Inductors

Hey guys. In this video we're going to be talking about these things called inductors. This is going to rely a bit on our discussion on self inductance so just make sure that you're comfortable with that before continuing, alright? If you take a coil of wire, like a solenoid, and you put it inside of a circuit that coil is known as an inductor, okay? An inductor or something that can undergo self inductance, a resistor or something that has resistance, a capacitor is something that has capacitance. So, just follows the same type of wording, there are two common symbols for inductors in circuit diagrams, this one on the left and this one on the right I just had to choose one of them and I decided to go with the one on the left but if you see the one on the right in your classroom or in your textbook note that it's still just an inductor, okay? In order to use inductors in circuits though, we need to know how to apply Kirchhoff's loop rule to them, we know how to do it for batteries, we know how to do it for resistors, we know how to do it for capacitors. Now, it's time to learn how to do it for inductors.

First of all, if an inductor is just sitting there with a constant current passing through it there's no voltage across the inductor, imagine that the inductor has no resistance, there's no voltage because it's just a current passing through a wire, okay? Now, if the current for instance was increasing then it would need some sort of extra push to increase, currents want to go from high potential to low potential. So, this should be a high potential point And this should be a low potential point, that's going to provide some extra motivation for the current to go forward to increase. Now, this means that the voltage or in this case the induced EMF points to the left, opposite to the direction of the current, okay? Because it always puts from low to high, sorry, yeah the low to high. Now, on the other hand, if the current is decreasing then we need some sort of impeding force, some sort of impeding electromotive force, current wants to go from high to low and it doesn't want to go from low to high, going from low to high will impede it. So point A should be a low potential point and point B should be a high potential point, in which case the EMF would point to the right? The shorthand way to remember this, is that if the current is increasing the EMF points opposite, if the current is decreasing that the EMF points with it, this is kind of like four resistors, that if you have a resistor and your loop points with your current it's negative, and if your loop points against your currents it's positive, okay? if your current is increasing your EMF points against it, if your current is decreasing your EMF points with it, okay? However you can figure out to remember that. Now, remember like we talked about during our discussion on self inductance by Faraday's law, the induced EMF on an inductor is just the inductance times the rate at which the current is changing through it, okay? Let's do an example, right out to your chops loop rule for the following circuit, treat the capacitor as initially charged, okay? So, this capacitor is initially charged, what does it want to do, it wants to release current, okay? Coming off of the positive plate going to the negative plate. So, here is current and here is current pure kirchhoff's loop rule is just adding up all the voltages but we do have to choose a loop direction first, I'm just going to choose this as the loop direction, so the two that are easiest to do are going to be the capacitor and the resistor so let's do them first, the loop goes with the polarity of the capacitor it goes from the negative to the positive, so the voltage of the capacitor is positive, the current, the loop goes with the direction of the current through the resistor, so the voltage of the resistor is going to be negative. Now, what about the inductor? Well, the current in this circuit is going to be dropping, okay? initially the resistor has all of the voltage of the capacitor so the current is very high but as the capacitor loses charge the current, sorry, the voltage of the resistor drops, so the current drops in the circuit. So, what we have is the current to the right that's decreasing, this means that the EMF is with the term, right? If it's decreasing is with it since the EMF is with our loop is positive so this is plus VL and that equals 0, okay? Now, all we have to do is plug in what those values actually are, the capacitance, sorry, the voltage of the capacitor is the charge divided by the capacitance the voltage of the resistor is the current times the resistance and the voltage of the inductor is the inductance times the rate at which the current is changing and that is very simply our answer. Now, what our inductor is actually used for by Lenz's law, we know that coils of wire resist changes to their magnetic flux, okay? The magnetic flux through an inductor depends upon its own current. So, if it wants to resist changes to its own magnetic flux by extension it wants to resist changes to its current, okay? So, that's what happens, that's what and inductor is used for, they're used to resist changes in currents, a very, very common application of them is adding a bunch of inductors to transmission power lines, if they're ever struck by lightning or the transformer or the pole that holds them, is never struck by lightning that could increase the voltage across a wire, dramatically, by millions of volts, okay? And that could really, really increase the current in that wire, which could be bad, it could be bad at the end of where the transmission line ends, because too much current is delivered, that current is going to generate heat, it could melt the wires, whatever reason, inductors are put in to reduce the effect on the current that that increase in voltage has, just because the voltage spikes doesn't mean the current is going to jump up with it, alright? So, that wraps up our discussion on inductors as circuit elements, thanks for watching guys.