 In this video we're going to do a walkthrough of a series parallel combination circuit. They can look a little daunting at first. You have all these resistors all over the place and from there you're going to be asked to determine what all the voltage drops are and possibly even what the power dissipated across all these guys are and it can be a little confusing. If you follow what I like to call the accordion method though it's actually quite simple. All you have to do is you're squishing a bunch of resistors together then you're squishing them into there and you're squishing those resistors together and you're squishing them into there and you're squishing those resistors in together and getting a total circuit resistance. From there we're going to get a current and we're going to start determining volt drops across these guys and kind of stretch our way back out. It's not as difficult as it sounds and it's not as difficult as it looks. So let's get started here. Now to begin what we need to do is we're going to start on the far right on this side here and I can notice that I've got these three resistors are in series with one another. Now when resistors are in series from our previous lessons we've learned that all you have to do is add the resistors together because it gives us a total circuit resistance. These guys being in series will be 5 ohms plus 4 ohms plus 8 ohms which is going to give us 17 ohms all together. So this 17 ohms that I've got here that was what those three guys were before. So next what we have to do is we've got to squish this 17 ohms up with this 10 ohms. Now in order to do that we have to remember how we add resistors in parallel and in order to do that we have to add them reciprocally so it'll be 1 over 10 plus 1 over 17 gives me 1 over the answer. So we switch those guys together because when we have resistors in parallel the overall resistance goes down we end up with an answer of 6.3 ohms. 1 over 10 plus 1 over 17 gives me 1 over 6.3 ohms. So I've squished all those resistors together. Now look at this we have a 5 ohm resistor in series with the 6.3 ohm resistor in series with a 9 ohm resistor. We just have to add these three guys up together and that will give us one total branch resistance of 20.3 ohms. See how easy this is? We're just squishing and adding, squishing and adding. Now I've got myself a 20.3 ohm resistor in parallel with an 8 ohm resistor. 1 over 8 plus 1 over 20.3 gives me the answer of 1 over what the other the answer is which in this case is 5.74 ohms. And now you know when we had it before we had all these resistors all over the place over here now we switch down to three series resistors. This 5.74 ohms represents all that other resistance that was previous to this. Now all we have to do is add 10 ohms plus 5.74 ohms plus 20 ohms in series and we get a total circuit resistance of 35.74 ohms. So we have determined what RT is which is another fancy way of saying the total circuit resistance. All this gobbledygook over here all crunched down and added and crushed and added and crushed works out to be 35.74 ohms. Why is that important? Because in this circuit once we determine what this is we can determine what the current is. We have voltage and we have resistance and using ohms law we can determine what the circuit resist or sorry circuit current is and from there we can start spreading our accordion back out. So let's do that. Our current for this circuit is 3.36 amps. I did that by going 120 volts divided by 35.74 ohms gives me a current of 3.36 amps. Now we're going to start determining and spreading it back out what our volt drops across each one of these are using ohms law 3.36 amps times 10 3.36 amps times 5.74 3.36 amps times 20 that gives us volt drops of 33.6 volts on this guy 3.36 times 10 19.29 volts on this guy 3.36 times 5.74 and 62 point sorry 67.2 volts because that is 3.36 times 20 ohms or double what this is because that's a 10 ohm resistor that's 20 ohm resistor. Now we have determined that this guy over here is 19.29 volts. Now we have to remember that we got this 5.74 ohms by taking 1 over 8 ohms plus 1 over 20.3 ohms so we're going to take this 5.74 ohms we're going to stretch it back out to the two parallel resistors that it was before. Let's do that. Now we've got 8 ohms and we got 20.3 ohms before. Remember voltage when it's in parallel stays the same so we have 19.29 volts over the 8 and we have 19.29 volts over the 20.3. Now if you remember back from the picture we had before this guy is what it is it was 8 ohms so we've determined this voltage determine this voltage and we determined this voltage. Now we need to stretch this guy out this guy was three different resistors before. Now before we do that in order to do that we need to determine what this branch current is so we have to remember that we go 19.29 volts divided by 20.3 ohms gives us a current of 0.95 amps. Now that current is important because what we're going to do is we're going to stretch these guys out again. This one resistor was actually three resistors. We're going to take this resistance stretch it back out to the three resistors that it was before and then use this current to determine what our volt drop is. Here we go. So that 20.3 ohms resistance was actually 5 ohms plus 6.3 ohms plus 9 ohms. We determined before that it was 0.95 amps running through this branch here which means now we can determine what the volt drop is across this guy this guy and this guy by using ohms law. 0.95 amps times 5, 0.95 amps times 6.3, 0.95 amps times 9 will give us the volt drops across these three resistors. In this case we end up with 4.75 volts across the 5 ohm resistor. We end up with 6 volts across the 6.3 ohm resistor and we end up with 8.55 volts across the 9 ohm resistor. So that's determined the volt drops now. Now we're going to stretch it back out one more time. This resistor was what it was if you go back to the previous drawing the first drawing that we started out with. This was 9 ohms so that's going to stay there as it was. We do know however that this 6.3 ohms was something that was stretched out. So what we're going to do is stretch it back out to what it was before. This 6.3 ohms was 1 over something plus 1 over something gave us 1 over 6.3. We've now determined that it is 6 volts. Let's stretch that out. So we have a 1 over 10 plus 1 over 17 gave us 1 over 6.3 from before. We know that that voltage was 6 volts that was over that 6.3 ohms. Therefore the voltage stays the same because we're talking about a parallel circuit. The voltage is 6.6 volts on this line and 6 volts on this line. Now if you go back to our very original drawing then when we started out with this stayed the way it was. It was 10 ohms from before. Now over on this leg here we have 17 ohms. This 17 ohms was actually three resistors in parallel sorry in series together. So this resistor plus this resistor plus this resistor gave us 17 ohms. So what we're going to do now is determine what the branch current is because we need that as we saw from before. We're going to go 6 volts divided by 17 ohms and we're going to get the branch current flowing through here. That branch current is 0.353 amps or 353 milliamps. So we know that this branch here has 353 milliamps running through it. We know that this branch contained a total of 17 ohms. Let's break them down to their individual resistances those three resistors that equal 17 ohms. In this case we had 5 ohms plus 4 ohms plus 8 ohms give us a 17 ohms. Now we're on the home stretch. All we have to do is take this current times each resistance because it's a series circuit and in a series circuit the current remains the same and we'll get a volt drop across each of these resistors. 353 milliamps or 0.353 amps times 5 ohms gives us 1.77 volts, 353 milliamps times 4 ohms gives us 1.41 volts and 353 milliamps times 8 ohms gives us 2.82 volts. We are done. We have determined exactly what the volt drop across each resistor is. So just to recap again all we have to do is add these three resistors to get a total resistance, squish it with this guy, add these three resistors, squish it with this guy, add these three resistors, get a circuit current, get this voltage, this voltage, this voltage, spread it out, determine what this guy's current was, then determine the volt drops across each one of those and just keep spreading it out and spreading it out until you get your volt drops. What I would suggest is drawing out each one of these steps and just walking through it step by step as it was drawn out and that will help you determine just practice as you go what each volt drop will be. Now if somebody asks you what the power dissipated across each one of these resistors are, all you would have to do is go E squared over R to get that. E squared over R to get that. E squared over R to get that. That would give you individual powers across every single one of them using the E squared over and then the nice thing about power is if we added up all of our powers together whether in their series or in parallel you would have your power total and that is how you solve a combination series parallel circuit.