Circuit Fundamentals - Inductors in DC Circuits

what the heck is up guys it's Jacob here and in this video I want to look at the behavior of inductors in DC circuits and it's very important to understand these very fundamental kind of understanding of inductors in DC circuits before we get into how inductors behaving AC circuits and it will play a major role in to AC circuit analysis and also circuit design and kind of giving us an intuitive understanding as to how these components will behave in a circuit and how they will influence a circuit so to start I'm going to start with a simple circuit a very simple circuit just a battery here and I'll draw a little switch there and there's our inductor and so there's our battery complete that switch okay so when the switch is open obviously it's pretty obvious that no current is flowing right no current can flow through this and basically what happens what I want to really analyze here is when when we close the switch what happened and your mind kind of tells you because if you know what an inductor actually looks like right I have an inductor right here let me show you guys an inductor is just really a coil of wire right it's just all it is a coil of wire it can be just a coil of wire in the air or it can be coil of wire around some core like this and you really your intuitive sense says well it's just a piece of wire right so it's going to act just like this and so what you think is as soon as this switch is closed current will just start to flow through this circuit right and that is not the case so let me explain I'm going to try and explain this is a little bit more difficult than the capacitor to explain because it actually has to really explain how an inductor works briefly not in too much detail but I want to explain what's going on here how this inductor works and why no current flows as soon as this switch is closed so let's go ahead and look at it as soon as this switch is closed you would this end of the inductor would have a more positive voltage in this end we have a more negative voltage right relative to each other and so let's take a closer look at this inductor let me draw this this conductor right here in more detail and so this side is at a more positive potential and this side is that a more negative potential and so I don't really want to get into too much details about this because I will make another video about exactly how inductors work but I don't want to do all that right here because I just want to explain how they behave in circuits but essentially we use the right hand rule and that's just going to tell us okay current conventional current would flow from positive to negative through this inductor right so if I point my thumb in the direction of the currents my fingers I curl my fingers that will show me what direction my Kirk my thumb the shows the direction of the current my fingers will show me what direction magnetic field points and so there's going to be a magnetic field that's going to point to the left right so there's going to be a magnetic field pointing to the left due to the actual current flowing due to the actual potential difference from this inductor but if you know anything about lenses law or anything like that you'll know that this inductor is really going to try and it's going to try and maintain the actual flux the magnetic flux in this inductor originally the magnetic flux in this inductor was zero right because there was no current flowing through here so the magnetic flux through this inductor was zero all of a sudden you put a current through this inductor and it has a magnetic field pointing this direction so what this inductor is actually going to do is it's going to try and maintain the flux in this inductor so it's going to actually produce its own it's going to have its own induced magnetic field which is going to point the exact opposite direction so you basically you apply a voltage through this inductor what happens is it creates a magnetic field this magnetic field is going to be opposed by an induced magnetic field this induced magnetic field is going to actually produce another voltage within the inductor which is going to oppose this inductor is going to produce its own voltage across it which is going to oppose the actual voltage that was originally across it which basically cancels it out and makes it so that no current can flow through this inductor okay so there's a lot going on there I'll explain it one more time just so you guys understand it this is the actual original potential difference or voltage applied across this inductor we use the right hand rule that tells us that it's going to produce going to produce a magnetic field pointing this way this inductor wants to maintain the flux that was originally and it had a Plus because there's no current or anything flowing through it so there's no magnetic fields at all this inductor is going to have a magnetic field across it from this potential difference it will induce a magnetic field its own magnetic field in the opposite direction to maintain that stem flux is zero that will cause it to produce its own voltage which will oppose the current and make it so that no current initially flows through this inductor over time this inductor will not be able to induce as strong of a magnetic field so this its own induced magnetic field will begin to weaken and eventually current will be allowed to flow through this inductor so that is what that is what's initially going on and that is why no current initially flows through an inductor so if we looked at if we graph the actual current I versus T the the current versus time right initially your current is going to be zero and so over time it will allow current to flow through it and the current will max it after a given amount of time enough time the current will taper off at its maximum current level and that can be determined by Ohm's law right and so you would just use this