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# OCR physics watch

1. Spread 2.2.6 the effect of temperature on resistivity......
Question is:
A thermistor is connected through a switch to a 12 V power supply of negligible resistance. When the switch is closed the initial current is 14 mA a few minutes later it has risen to a constant 20mA
a) explain why the current rises
b) does the thermistor dissipate more power initially or after it has been left for some tim? Why?

I'm stuck and the textbook has no answers. I have a rough idea if what it may be but I'm not really sure, if you can help it would be really appreciated
2. For this question, it tells you there is a thermistor. It also tells that the current has risen from 14mA to 20mA.

Thermistors resistance decreases with a rise in temperature. The current flowing through a circuit increases the temperature of the components in the circuit the longer it is left on.

a) I=V/R, If R(Resistance), decreases then I(current) will be greater.
b)For this you should be able to refer to the equation P=V^2/R. Again a smaller R will give a greater P(Power). Since the resistance is lower for the thermistor after it has been left for time, it will be the latter.

(sorry if theres any mistakes)

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