Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Calculating current

  1. Jan 24, 2010 #1
    1. The problem statement, all variables and given/known data

    A glass tube with a cross-sectional area of 8^-2 m^2 contains an ionizd gas. The densities are 10^15 positive ions/m^3 and 10^11 free electrons/m3. Under the influence of an applied voltage, the postive ions are moving with an average axial velocity of 6 x 10^3 m/s. At that same point the axial velocity of the electrons is 1000 times as great. Calculate the electric current moving through.

    2. Relevant equations

    A= c/s = dq(+ ions)/dt + dq(- ions)

    e = 1.6602E-19

    3. The attempt at a solution

    10^15*6x10^3*.08=4.8e17
    4.8e17 * 1.602x10^-19 = .0769c/s
    10^11*6x10^6*.08
    4.8e16 * 1.602x10^-19 = .00769c/s

    .00769+.0769= .0845c/s = 84.5mA


    This isn't correct, but i don't know why? please help.
     
  2. jcsd
  3. Jan 24, 2010 #2
    If you wrote it right, you're using .08 which is 8 x 10^-2. The problem says 8^-2 = 1/64
     
  4. Jan 24, 2010 #3
    thanks, you are correct about that mistake with the .08 thing. i fixed that, but there is still something wrong with my method. any thoughts?
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook