four coils there but in real inductor there might be a 100 terms or 75 turns right the more turns basically is going to concentrate the magnetic field inside of this hollow region inside of there right now the reason why it concentrates magnetic field I could teach you but it's it's not really it's not really the point of this class to teach you the physics there but if you look at the magnetic field on a wire and you look at the geometry of a coil then when you run an electric current down there and you do the physics then inside of that coil the magnetic field is going to be concentrated inside of this coil right so it's going to kind of go it's like a doughnut but its strongest inside of this coil the more turns you have the stronger that magnetic field is going to be right and also if you have a metal core like an iron core inside of there it also tends to concentrate that magnetic field also so that's why a lot of real inductors that you see are not going to have air inside they'll actually have a piece of metal in there and that the purpose of that is to try to concentrate that magnetic field there all right so the units are Henry don't worry too much about that right now there's you know lots of physics we could go in and show you what a Henry's really equal to but the point is on a circuit diagram you might see 50 ohm resistor 75 kilo ohm resistor you might see a 50 milli Henry inductor or a 100 milli Henry inductor or a half of a Henry inductor or something like that so all the metric prefixes apply but the base unit is Henry all right now just to kind of totally reiterate so we have this inductor we send current in it we know from physics that we run current in a wire a magnetic field is generated by coiling it like that we're concentrating that magnetic field inside of that coil right there all right so the other thing is if you believe me and I know that you all do that by putting a current in a wire like that especially a coil of wire we get a concentrated magnetic field inside that coil then it stands to reason that if I change that current if I make the current go up and down let's say I have like a knob on my current source and I go up and then down and I literally turn it with my hands so I can vary with time the current going through here maybe I can make the current go bigger and smaller and bigger and smaller then you kind of have to use your x-ray goggles to kind of visualize it but if you can visualize this magnetic field here if I crank the current higher that magnetic field gets stronger so you can kind of see it getting stronger in there if I back that current off closer to zero that magnetic field gets weaker in there so by changing the current we can change the strength of the magnetic field now I'm going to pause here for just a second before I write anything else because I've got something else very important to tell you about inductors I'm going to pause here and tell you that later on when we get to capacitors I'm getting way ahead of myself and I'm giving you a preview capacitors they don't store energy as a magnetic field capacitors store energy in an electric field inside the capacitor plates we'll talk about that later but inside the capacitor so I'm kind of giving giving you a little bit of a preview ahead of time you know you have the good old resistor we've learned so far it doesn't really store energy at all it just dissipates energy it heats up whenever you put current in there but I'm trying to tell you that inductors they store energy in the form of the magnetic field right that you can then remove later if you'd like - almost like a battery kind of right and then you have capacitors which store energy in a circuit in electric field so they're kind of opposites of one another here you store energy as a magnetic field here you store energy as an electric field and will get in all the calculations of how you figure all that stuff out later but I'm just trying to let you know the big picture is that you have inductors and capacitors they're kind of like peanut butter and jelly right there they're their cousins right they operate in a circuit in with slightly different fields but their purpose is to store energy right so you can build complicated circuits that use these phenomena to do what you want like transmit radio waves or operate a microwave oven or whatever because you're obviously you're trying to deal with handling energy in those circuits all right so we've talked about the symbol for an inductor we've described what an inductor would look like we've talked about the fact that it stores energy and magnetic field now it's time to look at what an inductor might look like from the point of view of circuit analysis right so let me go and draw a little line here I'm going to write down an incredibly important equation on the board and I want you you know to not be too scared of it but we're going to learn from it so let me go and write this inductor down again and I'm just going to write the same inductor down so here is inductor coil of wire right now this inductor we're going to call it inductance L which could be 10 milli henries 20 milli henries whatever all right so much like much like resistors right we might send a current down this inductor now when I draw it like this what I mean is it's going through the coil over and you know round around around then it comes out the other side that's what that means but let's say we send a current I down this inductor now much like resistors you might expect there to be a voltage across that inductor right remember Ohm's law we talked about Ohm's law before if you have a resistor you're sending a current down there there has to be a voltage across it and we use the passive sign convention the direction of the current flow we have a voltage drop plus to minus right if you just make a resistor here this drawing is something we've covered many many many times before when you have the passive sign convention you have current going you should have a voltage drop across your circuit element so that's the sign convention for an inductor when you have a current going through it you should have a voltage drop across it but the major catch the major difference the thing that makes you scratch your head is if it were a resistor I'll just write this here if it were a resistor if it were if it were a resistor I'll put if here now what you would have is V is equal to IR where R is the resistance of course we don't have any are here we have inductance L which actually I forgot to tell you the the unit of inductance is a Henry but we we refer to instead of R for resistance we refer to it as L right so for L it means inductor and by the way you use L because you know it's