 I'm going to help you out here going through the circuit analysis handout for science 30 What we're doing here is taking a look at what an electron would be doing if it was to go around a circuit like this And we're going to take a look at some of the Changes in voltage and current in the circuit as we move around through a series and a parallel circuit Afterwards, there's a couple of problems that I'll probably tackle in a different video. You can watch that if you use the help with those So here we've got a series circuit. How do you know it's series? Well, these two resistors here are right next to each other There's only one pathway for the electrons to go through this circuit and through both those resistors They kind of don't have a choice as to where to go. That's what makes this a series circuit So let's take a look what's happening here First of all, it says that in a series circuit, this is really important This is what you want to remember currents the same everywhere That mean it every that means that every resistance the current or every resistor the current is going to be the same The voltage changes depending on which resistor you're going through. All right, let's see what this means Electrons coming from the battery pass through point one right here. Let's draw a little electron in What's the voltage of the electrons here, it's going to be 25 volts Reason for that those electrons haven't gone through a resistor that has taken away any of their energy yet And the current in the circuit here is going to be 5 amps Reason for that. There isn't any other position for the electrons to go through. There isn't like a fork in the road Current is kind of like the number of electrons passing at any point and they all have to kind of go through point one They don't have any choice. So the current resistance and the voltage hasn't changed from the battery So they go through point two They're going to meet a resistor and that resistor is going to make those electrons give up a little bit of their energy So the current in the resistor here is Still going to be five amps because the number of electrons going through the resistor is the same as the number coming out of the battery here But to figure out how much voltage that resistor is going to take from the electrons We're going to do a little calculation. We're going to do a little ohms law v equals i r calculation And it's pretty easy to do. We're just going to take the current That's five amps and we're going to multiply that by the resistance, which is two ohms This is the symbol for ohm. That's the Greek symbol omega and Kind of a weird one 10 volts is the voltage that we're going to see That first resistor taking so that first resistor takes 10 volts of energy away from the electrons So now we're going to take a look at part C which is when we're going to go through the Spot in between the two and the three ohm resistor and in that spot there We're still going to have the same amount of current as before right because again the electrons haven't gotten any choice as to where to go They're still kind of stuck on that pathway, but the voltage of the electrons is going to change those electrons came in with 25 volts from the battery and Now they lost 10 volts as they went through the battery so that means they're going to have 15 volts of Voltage kind of like energy per unit of charge energy per electron left over So 15 volts all left over once that's said and done What about a point four well here at point four we have another resistor and it's a three ohm resistor The electrons going through this little resistor are going to Experience a voltage drop again now the current hasn't changed keeping in mind currents always the same in a series circuit But I can figure out what the voltage drop is so it's going to be five amps of current times three Ohms of resistance, so that's a voltage drop of 15 volts All right, so that means that we're going to lose the last 15 volts of energy or potential difference as we go through that last resistor So the voltage at point five as they come out of the resistor is zero Now all of the energy per unit of charge of those electrons is gone, but the current is still five amps That's a series circuit. Let's take a look at a parallel circuit here How do we know this one is parallel take a look at the resistors now those resistors are kind of next to each other as opposed to Kind of being connected in to end and maybe more importantly is the electrons have a choice when they're going through the circuit They can go one way or another way. There's sort of a juncture or a fork in the road So let's take a look at what's happening with this circuit The electrons again come out of the battery At position one and at position one they have a voltage of 25 volts and here I've got a current of 18.75 amps Now the thing to keep in mind when we get into a parallel circuit is the voltage is going to be the same everywhere But the current is going to change as we go from one spot to another. I'll show you how that works So let's say the Some of the electrons are going to go through this pathway here. Some of them are going to choose to go down the two ohm resistor What voltage would be there? Well 25 volts 25 volts is the amount of energy per unit of charge Each the electrons have at this point on the circuit. So the electrons who choose to come down this way they still have 25 volts of Kind of energy per unit of charge as well and the ones that go this way also have 25 volts Of energy per unit of charge. They have the same amount of energy. The voltage is the same everywhere in the circuit Now let's figure out what the current is in this position Here at resistor two. So I'm going to do another ohms law calculation V equals IR the voltage is 25 volts and I'm going to calculate the current by substituting in two ohms of Resistance so I'm going to divide both sides by two here and the current is going to be 12.5 volts Pardon me 12.5 amps for the current So that means 12.5 of The original 18.75 Amps of current 12.5 amps chose to go down that first pathway through the resistor that was a two ohm resistor Let's think about the electrons that chose to go down through the pathway through the four ohm resistor The voltage here is still 25 volts. That hasn't changed any But the current now this is interesting if you had 18.75 amps originally coming out of the battery Kind of like the number of electrons roughly Coming out of the battery and out of that 18.75 12.5 chose to go down through the first branch That means whatever is left over in this case That's going to be six point two five has to go down through this branch So that means through the next resistor There's six point two five amps so that the total current is the same It's a conservation law. It's called the law of conservation of charge But that's the idea that's going on there so that this current in the circuit at that second resistor is a six point two five amps Point four is when we're coming right back into the battery again So the total current here adds up To 18.75 again. We've got the six point two five from the four ohm resistor being added to the 12.5 from that two ohm resistor and all together adding up to give 18.75 amps Now how much voltage does the resistor take from the electrons? We can do another Calculation here, so we're going to do a v equals i r calculation How much voltage does the resistor take from the electrons? So we're going to do a little calculation here another ohm's law calculation v equals i r We've got the voltage equals the current now. I'm going to use the overall current in the circuit here so that's 18.75 amps and I'm going to put in the resistance now there's a problem here. We don't know What the resistance is of the whole circuit? We know what the resistance is of the first resistor and of the second resistor, but what's the total of those? So we have a formula for figuring this out what we can do here Is we can use the total resistance formula, which is found in your data booklet If you have resistors in series the total resistance is very easy The total resistance is just what you get when you add together the little individual resistors Resistances just total it up But if it's in parallel we have to add the reciprocals and what we end up with is the reciprocal of the total resistance And a great way of doing this is to use the x to the negative one button key on your calculator So we're going to take a look at a few of those examples when we work through in the next video the Questions that you'll find in the next couple of problems. So check out the next video to see an explanation of that