this inductor would essentially act like a wire so after enough time this inductor would be it would act just like the wire you initially thought it was going to behave like because it's going to build up this magnetic field its induced magnetic field is going to disappear and this this inductor will act just like a wire in a DC circuit so the current the current through this inductor will be zero initially it will increase exponentially and it will taper off to a maximum value let me show you guys the actual voltage across this inductor initially the so if we have this if we plot a voltage versus time the voltage would be maximum right and over time that voltage would would decrease so initially you have a maximum voltage across the inductor that's that voltage that we applied here but there is still no current initially so initially there's no current there's a loss of voltage and then over time that current starts to increase in the voltage across that inductor we'll start to decrease to eventually zero right assuming this inductor had no resistance in the wire and set that in the earth yes the inductor had no resistance in that little wire coil that I was made of and so that inductor will behave just like a wire in a DC circuit after this after this time period and this is a very generally a very fast transition this all happens in a very short period of time so generally we would say that this that if I drew this a quit an equivalent circuit for this let me go ahead and draw that so if we drew an equivalent circuit this was our battery and we drew a an inductor right kind of looks like a resistor but that is an inductor L that's a symbol for an inductor it would be equivalent it would be equivalent to a circuit that looks something like this there would be essentially nothing there that is what an inductor looks like in a DC circuit because really initially it's going to it's going to act like an open circuit right there's going to be no current flowing through it so it'll just act like a break in the wire initially but after a very short period of time that inductor will allow current to pass through it and it will actually act just like a wire so it'd be as if that inductor was not even there in that DC circuit also guys one very important concept that I almost forgot to explain was what happens when this switch is open because it's very important we actually have to look at what happens when the switch is opened unlike in a capacitor circuit something very interesting happens when the switch is actually open so let's go ahead and take a look at that so again let's go back to the very fundamentals I want to just blow up this coil and show you guys it in a bigger picture right and so we have some voltage which we've applied across this inductor right there there is some potential difference across this inductor to have current flowing through it again this is after the switch has been closed for a while so this inductor is now acting like a wire and basically this inductor it has some some induced magnetic field right again that can be determined by the right-hand rule and or not some it has it has a magnetic field created by the current and it's induced magnetic field has already faded away it's disappeared and it's allowing current to flow through at this point again the switch has been closed for a significant period of time to the point where this inductor is allowing current to flow through it and so what is important to understand is like I said these these inductors really want to maintain their flux initially that had that when there when the switch was initially open there was no current flowing to his conductor there was no flux or anything right there was this looks we could say was zero and so we closed the switch and all of a sudden there's a magnetic field pointing to the left this conductor is going to want to point one to the right to cancel that out and maintain that flux of zero well we can see again eventually in the end it loses and it can't maintain that flux so a current will begin to flow through it and it will have an overall magnetic field pointing to the left here just like this and so this is the state that the inductor would be in for after a long period of time after current has been flowing through this inductor when we open up this switch the overall flux there is a flux pointing overall to the left right so there is a flux in this and this inductor is going to want to to maintain that flux and so what happens is this was the this was the original this was the actual potential difference across this inductor while it was conducting current once we actually removed once we open up the switch now relative to what it was before this side of the inductor will become more negative right and this side will become more positive because before it was negative now it's before the side was positive now it's going to become more negative before this side was negative now it's going to become more positive because that switch is opening up and there's now no more potential difference across this inductor right and so that's actually going to cause a really we can think of it it's going to do to let the right hand rule if you pointed your finger in the direction of the of the currents right so the direction of the current is this way and the magnetic field is going to point this way so this is going to be the actual magnetic field due to the change in current and so the inductor is going to induce its own magnetic field because it's going to want to maintain that that that a magnetic field that had there before it had a magnetic field pointing to the left so it's going to induce even more magnetic fields and pointing them to the left to try and maintain that because because of that current chain so it's kind of the opposite of what was originally happening and so this inductor is going to want to make its own voltage to maintain that current so if we looked at