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Curent densitys relation to drift velocity

  1. Feb 13, 2012 #1
    1. The problem statement, all variables and given/known data
    A beam contains 4.0x10^8 doubly charged positive ions per cubic centimeter, all of which are moving north with a speed of 1.2x10^5 m/s.

    (a) What is the magnitude of the current density ?
    (b) What is its direction?
    (c) What additional quantity or quantities are needed to calculate the total current in this ion beam?

    2. Relevant equations
    J=I/A=nqv (v = drift velocity)

    3. The attempt at a solution
    Well I tried this (4.0e8)(2)(1.6e-19)(1.2e5) which I thought would give what I wanted but apparently not. I was thinking by looking a dimensional analysis that it has something to do with the seconds in the velocity but im not sure?
  2. jcsd
  3. Feb 14, 2012 #2


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    Take care with the units of length. The ion density is given in particles/cubic centimeters, the velocity is given in m/s. The current density has to be in C/m2.

  4. Feb 14, 2012 #3
    Ok I did that I converted (4e8) in particles/ cubic centimeter to 400 particles per cubic meter but it still did not give the right answer?
  5. Feb 14, 2012 #4


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    There are 400000000 particles in one cm3. How many cm3 makes one m3? (which one is bigger? 1 cm3 or 1 m3?) Where are there more particles? In a big volume or in the small volume?

  6. Feb 14, 2012 #5
    Huh wait your confusing me I though 400 cubic meters was correct is it not?
  7. Feb 14, 2012 #6


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    What was 400 cubic meters?

  8. Feb 15, 2012 #7
    im saying the conversion from cubic centimeters to cubic meters is 400
  9. Feb 15, 2012 #8
    I got it thanks for all the help!!! WOOT
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