[Music] Hi and welcome back to freessons.co.uk. By the end of this video, you should be able to determine the total resistance of several resistors in series. You should then be able to use this to calculate the current or potential difference in a circuit. And finally, you should be able to describe the total resistance produced by resistors in parallel. In a previous video, we looked at resistors and circuits such as this one. We've got a 100 ohm resistor with a potential difference of 5 volts across it. The questions asking us to calculate the current in the circuit. And to do that, we're going to use this equation. Remember, you're not given this in the exam. The resistance equals the potential difference divided by the current. To calculate the current, we divide the potential difference by the resistance. Looking at the circuit, we've got a potential difference of 5 V and a resistance of 100 ohms. Putting these into the equation gives us a current flowing in the circuit of 0.05 amp. Now, in this video, we're looking at circuits which contain more than one resistor. Let's start by looking at resistors in series. Take a look at this circuit. Here we've got a 100 ohm resistor in series with a 50 ohm resistor. The potential difference is 20 vol and we've got to calculate the current. So here is the key fact you need to learn. Resistors in series add together in our circuit. We've got a 100 ohm resistor and a 50 ohm resistor. So the total resistance is 150 ohms. Now it makes sense that resistors add in series. That's because the current has to pass through each resistor in turn. it cannot bypass any resistor. What that means is that we could replace these two resistors with one 150 ohm resistor like this. Scientists call this equivalent resistance. Going back to our equation, we can now calculate the current in this circuit. The potential difference is 20 volt and the combined resistance is 150 ohms. Putting these into the equation gives us a current of 0.13 amp to two decimal places. Here's a question for you. Determine the current in the circuit. Pause the video and try this yourself. Okay, we've got a 20 ohm resistor, a 15 ohm resistor, and a 10 ohm resistor all in series. Adding these together gives us a total resistance of 45 ohms. The potential difference across these resistors is 10 V. We can calculate the current by dividing the potential difference by the total resistance. So 10 / 45 gives a current of 0.22 amp to two decimal places. Here's another question for you. Calculate the potential difference across the resistors. Pause the video and try this yourself. Okay, so first we've got to calculate the total resistance. 200 ohms + 75 ohms gives a total resistance of 275 ohms. Now to calculate the potential difference, we need to go back to the equation we saw before. The potential difference is calculated by multiplying the resistance by the current. 275 multiplied by 0.5 gives a potential difference of 137.5 vol. We're going to finish now by looking at resistors in parallel. Now, I need to point out that this is more complicated than resistors in series. But the good news is that you're not expected to calculate the total value for resistors in parallel. Here's a key fact you're meant to know. The total resistance of two resistors in parallel is less than the resistance of the smallest individual resistor. Now, that sounds a bit tricky. So, let's look at an example. We've got two resistors in parallel. One resistor has a resistance of 150 ohms and the other has a resistance of 200 ohms. The total resistance of these will be less than the smallest resistor. In other words, less than 150 ohms. Now, that's because with two resistors in parallel, we've got two pathways for the current to take. This means that more total current will flow through the circuit. Going back to our equation, we can see that if the current has increased but the potential difference has not changed, then the total resistance must have decreased. Remember, you'll find plenty more questions on resistors in series and parallel in my vision workbook and you can get that by clicking on the link above. Okay, so hopefully now you should be able to determine the total resistance of several resistors in series. You should then be able to use this to calculate the current or potential difference in a circuit. And finally, you should be able to describe the total resistance produced by resistors in parallel. [Music]