February 5th, 2019, 02:41 AM  #1 
Member Joined: Feb 2018 From: England Posts: 61 Thanks: 0  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. 
February 5th, 2019, 03:10 AM  #2 
Global Moderator Joined: Dec 2006 Posts: 21,110 Thanks: 2326 
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? 
February 5th, 2019, 08:56 AM  #3 
Banned Camp Joined: Mar 2015 From: New Jersey Posts: 1,720 Thanks: 126  Microscopic View of Electric current Surely you are not posting to have someone check your algebra. That would be in the HS Algebra forum. 
February 5th, 2019, 09:01 AM  #4 
Member Joined: Feb 2018 From: England Posts: 61 Thanks: 0 
Hi skipjack, Here was my attempt. 8.10A / (1.00x10^28) x (1.60x10^19)x(0.0616x10^6) = 0.082183441 mm s1 Last edited by skipjack; February 5th, 2019 at 11:14 AM. 
February 5th, 2019, 11:08 AM  #5 
Banned Camp Joined: Mar 2015 From: New Jersey Posts: 1,720 Thanks: 126 
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. Last edited by zylo; February 5th, 2019 at 11:21 AM. 
February 5th, 2019, 11:32 AM  #6 
Global Moderator Joined: Dec 2006 Posts: 21,110 Thanks: 2326  
February 5th, 2019, 11:45 AM  #7 
Banned Camp Joined: Mar 2015 From: New Jersey Posts: 1,720 Thanks: 126 
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. 
February 5th, 2019, 12:16 PM  #8 
Global Moderator Joined: Dec 2006 Posts: 21,110 Thanks: 2326 
NAC54321 specifically asked about the two answers obtained and didn't give all the units involved.

February 5th, 2019, 12:18 PM  #9 
Member Joined: Feb 2018 From: England Posts: 61 Thanks: 0 
Hi Skipjack and Zylo... I used 3.14. Am I going in the right direction with the answer or have I gone wrong? 
February 5th, 2019, 01:32 PM  #10 
Global Moderator Joined: Dec 2006 Posts: 21,110 Thanks: 2326 
pi = 3.14159265... I think you made a slip when using 3.14. 

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