I'm going to run through the required practical which is the VI characteristics of filament bulbs versus fixed resistors. Go to my power pack here which I'm just going to be varying in the supply potential difference. Got a voltmeter. Remember volt meters go in parallel with the component that you're measuring across and my ammeter. And I'm just going to collect all my results in my computer. And I'm going to see the VI characteristics graphs that we get. So for the fixed resistor we have a straight line. Voltage and current are proportional. Potential difference is proportional to the current. That means for an equal change in potential difference we get an equal change in current. If you double the potential difference you double the current. Now a couple things to say about that practical there. I was just increasing the supply PD by equal amounts roughly. I wasn't looking for exactly one, two, three, four volts. I just made sure I have a set of results roughly equally spaced so that I can see my straight line. And the other thing to say is that this resistor will have got really, really hot. So if you do do this practical yourself in the lab make sure that you're aware that you don't touch that resistor once it's been running because it will have got really, really hot. And that heat is a really important thing to understand what is going on with these different shapes of voltage current characteristics, these VI curves as you'll see in a moment with the lamp. Remember current causes heating. The heating effect of current is what we're really interested in right here. And the last thing I want to say about that's that the result is that the gradient of the graph will be inverse the resistance. So I'm going to use Excel now to get a gradient of the graph and then one over that the reciprocal of the gradient will be the resistance. It should be 4.7 ohms. That's what's marked on the resistor. So let's find out. Yeah, okay. So it's come out of 4.8 so that's quite close. We can say that that resistor is omic. That means that for our range of potential differences you get potential difference proportional to current. V equals IR, where R is essentially the coefficient of proportionality. The 60 thing which governs the relationship between V and IR is fixed for our range. We can call that a fixed resistor or we can call that an omic resistor. So now let's go ahead and replace the resistor with the bulb and see if we get the same relationship. If you this time you can see that something else is going on. It isn't potential difference is proportional to current. So the lamp something else happens. Now what is going on is the resistance is changing. Remember I said the gradient of the graph is actually inverse the resistance. So in other words the steep of the graph the lower the resistance is. This graph is getting less steep as it goes through the range of potential differences. So what's actually happening is the resistance is increasing and the reason for that is because of the temperature is increasing as well. And the reason for that is because of that current causing heating effect. The temperature is increasing as well so therefore we're getting an increase in the resistance. What's going on inside the metal, the metal filament of the lamp is that actually as it gets hot is that as it gets hot the metal ions are actually getting in the way of the charge trying to flow through it. So that's the practical that's all it really is. It's the case that you need to be able to describe that method and you need to be able to compare the different graphs. So I haven't done positives and negatives and I haven't done diodes. This is just a brief introduction of how this practice works and how to analyse the results. And the key conclusion is that for a fixed resistor the resistance is the same throughout the range of potential differences and you get that straight line graph through the origin, voltage is proportional to current or potential difference is proportional to current as I should say. But for a filament bulb which is a non-omic resistor the resistance actually changes. You get a higher resistance when you have a higher current flowing and that's because the metal in the filament heats up and actually the ions vibrate more and then get in the way of the charge carriers.