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# Physics CIE 9702 Paper 21

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1. 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?
2. 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)?
3. (Original post by Paranoid_Glitch)
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?
Thought that was a very hard question. The rest was okay though. How did u prove the V=I/nAe one?
4. (Original post by olmomiau)
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)?
But doesen't optimizing the cross sectional are change the amount of electrons in the wire?
5. (Original post by olmomiau)
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)?
I got 2 for the power and 4 for the mean velocity.
Thought that was a very hard question. The rest was okay though. How did u prove the V=I/nAe one?
I proved that i=nave, then simply rearranged it to prove v=i/nae.
7. (Original post by Paranoid_Glitch)
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. :/
8. (Original post by ZK123)
I got 2 for both the ratios we had to calculate. :/
i think it was 2 for both resistance and power. not sure about mean drift velocity.
9. (Original post by Paranoid_Glitch)
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?
10. (Original post by olmomiau)
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.
11. (Original post by ZK123)
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.
Yup, that's right. It's a series circuit, so the current in each resistor is the same.
12. So compared to old exams do you think its easier or harder ? So grade boundaries are going up or down and by how much ? DON'T FORGET THAT WE HAD AN EXTRA 15 MINS COMPARED TO ALL PREVIOUS EXAMS!
13. (Original post by olmomiau)
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?
14. (Original post by Paranoid_Glitch)
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.
15. 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?
16. I proved v=i/nAe by using the speed = distance/time formula.

v = l/t = l / Q/i = il/q = il/Ne = il/nVe = il/nAel = i/nAe

Hope it's correct
17. I proved it by using the speed = distance/time formula.

v = l/t = l/Q/i = il/q = il/Ne = il/nVe = il/nAel = i/nAe
18. (Original post by olmomiau)
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.
Oh. thanks.
19. (Original post by Silver-)
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
20. (Original post by Paranoid_Glitch)
I think you were finding v=I/nAe not I=nAve
It's the same thing, just a matter of rearranging formulae.

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