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Homework Help: HELP! drift velocity question

  1. Jan 10, 2006 #1
    In a circuit, if the voltage is doubled, what happens to the drift velocity?
  2. jcsd
  3. Jan 10, 2006 #2


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    It will essentially double. The Coulomb scattering cross section decreases with speed but for typical voltages the change is very slight.
  4. Jan 10, 2006 #3
    thanks. Does the drift speed half if the length of wire doubles?
  5. Jan 10, 2006 #4


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    If you couldn't tell from the previous response,
    You are supposed to at least tell us how you're thinking ...

    How would a tiny charge deep inside an opaque wire find out
    how long the wire was?
  6. Jan 10, 2006 #5
    OK i just need to know what factors vary drift velocity. I am not very smart, my teacher is rubbish and I have exams in a week so im quite frustrated. So length doesnt change the speed? Voltage increases the drift speed as electrons gain more energy? Increasing cross-sectional area decreases the drift speed as there is a larger area for more collisions therefore slowing down the electrons
    Last edited: Jan 10, 2006
  7. Jan 10, 2006 #6


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    No, the drift velocity is causally determined by LOCAL quantities:
    the Electric Field, the charge carrier density, the density, size,
    and mass of the NON-mobile atoms (and the Temperature).

    Voltage only has an effect on drift velocity if it influences the local E-field.
    It's the E-field that accelerates electrons, before they collide and start again.

    Cross-sectional Area only affects drift velocity if it decreases R thereby decreasing V (if current is held constant) thereby decreasing the local E-field.

    Increasing the length might (if Voltage is held constant) change the local E-field, in what way?
    Last edited: Jan 10, 2006
  8. Jul 27, 2007 #7
    I have a question regarding drift velocity. Paul Drude originally had the drift velocity given by

    v =(q*E/m*2)*t (1)

    but then the factor of 2 was dropped and now most textbooks write the drift velocity as

    v = (q*E/m)*t (2)

    My question is was it measurement of the actual current in a wire that led to the dropping of the factor of two ? In other words Drude's formula disagreed with observation so equation (2) was adopted.
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