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# thermistors Watch

1. QP
http://www.edexcel.com/migrationdocu...e_20110606.pdf

I need help on question 5. A is the correct answer, but I need an explanation for why. Decreasing temperature makes the thermistor's resistance increase. Therefore pd across the thermistor increases. So why isn't D the right answer? Also isn't the current in the circuit constant?

Thank you
2. What is it that makes you think that the current would be constant? What determines the current in a simple circuit like this, where there is just a single component and an ideal battery?
3. (Original post by krisshP)
QP
http://www.edexcel.com/migrationdocu...e_20110606.pdf

I need help on question 5. A is the correct answer, but I need an explanation for why. Decreasing temperature makes the thermistor's resistance increase. Therefore pd across the thermistor increases. So why isn't D the right answer? Also isn't the current in the circuit constant?

Thank you
Considering a series circuit, if resistance increases, current decreases. Voltage of source remains unchanged, and potential drop across resistance is unchanged. But this is only true if they're in series..
4. So is voltage across the thermistor constant and equal to the cell's emf? Since sum of all potential differences=emf of cell. If voltage is constant across thermistor, by V=IR, a resistance increase means a decrease in current. So A is correct. I thought voltage was affected by resistance. Sorry I'm terrible at electricity
5. The potential difference across a component may well be affected by its resistance, but you have to look case-by-case at which quantities will change and which will stay the same. In this case, you've now got it - Kirchhoff's Second Law tells you that the p.d. across the thermistor will always be the same as the emf of the cell, so the change in resistance will bring about a change in current.
6. (Original post by Pangol)
The potential difference across a component may well be affected by its resistance, but you have to look case-by-case at which quantities will change and which will stay the same. In this case, you've now got it - Kirchhoff's Second Law tells you that the p.d. across the thermistor will always be the same as the emf of the cell, so the change in resistance will bring about a change in current.
Thanks

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