The Student Room Group

URGENT A level physics question

A potential divider circuit is shown below.
V
9.0 V
The battery has electromotive force (e.m.f.) 9.0 V and negligible internal resistance.
At room temperature the potential difference (p.d.) across the thermistor is 4.5 V.
The temperature of the thermistor is increased and its resistance decreases by 20% from its
previous value.
What is the p.d. across the thermistor now?
A 3.6 V
B 4.0 V
C 5.0 V
D 5.4 V

How is the answer B?
first you have to set up the potential divider equation
so the pd across thermistor = 9 * R_t/(R_t+R_r)
where R_t = resistance of thermistor and R_r = fixed resistance of resistor

then R_t decreases by 20% so
now, the pd across thermistor = 9 * 0.8*R_t/(0.8*R_t+R_r)

Solving equation 1), you get R_t = R_r
then subbing that into equation 2) you can work out the pd = 4V

hope that helps and feel free to ask more qs if youre stuck :smile:
Original post by mosaurlodon
first you have to set up the potential divider equation
so the pd across thermistor = 9 * R_t/(R_t+R_r)
where R_t = resistance of thermistor and R_r = fixed resistance of resistor
then R_t decreases by 20% so
now, the pd across thermistor = 9 * 0.8*R_t/(0.8*R_t+R_r)
Solving equation 1), you get R_t = R_r
then subbing that into equation 2) you can work out the pd = 4V
hope that helps and feel free to ask more qs if youre stuck :smile:

thanks
Original post by mosaurlodon
first you have to set up the potential divider equation
so the pd across thermistor = 9 * R_t/(R_t+R_r)
where R_t = resistance of thermistor and R_r = fixed resistance of resistor
then R_t decreases by 20% so
now, the pd across thermistor = 9 * 0.8*R_t/(0.8*R_t+R_r)
Solving equation 1), you get R_t = R_r
then subbing that into equation 2) you can work out the pd = 4V
hope that helps and feel free to ask more qs if youre stuck :smile:

why does it not work if you use V1/V2=R1/R2
Original post by tpugh1265
why does it not work if you use V1/V2=R1/R2


It works. I would say using this ratio to solve the problem is easier.

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