Physics CIE 9702 Paper 21

How did you guys find it? Personally i felt that it was great but now i have lost a potential 3 marks because i never took into account the number density when working out the drift velocity ratio. Does anyone remember what the question specifically asked?

It said that the wires were made of the same material, so the number density is the same.
For the first ratio (mean velocity in y/ mean velocity in z) and the last one (power in y/ power in z), did you get 2 (for both of them)?

It said that the wires were made of the same material, so the number density is the same.
For the first ratio (mean velocity in y/ mean velocity in z) and the last one (power in y/ power in z), did you get 2 (for both of them)?

Thought that was a very hard question. The rest was okay though. How did u prove the V=I/nAe one?

How did you guys find it? Personally i felt that it was great but now i have lost a potential 3 marks because i never took into account the number density when working out the drift velocity ratio. Does anyone remember what the question specifically asked?

I got 2 for both the ratios we had to calculate. :/

I got 2 for the power and 4 for the mean velocity.

You got both right. I just realized that length of wire does not affect mean drift velocity as, if the current is to remain constant, the same number of electrons will have to pass a point per unit time, not travel the whole length of the wire.
Do you remember how many marks this first
ratio was worth? Was it 2 or 3?

It was for 3 marks.
How did u all calculate the pd of resistor Y.?
I wrote that as the ratio for resistance in 1:2, pd ratio should also be same.

You got both right. I just realized that length of wire does not affect mean drift velocity as, if the current is to remain constant, the same number of electrons will have to pass a point per unit time, not travel the whole length of the wire.
Do you remember how many marks this first ratio was worth? Was it 2 or 3?

Though if the cross sectional area were to change, would that not change the number of charge carriers passing through the conductor? So wouldn't that change the mean drift velocity, even though the current were to remain constant?

Yes, the cross sectional area was 4 times greater in Z than in Y, so the electrons in Z travel 4 times slower than in Y.

I knew I did this one wrongly.

I went by the formula :-

I = n x A x v x q
Area in Z was 4 times greater so 4A
Speed was twice as much since length was twice as much.

So overall I did:- Current in Z = n x 4A x 2v x q = 8 (nAvq)

As a result v = I /(8 x nAq)

So Y divided by Z = 8:1

But this is wrong since I tried this question by the resistance formula and answer came 2:1. Can someone point out why isn't the above method correct?

I think you were finding v=I/nAe not I=nAve