it this circuit before had a positive voltage here and so this was positive and this was negative right when the switch was closed this side of the inductor was more positive this side was more negative now when this switch is open and there was a current right there was a current flowing this way this is conventional current flow so this was our current I so there were the current flowing through this inductor when that switch was closed for a while as soon as we release this switch this inductor is going to induce a voltage which will create it it's going to want to keep that current it's going to want this current to maintain the inductor itself is really opposing current changes in any direction it doesn't want current to start flowing through it and it doesn't want current to stop flowing through it once it has started flowing through it so the inductor will oppose these changes in any direction and so it's going to want to keep this current flowing through this inductor right and so what's going to happen really is it's going to create a positive voltage on this side and a negative voltage on this side this is going to be very high voltage it'll create voltages far higher than what this original voltage across this battery was so it'll create very high voltages and it will try to keep a current flowing and it's some in most cases it will actually create a voltage so high that it'll actually be higher than the actual breakdown voltage of the air and it will create a spark which will jump across these switches and stuff like that so you gotta be very careful in electronics design if you're ever doing like switching with inductors it can actually create high voltage transients which can damage transistors and other solid state components but yes it will try to create a voltage as high as it possibly can to maintain that current so basically what you have is you have this magnetic field which is around this inductor in both directions and this magnetic field is actually collapsing in so when you first charged up the inductor originally when you when you closest which you had magnetic fields building up around the inductor right and then so when we open up this switch it's going to produce its own EMF or it's going to produce some voltage across this inductor and trying maintain its current and it's going to use its magnetic fields that it built up and it's going to trade those off those magnetic fields are going to collapse and it's going to try and turn those back into electrical energy so that's what the inductor does when the switch is open alright guys so I just want to show you guys the actual circuit I set up here to demonstrate this so I have a twelve volt power supply it's not a battery but a power supply same thing here is a hundred ohm resistor so there's a hundred ohm resistor in series with this circuit and then here's our inductor and then here's the switch so there's a switch so I'm going to open and close I'm actually just going to use a wire not a switch but it's the same thing and then basically so the first channel of our solar scope is actually going to measure the voltage across this inductor so this will show us the actual voltage across the inductor and then the second channel of the oscilloscope is going to measure the voltage across this resistor and this is actually going to represent the current flowing in the circuit right if current if there is no current flowing in this circuit there will be no potential difference across this resistor if there is current flowing through this circuit there will be a potential difference across this resis resistor and then we can actually measure the amount of current flowing through this circuit by using Ohm's law so I is equal to V over R if we took this voltage that we got out of channel two divided by the hundred ohm resistor that would tell us the actual current flowing to this circuit we don't really need to know the exact current flowing through the circuit I just want to demonstrate this but just know that channel twos voltage is directly proportional to the current flowing through the circuit so we could think of channel 2 as actually representing the current flowing in the circuit so here's our power supply at 12 volts there's a lot going on here and here's our little hundred ohm resistor we have both of the oscilloscopes hooked or both of the channels of the oscilloscope hooked up here and then I'm just using this as a switch kind of just touching it and this is the power coming from the power supply and there is our inductor so there's it's quite a busy set up here but let me show the actual data we captured from this alright guys so here's the actual data that we captured the meter some of those display features off because it's getting a little busy alright so here's what's going on this right here is the very instant that that switch was closed and as you can see the voltage across that inductor is maximum it's almost 12 volts it is 12 volts s supply voltage right but the current at that point is still zero there's no current flowing so that inductor is resisting that current flow over a short period of time that inductor will begin to allow current to flow through it right those magnet as those magnetic fields build up to their maximum value that inductor will start to allow current to flow through it and the potential difference so the voltage across that inductor will start to decrease and it will become zero and at this point the inductor is pretty much just like a closed switch or exacting like a wire in this DC circuit all right guys that's pretty much it for this one I hope you guys enjoyed it if you did be sure to give it a thumbs up and subscribe for more content like this as always guys have a good one

Channel: Jacob Dykstra

Views: 23,697

Rating: 4.760479 out of 5

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Id: kyy8VO6b_ro

Length: 16min 22sec (982 seconds)

Published: Sun Jan 29 2017