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# Average Electron drift velocity question watch

1. I'm not sure of the method in answering this question:

Q. A piece of element x is 2.0mm wide and 0.25mm thick. The number of conduction electrons per (m^3) is (6.0 x 10^20). What is the average drift speed of the electrons when a current of 1.5 mA flows ?

Here is what I have so far:

Rearranging I=nAve gives v = I/nae and the cross section area is 3.14x10^-6

This gives the drift velocity at 4.98 ms-1. Is this correct or not ?

Thanks
2. The cross section is rectangular not circular.
3. No. If the element is 2.0mm wide and 0.25mm thick, then A = 5 x 10-7m2.

I got 31.2ms-1.
4. (Original post by teachercol)
The cross section is rectangular not circular.
^^i doubt it

youre rearranging is correct, but your answer is wrong

If you meant that the element is 0.25mm thick and 2.0mm long then the area in m^2 is pi x (0.00025)^2

e= 1.6 x 10^-19 (i think)

sub in these values and v will be in m/s (and then you can convert it into whatever you want)

[i got an answer of 79.6m/s (3sf)]
5. The question clearly states wide and not long. I did the question again and got 31.25 ms-1 as Morbo got. So I guess it depends on whether it is circular or rectangular.

Anyway, thanks for all your help
6. (Original post by teachercol)
The cross section is rectangular not circular.

(Original post by Adhavan)
^^i doubt it
Hall probe elements are normally rectangular slices of silicon - which looks like the case here.

Area = width x thickness.

Trust me ...
7. (Original post by teachercol)
Hall probe elements are normally rectangular slices of silicon - which looks like the case here.

Area = width x thickness.

Trust me ...
Fair enough mate

pretty much every average drift velocity question ive ever done has involved a cylindrical wire of some kind (which is why i was sceptical to the idea of a rectangular element)
8. I've put my final answer as 31.25 ms-1. I'll ask my teacher to see if this is right tomorrow. Thanks once again for all your help.

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