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February 5th, 2019, 02:41 AM   #1
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Drift Velocity.

Hi All,

I am getting two 'slightly' different answers for this question, any help would be much appreciated.

Density of electrons - 1.00 x 10^28
Diameter - 0.0280 mm
Current - 8.10
Charge of electrons - 1.60 x 10^-19

After trying several times I seem to have the two following answers:

0.08207
0.08218

Thanks.
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February 5th, 2019, 03:10 AM   #2
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Can you post your working? Did you use the same approximation for pi for both attempts?

Also, are you sure the diameter units are correct, and what are the units for the current and the electron density?
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February 5th, 2019, 08:56 AM   #3
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Microscopic View of Electric current

Surely you are not posting to have someone check your algebra. That would be in the HS Algebra forum.
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February 5th, 2019, 09:01 AM   #4
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Hi skipjack,

Here was my attempt.

8.10A / (1.00x10^28) x (1.60x10^-19)x(0.0616x10^-6)

= 0.082183441 mm s-1

Last edited by skipjack; February 5th, 2019 at 11:14 AM.
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February 5th, 2019, 11:08 AM   #5
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i=envA

i: Coulombs/sec
e: electron charge, Coulombs
n: mobile electron density, electrons per m^3
v: mobile electron velocity, m/sec
A: area, m^2

Why are you worried about the algebra? If you get the formula and units right, your answer will be right, assuming you have had high school algebra.

What you should really be thinking about is, how to calculate drift velocity formula from its definition. The key concept is: Volume of particles travelling at velocity v and crossing an area A in time dt is Avdt.

EDIT: You could do the calculation in mm if you assumed density was per mm^3. Then your answer would be in mm/sec.
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Last edited by zylo; February 5th, 2019 at 11:21 AM.
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February 5th, 2019, 11:32 AM   #6
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Quote:
Originally Posted by NAC54321 View Post
= 0.082183441 mm s-1
To get that, you used pi = 22/7 (which is only roughly correct). What value for pi did you use when you got the slightly lower answer?
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February 5th, 2019, 11:45 AM   #7
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Solving physics problems is not about manipulating numbers to try and get right answer in book.

Checking his work involves checking to see if his equation is right and he has made the right substitution with correct units.

Last edited by zylo; February 5th, 2019 at 11:55 AM.
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February 5th, 2019, 12:16 PM   #8
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NAC54321 specifically asked about the two answers obtained and didn't give all the units involved.
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February 5th, 2019, 12:18 PM   #9
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Hi Skipjack and Zylo...

I used 3.14.
Am I going in the right direction with the answer or have I gone wrong?
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February 5th, 2019, 01:32 PM   #10
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pi = 3.14159265...

I think you made a slip when using 3.14.
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