GCSE Physics Revision "Thermistors"

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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 themister then you should be able to

explain how the resistance of a theist

changes with temperature and finally you

should be able to describe applications

of theist for example in

thermostats in the last few videos we've

been looking at resistors and and here

are the symbols for a resistor and a

light dependent

resistor we saw that light dependent

resistors have a high resistance and

dark conditions and a very low

resistance and light conditions we find

ldrs in devices such as these Garden

lamps which come on automatically at

night in this video we're going to look

at another type of resistor which is

extremely useful it's called a ther and

I'm showing you the symbol for a

themister

here now the key feature of a theist is

that its resistance decreases if the

temperature increases and we can see

that on this graph when the temperature

is low the resistance of the themister

is very high however at high

temperatures the resistance of the The

Falls to a low level now theists are

really useful as thermostats these are

devices which turn appliances on or off

depending on the

temperature take a look at this circuit

we'd find a circuit like like this

inside computers now computers have a

big problem which is that they can get

hot to reduce this computers usually

have a fan when the temperature of the

computer Rises the fan switches on and

cools it back down so in this circuit

we've got a themister here and that's

connected in series with a cooling fan

under cool conditions the resistance of

the theist is high and that means it

takes a lot of energy for the current to

pass through the themister because of

that the potential difference across the

theist is high and remember that the

potential difference is shared between

components in series so the potential

difference across the fan is

small this means that the fan operates

at a very low

speed if the computer gets hot the

resistance of the theist Falls now it

takes much less energy for the current

to pass through the the this means that

the potential difference across the

theist is now very low so more

electrical energy is available for the

fan this makes the potential difference

across the fan very high so the fan

Powers up to high speed cooling the

computer back down now there are plenty

of other examples where thermas are used

as thermostats I'm showing you here an

incubator for premature

babies premature babies need to be kept

warm so the incubator contains a system

to detect faults if the temperature

Falls then that's sensed by a themister

this causes a lamp to turn on and an

alarm sound

remember you'll find plenty of questions

on the use of theists 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 a symbol for a

themister then you should be able to

explain how the resistance of a

themister changes with temperature and

finally you should be able to describe

applications of theists for example in

thermostats

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