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# Thermistor question watch

1. When doing a thermistor experiment using the thermistor as a temperature sensor, why would you need to have a resistor in series with the thermistor?
The question shows a cell in series with a thermistor and resistor. It has a voltmeter connected in parallel with the thermistor.

What it wants me to answer is how I could use this experiment to see how the voltage of a thermistor varies with temperature.
There are three parts of the question I don't understand:

• 'an explanation of the need for resistor R'
• 'an explanation of how the thermistor can then be used to measure the temperature of a room'- on the mark scheme my teacher gave me, it mentions a calibration curve which i have no idea about
• and how the reading on the voltmeter would be different if the voltmeter was connected across R instead.

This is part of the June 2014 paper I got so you won't be able to find it online, but I'm guessing you can picture the circuit.
2. Well, think about the circuit without the resistor in series. This will just be a power source connected across a thermistor. As a result, the voltage across the thermistor will always be constant as it will equal the terminal voltage of the cell. Resistance and hence current will change, but your voltmeter won't give you any useful information.
Hence you need a constant resistor in series, which will give you a potential divider circuit, causing the voltage across the thermistor to change. Think about the equation for output voltage of a potential divider circuit to help with the third part. And finally, a calibration curve is a graph that will relate temperature on one axis to resistance on the other - its use should be pretty clear.
3. (Original post by Actaeon)
Well, think about the circuit without the resistor in series. This will just be a power source connected across a thermistor. As a result, the voltage across the thermistor will always be constant as it will equal the terminal voltage of the cell. Resistance and hence current will change, but your voltmeter won't give you any useful information.
Hence you need a constant resistor in series, which will give you a potential divider circuit, causing the voltage across the thermistor to change. Think about the equation for output voltage of a potential divider circuit to help with the third part. And finally, a calibration curve is a graph that will relate temperature on one axis to resistance on the other - its use should be pretty clear.
Thanks! So when I plot temperature against voltage, I can then measure the room temperature separately and read along the graph and check what voltage it is?
4. (Original post by particlestudent)
Thanks! So when I plot temperature against voltage, I can then measure the room temperature separately and read along the graph and check what voltage it is?
Room temperature is a constant (we assume) and that is what you want to measure using the thermistor. So if you find the resistance of the thermistor experimentally at room temperature, you can look at the calibration graph to see what exact temperature produces that resistance

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