Physics question - Resistance

engstrom0304 · 14 Nov 2011 at 22:14

Hi guys...

heres the question...

In use, A potential divider becomes hot and its resistance increases from 10 to 11 K Ohms. If the input p.d is 2V, show that the current though it changes by 0.02mA. Explain why, if the sliding contact stays in the same place, the output p.d does not change.

Now... ive found that the current decreases by 0.02mA when the resistance increases from 10 K Ohms to 11 K Ohms...

Firstly... Why does the temperature rise? Id always thought that an increase in current results in a higher temperature, but here the current is decreasing and the temperature is rising?

Secondly... Why is the sentence in Red the case?

Thanks!

engstrom0304 · 14 Nov 2011 at 22:28

RomanNose said:
Red sentence: Because the resistance of the variable resistor will not change, therefore the PD across will remain the same.
The temperature rises as Power= V^2/R as the energy is dissipated as heat, the temperature will rise.Though I am not sure about this, it is rather confusing.

But the resistance does change doesnt it? From 10 to 11 K ohms?

engstrom0304 · 14 Nov 2011 at 22:32

geuben said:
but if the slider is 1/4 along the length then there are 2.5ohms on one side and 7.5 on the other. If the resistance goes up by 1 ohm then you get 2.75ohms on one side and 8.25ohms on the other. The ratios are the same in both cases.

Oh ok! Thank you!! That explains it!

engstrom0304 · 14 Nov 2011 at 22:33

PermaBanned said:
I don't think the temperature rise in this question is to do with what's going on in the circuit, i think it's an outside environmental factor, so you wouldn't have to use it in your working. If you want help with formula then this is brilliant, probably one of my most used notes so far

The sentence is in red to confuse you

Thanks! Ive printed out that picture and added it to my folder

Great way of summarising the equations!