 In this video, we're going to talk about Ohm's Law. Let's get started here. So we're going to get started with the 120-volt battery. The long side of the battery is your positive terminal. The small sides are your negative terminal. We have a 20-ohm resistor in series with a 5-ohm resistor in series with a 15-ohm resistor. Now remember, current flows from negative all the way through these resistors to positive. This is like a pump that's got 120 volts of pressure. Often times we can refer to voltage as electrical pressure. These guys here are resisting the pressure in the circuit. So the current, which is the flow of electrons, again using the water analogy, has to flow through this resistance and this resistance and this resistance. In order to do that, it has to overcome all three resistances, which means that we need to add these resistances together to get our total circuit resistance. That now gives me 40 ohms of total circuit resistance by going 20 plus 5 is 25 plus 15 equals 40. So that means that this entire circuit has a total resistance of 40 ohms. Now we can see that we have 120 volts and 40 ohms. We can determine the current using Ohm's Law. Let's take a look at what that looks like right now. Ohm's Law tells us that resistance is directly proportional to the voltage and inversely proportional to the current. That's its basic Ohm's Law. Now we can transpose this formula here. This r is equal to v over i. We can transpose that into a formula that looks a little better to us and the one that you're probably learning about in school, which looks like this. v is equal to i times r, meaning that the current in this circuit is equal to the voltage in this circuit is equal to the current times resistance. I'm going to keep this here because we're going to use this little formula later on. But right now what we're trying to do is figure out what the current is in this entire circuit. So in order to figure that out, we're going to use the total circuit voltage and the total circuit resistance to figure out what the total circuit current is. So we're going to take this formula again and we're going to transpose it to something else. i is equal to v divided by r, which is the same thing as this. We just move the formula around. We've transposed. If you don't know how to transpose, there's a video on transposition. So make sure you go back and take a look at how that works out. So here we are. We now know that we have 120 volts, 120 volts. We have 40 ohms of total circuit resistance, 40 ohms of total circuit resistance. I'm going to go 120 divided by 40 and that will give me a total current of 3 amps. So there you go. We've figured out what the current in the circuit is using Ohm's law. All we've done is transpose it and move it around a bit. Now here's the deal. Now that we have 120 volts in this circuit total, we have a total resistance of 40 ohms. We also have a total current of 3 amps. In this circuit, the one thing that doesn't change, and I always tell this to my students, the one thing that won't change is my love for them and current in a series circuit. The current is what it is. It is the flow that is going to be pushing through this circuit due to this resistance here. So once you've figured out what this 3 amps is, we now know what the current is at all points in the circuit. It will be 3 amps here, 3 amps here, and 3 amps here. So in order to figure out what our volt drops are going to be across the resistors here, all we have to do is take this 3 amps, look up here and let this part of the formula, V is equal to I times R, well I can go 3 times 20, 3 times 5, and 3 times 15 will give me my volt drops across each resistor. So we take 3 times 20 gives me 60 volts, 3 times 5 gives me 15 volts, and 3 times 15 gives me 45 volts. And there's a way to double check this, as we know that if we have a total circuit voltage of 120 volts, Kirchhoff's law of volt drops in a series circuit tells us that the sum of these voltages basically has to equal this voltage. So if I go 60 volts plus 15 volts plus 45 volts, I get Shazam 120 volts. And that is Ohm's law in a nutshell in regards to a series circuit.