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# Thermistors question Watch

1. This may sound really stupid in some ways, maybe I'm overthinking it but even though we finished the whole topic in class this question in the exam really got to me and I still can't figure it out (OCR A G482 AS)

I can't remember it exactly but it had a thermistor in a circuit and it asked what happens to the voltage reading across the thermistor (NTC) as temperature rises.

My way of thinking through this is that if the temperature increases, resistance decreases so current increases, but when I looked at the equation V = IR. Considering the rise in current is proportional to the fall in resistance. Voltage shouldn't change at all, it didn't give a particular temperature increase/decrease values so I didn't know if it would decrease resistance dramatically or gradually, maybe I am just overthinking this or looking at it in the wrong way, but can someone please explain!

Thanks!
2. (Original post by sohailkm96)
This may sound really stupid in some ways, maybe I'm overthinking it but even though we finished the whole topic in class this question in the exam really got to me and I still can't figure it out (OCR A G482 AS)

I can't remember it exactly but it had a thermistor in a circuit and it asked what happens to the voltage reading across the thermistor (NTC) as temperature rises.

My way of thinking through this is that if the temperature increases, resistance decreases so current increases, but when I looked at the equation V = IR. Considering the rise in current is proportional to the fall in resistance. Voltage shouldn't change at all, it didn't give a particular temperature increase/decrease values so I didn't know if it would decrease resistance dramatically or gradually, maybe I am just overthinking this or looking at it in the wrong way, but can someone please explain!

Thanks!
You are assuming that ohms law applies and hence a graph of voltage against current for a thermistor is linear.

Both assumptions are false in such a scenario. It should be noted that resistance will decrease in the circuit and as such the voltage drop across the resistor will fall.

http://www.passmyexams.co.uk/GCSE/ph...Thermistor.jpg
3. (Original post by Ari Ben Canaan)
You are assuming that ohms law applies and hence a graph of voltage against current for a thermistor is linear.

Both assumptions are false in such a scenario. It should be noted that resistance will decrease in the circuit and as such the voltage drop across the resistor will fall.

http://www.passmyexams.co.uk/GCSE/ph...Thermistor.jpg
Actually I just got what you meant, thanks!
4. (Original post by sohailkm96)
I get that it doesn't obey ohm's law so from looking at that graph solely would the increase in temperature lower the voltage reading across the thermistor?
Yes, increasing temperature = lower resistance = lower potential drop across thermistor

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