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# Basic question regarding I=nAvq watch

1. Two pieces of wire A and B are made of the same material but have different diameters. They are connected in series with each other and a power supply.

(i) Which terms from the equation will be the same for both wires?
(ii) The diameter of A is twice that of B. Calculate the ratio vA:vB

For (i), I know it isn't A because A is area, and they have different areas so they also have different areas. However, I don't know which term it is.
Please can you help me?

(ii) I have no clue how to do this. Again, any help would be greatly appreciated.

Thanks!
2. resistors in series have the same current flowing through them... That's often useful to know
3. (Original post by Ravenous food)
What? Can you help me please?
4. assume the wires are circular in cross section... what happens to the area of a circle if you double it's diameter?
5. (Original post by blobbybill)
What? Can you help me please?
6. (Original post by Joinedup)
assume the wires are circular in cross section... what happens to the area of a circle if you double it's diameter?
The area of the circle would double. I still don't get either of the questions though.
7. (Original post by blobbybill)
The area of the circle would double. I still don't get either of the questions though.
I'm pretty sure the Area will be as of 4 times the initial one.
eg. A square with sides of 2cm. Area would be 4cm^2. If you double the length of the sides to 4cm, then the area would be 16cm^2.
Hope this helps!
8. (Original post by blobbybill)
The area of the circle would double. I still don't get either of the questions though.
+Mrwaffle is right...

also for circles where A=πr2

doubling the diameter also doubles the radius so the area goes up by a factor 4

this is interesting because the resistance of a wire depends on it's cross sectional area...
ρ=RA/l
ρ is a property of the material the wires are made of - and we're told they're made of the same material so ρ is the same for both.

ρ=RA/l
can be rearranged
R=ρl/A

so the wire that's got 1/4 the cross sectional area has 4 times more resistance for the same length.
9. (Original post by Joinedup)
+Mrwaffle is right...

also for circles where A=πr2

doubling the diameter also doubles the radius so the area goes up by a factor 4

this is interesting because the resistance of a wire depends on it's cross sectional area...
ρ=RA/l
ρ is a property of the material the wires are made of - and we're told they're made of the same material so ρ is the same for both.

ρ=RA/l
can be rearranged
R=ρl/A

so the wire that's got 1/4 the cross sectional area has 4 times more resistance for the same length.
How do I do the question"The diameter of A is twice that of B. Calculate the radio vA:vB"? I think it means the v in the equation, so the drift velocity, but I don't get how to work out the answer to the question.

Thanks
10. (Original post by blobbybill)
How do I do the question"The diameter of A is twice that of B. Calculate the radio vA:vB"? I think it means the v in the equation, so the drift velocity, but I don't get how to work out the answer to the question.

Thanks
OK so the current is the same in both pieces of wire because they're in series... I is the same for both.
n is a property of the material the wires are made of - and they're both made of the same material so n is the same for both wires.
q is the charge per charge carrier - in metal wires the charge carrier is the electron and an electron in wire A carries the same charge as an electron in wire B

A (area) is different for the 2 wires

The formula says I=nAvq, if one of the wires has A 4 times larger than the other... and the only other variable that's able to change is v... what has to happen to v to keep I the same?

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