GCSE Physics Revision "Light-Dependent Resistors"

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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

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