an inductor so you think you might use it call I you know I inductance or whatever but I is the current so you really can't use I so we use a different a different letter C is going to be for capacitor R is going to be for resistor L is going to be for inductance I know it's a little weird but we just have to do that because we can't use I again but anyway if this were a resistor you would have V is equal to IR like this but this is not a resistor so we're going to kind of X through this right and what you really want to know or what you really want to memorize is that the voltage across this inductor is equal to its inductance L times the derivative of the current with respect to time right and I'm going to circle this because really this equation here is really the point of this entire section to explain to you that the voltage across the inductor is L di DT so repeat this with me a few times I had to do it when I learned circuits it really does help v is L di DT voltage is L di DT V is L di DT so say that over and over again so when you're talking about inductors it's not current times resistance there's no resistance here we haven't discussed the concept of resistance in terms of inductors but we have talked about the concept of inductance the more coils of wire the higher the inductance value the more stores magnetic field right this voltage here is equal to or proportional to or equal to the value of the inductance times di DT now for those of you who have calculus under your belt you know that this means the derivative of the current I with respect to time for those of you who haven't had much calculate or don't remember any calculus that's okay I'm going to explain to you what this means basically what this means is how fast the current I changes through the inductor L right what this means is that all right what this means is that if I have a inductor with a value of so many Henry's or so many milli henries okay and I put a current through it let's say right and that current is steadily increasing in other words I have a knob on my meter and I increase it increase it increase and increase integral I'm increasing the current right that means that di DT the derivative or the change of current with respect to time is a constant because it's constantly increasing then I'll have a number for the rate of change of that current times the inductance that is the voltage across that inductor so if I actually go in the lab put an inductor on in a circuit hook a current source up to it and then slowly increase the current where I'm changing the current constantly at a certain rate that's di DT right and then I multiply that rate times the inductance then I'm going to have a voltage across that inductor that I can measure with a voltmeter I can put the probes across and see that it's so many millivolts or whatever right but it's proportional to the rate of change it's not proportional to the current it's not that the voltage is proportional to the current you have to get that out of your head it's not proportional to how many amps is flowing through it I could have a hundred million amps flowing through this inductor but no voltage right because it's not proportional to the current it's proportional to how fast the current changes right how fast the current changes to put this in perspective again let's say I put 10 amps through this inductor 10 amps that's a lot of current that current could kill you right but let's say I don't change the current it's a constant 10 amps so I have a breadboard here or a circuit and I have an inductor i hook a current source and I send 10 amps through it but I don't change it it's a constant 10 amp so the 10 amps is constantly flowing around and around around around around well even though it's 10 amps the rate of change of that current how fast the current is changing is zero if I put a constant current whether it's a constant 1 amp or constant 5 amps or constant 10 amps or a constant 100 amps or whatever if it's not changing then di DT is zero so that means zero times anything is zero so anytime I have a current flowing through an inductor right that's not changing that's not going up and down or changing or increasing or decreasing or whatever if it's just constant like if I hook a 9-volt battery up to an inductor right there's going to be current flowing but it's not going to be changing with time then that means the voltage across an inductor is zero anytime I have current flowing through it that's not changing I keep kind of saying the thing over and over again because you know when you do circuits later you're going to have to think about that you're going to have to think about okay is there current changing here is there current not changing through here you're going to have to start thinking in terms of that so let me ask you a question right if what I'm telling you is true that if I'm not changing the current that the voltage cross it is zero how could that be how could that really be how can I have 50 amps going through something but no voltage you're going across it does it Ohm's law apply well sure Ohm's law always applies but it's V is equal to IR we haven't said there's any resistance with this thing in fact if you notice it's a coil of wire right there's no resistor inside here it's literally just a coil of wire it's a coil of wire what is the resistance of a perfect wire resistance of a perfect wire is zero think about that the resistance of a perfect wire is zero so if I build a circuit where the only thing in it is an inductor and a battery and I let it run for a long time then that inductor is just going to look like a piece of wire to that battery even though it's closed up like that and that's what this is reflecting right so when you have a constant current going through it where everything's reach steady state and everything's kind of settle out and I have a constant current flowing through this cloak what we call an inductor then the the fact of the matter is that inductor is going to look like just a piece of wire it's going to look like a short circuit it's going to look like something that has zero volts across just like any wire this piece of wire right here has got zero volts across it because all perfect wires we say are perfect I don't have any resistance right so I want to kind of summarize everything because we're closing the section almo more or less I just want to kind of get across the voltage equation here in the next section we'll have a real circuit to show you how this works and to kind of solidify but I'm kind of doing a lot of talking here because this kind of concepts can can confuse you so let me kind of summarize from the beginning and then we'll close the section