[Music] hi and welcome back to freci lessons. co.uk by the end of this video you should be able to recognize the symbol for a light dependent resistor and this is also called an ldr you should then be able to describe how the resistance of an ldr changes in light and dark conditions and finally you should be able to explain how ldrs can be used in circuits in the last video we saw that we can use resistors to control the potential difference in circuits here's a symbol for a resistor if we change the potential difference across a resistor and we measure the current then we get a straight line passing through zero like this now this tells us that the current through a resistor is directly proportional to the potential difference and scientists call that an omic conductor however as we saw that's only the case if the temperature is constant so in this video we're looking at a different kind of resistor it's called a light dependent resistor or ldr for short here's a symbol for an ldr and it's important that you learn this now in dark conditions the ldr has a high resistance however in the light the resistance of the ldr is very low I'm showing you that on this graph as you can see we've got a high resistance when the the light intensity is low in other words in the dark and a low resistance when the light intensity is high now ldos are really useful as we can use them to detect light and I'm showing you a good example of that here when you put your mobile phone to your ear the screen switches off to save the battery we can do that using an ldr now I should point out that the circuit in a mobile phone is far more complicated than the one I'm going to discuss I'm going to show you an extreme simplified way that you could use an ldr to do this so take a look at this circuit we've got an ldr in series with a lamp the lamp is the backlight for the mobile phone screen we've got a cell in the circuit with a potential difference of 10 volts now in the light in other words when the phone is not held to your ear the resistance of the ldr is very low that means that it takes very little energy for the current to pass through the ldr because of that the potential difference across the ldr is very low now remember that the potential difference is shared between components in series so that means that the potential difference across the lamp is large so the lamp now lights up the screen if the phone is held to a person's ear then the ldr is now virtually in darkness the resistance of the ldr Rises sharply now it takes a great deal of energy for the current to pass through the the ldr so the potential difference across the ldr is very high this means that less electrical energy is available for the lump the potential difference across the lamp is now very low and the lamp becomes very dim this turns the screen off now we can also find ldrs in devices such as these Garden lamps here the circuit's designed so that the light comes on in the darkness and then goes off again in the light remember you'll find plenty of questions on ldr in my vision workbook and you can get that by clicking on the link above okay so hopefully now you should be able to recognize the symbol for a light dependent resistor you should then be able to describe how the resistance of an ldr changes in light and dark conditions and finally you should be able to explain how ldrs can be used in circuits [Music]