#1
Doing revision for my AS level physics exam tomorrow morning and I just got confused. For a filament lamp, as current increases, the temperature of the filament increase, increasing the resistance and so decreasing the current. That I understand.

However, for a thermistor, this seems to be the other way around, as current increase, temperature increases and the resistance decreases.

Is it as simple as filament lamp and thermistors just work differently?
Thanks
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#2
It's been a while since AS, but that's right, as far as I remember.

They have NTC (negative temperature coefficient), which basically means what you just said. Good luck!
#3
That reminds me that I need to start revising for physics tomorrow
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#4
I remember doing As physics..
Good luck bro. Study hard for physics. Almost everyone failed it in my class at the end of A2.

Remember these encouraging words.
#6
As thermistors heat up, the charge carriers available increase exponentially, increasing the current/reducing the resistance. The increase in resistance due to the exciting of the lattice ions from heating is nowhere near as significant as the increase in charge carriers, and so overall the resistance of the thermistor decreases.

As a filament lamp heats up, the lattice ions vibrate more, increasing the resistance due to them interacting with the electrons more often, reducing the drift velocity. As I=nAvq, clearly a reduction in drift velocity reduces the current passing through, so it is clear that as the filament heats up, its resistance increases. With filament lamps no charge carriers are released when heated (at least a negligible amount).

I hope that makes sense.
#7
Quote by quinn_1888
Temp Up Resistance Down


and the you know V=I*R

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#8
Quote by quinn_1888
Temp Up Resistance Down


That I will never forget!

Thanks for the clarification guys
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#9
Good luck TS. I took Physics also. I can hardly remember a thing of it.
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#10
Quote by quinn_1888
Temp Up Resistance Down

While that is a great acronym, it's also wrong in most cases.
#11
I have that exam tomorrow, i'm just going to write poems about pokemon and fairy's invading the exam hall.
#13
this confused me also. i have physics, but gcse, and i'm doing this. i have books that say with thermistors, temp up, resistance down, and with LDRs, light up resistance down, but there's also sources that say otherwise. can somebody clarify what i've just said?
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#14
Quote by laid-to-waste
this confused me also. i have physics, but gcse, and i'm doing this. i have books that say with thermistors, temp up, resistance down, and with LDRs, light up resistance down, but there's also sources that say otherwise. can somebody clarify what i've just said?


As far as I'm aware, with an LDR as light intensity builds, resistance decreases.
But with Thermistors it can be either:
1. Temp. goes up, Resistance down (negative temperature coefficient)
2. Temp. goes up, Resistance goes up (positive temperature coefficient)
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#15
Quote by maiden_man_666
As far as I'm aware, with an LDR as light intensity builds, resistance decreases.
But with Thermistors it can be either:
1. Temp. goes up, Resistance down (negative temperature coefficient)
2. Temp. goes up, Resistance goes up (positive temperature coefficient)


thanks. in gcse, i haven't heard anything about ntc/ptc, so i'll have to ask my physics teacher.
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