al all right so let me go ahead and write this um you wouldn't write it right underneath it me use green okay let me put a couple notes down just to kind of make sure everybody's on the same page if I have a constant current then what does that mean that means zero volts across well constant current no changes of current means I have zero volts across the thing all right let me switch colors here if I see that I have current through the inductor and the current Rises means it increases positively that means the derivative di DT is going to be positive if the currents rising it's going to be positive so that means I'm going to have a positive voltage a positive voltage across this inductor so if I have this inductor I'm sending a current down and I'm increasing the I'm increasing the current then the voltage here is going to be a positive value in terms of how I've drawn it here and then the final thing I want to say if the current Falls or decreases that means I'm going to have a negative voltage right make sure you understand that the same equation holds L di DT but in terms of how I've drawn it here if I have a current going this way let's say I have 10 amps and then I'm decreasing it that means di DT is getting smaller because I'm decreasing it right in terms of calculus when you have something you know a function that's decreasing and you take its derivative you get a negative derivative right so if I have something doing this and I'm decreasing the current that means di DT is negative so that means my voltage will be negative so that kind of brings me to another point I want to make sure you understand in this drawing we've drawn current going this way and we've drawn a voltage drop like this but you got to keep in mind that this is kind of a perfect picture it's assuming that this current is increasing it's assuming a positive increase of the current so the voltage could actually be flipped around when we say it has a negative voltage what we mean is the positives really over here and the negatives really over here if I if I put a current through it and decreased it where a di DT was small or going getting smaller then the voltage would be flipped from what I have right here so I want to summarize in a few minutes here everything we talked about and then when we get to the next section we'll draw a real circuit where we can really look at it and I can show you mathematically how this is really working we have this circuit element called an inductor it's a coil of wire the purpose of its journey in life is to store a magnetic field and there's lots of uses for that we'll get to later and you just have to trust me that there are lots of reasons you might want to temporarily store a magnetic field in a circuit all right but as a consequence of that whenever it's because it's a coil of wire if I just send a constant current through it where where nothing is changing then it just looks like a piece of wire so di DT is zero and the voltage across that thing is going to be zero anytime I have a constant current flowing through an inductor of any of any direction and of any size if it's constant its voltage across it's going to be zero all right but if I start increasing this current where I'm increasing it with time what's really happening is if I increase this current and make it stronger stronger stronger stronger then that means the magnetic field in here is getting stronger stronger stronger stronger if you remember back from physics when you have a magnetic field that's getting stronger stronger stronger the field lines are moving they're getting kind of bigger for lack of a better word and they're cutting across this inductor so the reason that a voltage is generated whenever di DT is positive is because when you Ram more current into this thing that magnetic field tries to get stronger and when it does that it's cutting across across these coils and from physics you know that whenever you have a coil of wire sitting in a change in magnetic field you're going to get a voltage that's going to try to oppose that right so the voltage is going to try to oppose that so a voltage pops up to do that so if it's a positive increase in current you're going to get this voltage if the current is decreasing di DT is going to be negative and so you're going to have a voltage basically is going to be opposite the way we drew it here all right but the point is when you think about inductors and later on when we talk about capacitors because it's going to be very similar the voltage is not proportional to the current the voltage is proportional to how the current changes it all comes back to basic physics from a long time ago that you've learned that whenever you have and by the way you know you can go and look at some of my physics lessons and learn even more detail why this happens basically when you increase the current you're trying to make that magnetic field get bigger bigger bigger bigger bigger stronger stronger stronger and it's sitting a coil of wire sitting in there when you have those field lines cutting across that coil you're going to get a voltage that's generated to try to stop it from from getting bigger you know from growing out to infinity so you're going to have that voltage there that's going to pop up the voltage that's generated we've drawn here right but in terms of circuits you don't really have to think about too much in terms of magnetic field all you need to think about is is current increasing or decreasing if it is I've got a voltage in this sense if the current is decreasing I've got a voltage in the opposite sense if the current is is not changing at all it's a fixed value then I have no voltage drop at all across this inductor alright so make sure you understand this section watch it a few times I can't say how important it is for you to have a good grasp of of this concept follow me on to the next section we're going to draw a real circuit on the board and show you how you can use the voltage drop across an inductor to learn a lot about how inductors actually operate

Channel: Math and Science

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Keywords: inductor, voltage across inductor, voltage, what is an inductor, how inductor works, how does an inductor work, basic electronics, electronics tutorial, basic electronics tutorial, physics of inductors, inductor tutorial, electronics course, what is inductor, basic electronics course, inductors in circuits, inductor circuit analysis, inductor coil, circuit analysis, learn, electronics, physics, mathtutordvd, capacitor, inductor voltage, coil, tutorial, circuit, inductance, current, resistor

Id: VyRWOGVEuUc

Length: 25min 16sec (1516 seconds)

Published: Thu Feb 04 2016