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# electricity question Watch

1. MS
http://www.scribd.com/mobile/doc/46124583?width=768
QP
http://www.scribd.com/mobile/doc/50654250?width=768

I need help on question 18.

With the first part why do you need a variable power supply/variable resistor?

In cii why can't you say
In I=nAvq the n value increases as there are more free electrons per metre cubed
With I, A and q constant, drift velocity decreases??
How do I know whether I am supposed to say this or say that there are more atomic vibrations, ausing more collisions between atoms and electrons, so resistance increases while drift velocity decreases?

Thanks
2. (Original post by krisshP)
MS
http://www.scribd.com/mobile/doc/46124583?width=768
QP
http://www.scribd.com/mobile/doc/50654250?width=768

I need help on question 18.

With the first part why do you need a variable power supply/variable resistor?

In cii why can't you say
In I=nAvq the n value increases as there are more free electrons per metre cubed
With I, A and q constant, drift velocity decreases??
How do I know whether I am supposed to say this or say that there are more atomic vibrations, ausing more collisions between atoms and electrons, so resistance increases while drift velocity decreases?

Thanks
n will only increase if this is a thermistor - which it is not. The number of electrons free in the wire remains constant.
You will need to know the bold in general, but what you said for a thermistor.
3. (Original post by joostan)
n will only increase if this is a thermistor - which it is not. The number of electrons free in the wire remains constant.
You will need to know the bold in general, but what you said for a thermistor.
THANKS!
4. (Original post by krisshP)
THANKS!
Anytime
5. (Original post by joostan)
Anytime
Any ideas about why you need a variable power supply/variable resistor for the first part in the same question?
6. (Original post by krisshP)
Any ideas about why you need a variable power supply/variable resistor for the first part in the same question?
The mark scheme doesn't state that it MUST be a variable power supply/resistor. . .

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