 So far we have covered current, voltage, resistance and ohms law, so now it's time to start putting all this knowledge together and start analyzing some circuits. So if you have a look here, I've created a circuit here. It's a very simple circuit that contains a 9 volt battery, which is connected to using this breadboard here. It's just a component that I can use to connect other electronic components. I've got two resistors. So there's one resistor, which is then connected to another resistor, and then back to the negative terminal. So it's positive terminal of the battery, first resistor, second resistor to the negative terminal of the battery. So if I wanted to draw a circuit diagram of this circuit, it would look like this. So here we go. We have two one ohm resistors. So current would flow from the positive terminal straight to the first resistor, then through the second resistor, and finally come round to the negative terminal. So how can I predict how much current is going to flow through this circuit here? Well, the way that I can do that is I can use a very special, well there's two very special walls, and they're called Kirchoff's laws. But the key thing is they derive from two fundamental concepts in physics. The first one is conservation of energy. The second is conservation of charge. So these two principles lead to two Kirchoff's laws. Conservation of energy states that if there's any loop in the circuit, so in this case our circuit forms a nice loop, then the total voltage, if you start at one point in the loop and you go all the way around, you must end with the same energy that you started with. So I'm going to start at this point here, I'll highlight it with yellow. If I start at this point in the circuit, if I go across the battery, I gain nine volts of energy. So if I gain nine volts here, then as I go around, I must lose nine volts of energy across those two resistors. So that there is what we call the loop rule, and that derives from conservation of energy. However, there's also conservation of charge. And this is what we call the nodal rule. What that means is if I have a particular point in the circuit, maybe let's choose this point here. Then the amount of current that flows through this particular point in the circuit, the amount of current flying into it, must be equal to the amount of current flowing out of it. So this is called the nodal rule. And all that means that there is going to be no charge building up at any point in the circuit. Therefore, the charge going in must equal the charge going out.