[Music] hi and welcome back to frees science lessons. co.uk by the end of this video you should be able to describe the current potential difference graph for a filament lamp you should then be able to explain the shape of this graph in the last video we saw that resistors are often used in circuits to control the potential difference across other components remember that resistors have got this symbol we saw that we can use this circuit to change the potential difference across a resistor and then measure the current flowing through it remember that if we change the potential difference across a resistor and measure the current then we get a graph like this the current through resistor is directly proportional to the potential difference across the resistor in other words the resistance is constant we call resistors such as these omic conductors remember though that this only applies if the temperature does not change now in this video we're going to do the same experiment but this time using a filament lamp we're going to look at the shape of the graph and compare it with the graph for resistor so let's stop by taking a closer look at filament lamps I'm showing you here a filament lamp that we sometimes find in homes these are being phased out as they're not energy efficient a torch bulb such as this one is also a filament lamp now the word filament means a very very fine wire and you can see the filaments in the lamp and in the torch bulbs here so this shows you a closeup of a filament and you can see that the wire is very tightly coiled in a filament lamp this wire gets extremely hot when an electric current passes through it this causes it to glow and give out light like this in this circuit I've got a filament lamp and I'm going to change the potential difference and measure the current just like I did before with the resistor so here's the graph that we get from that experiment and as you can see the current through a filament lamp is not directly proportional to the potential difference that's because the filament gets hot and that causes the resistance to increase at high temperatures the atoms in the filament vibrate more that means that the electrons in the current now Collide more with the atoms so more energy is needed to push the current through the filament now we can see that by looking at the graph as the potential difference increases the current no longer increases as much and that tells us that resistance is increasing so in other words a filament lamp is not an omic conductor now it's really important that you learn the shape of this graph it often comes up in exams remember you'll find plenty of questions on the resistance of a filament lamp in my vision workbook you can get that by clicking on the link above Okay so so hopefully now you should be able to describe the current potential difference graph for a filament lamb you should then be able to explain the shape of this graph [Music]