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Electricity: Drift Speed

  1. Jun 29, 2015 #1
    1. The problem statement, all variables and given/known data:
    When a steady current flows through a conductor, the electrons in it move with a certain average 'drift speed'. The drift speed of electrons for a typical copper wire is about 1 mm s-1 . How is it then that an electric bulb lights up as soon as we turn the switch on?


    2. Relevant equations:
    N/A


    3. The attempt at a solution:
    It cannot be that a current starts only when an electron from one terminal of an electric supply physically reaches the other terminal through a bulb, because the drift speed of electrons in conducting wire is very slow.
    In my opinion, when an electron moves through a conductor with its drift velocity, it collides with other electrons in a conductor, as a result of which other electrons come in motion too the same way until they reach the output.
    Please rectify me if I am wrong or right.
     
  2. jcsd
  3. Jun 29, 2015 #2

    SammyS

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    It's not a particularly good explanation. The phrase "until they reach the output" being particularly troublesome. It still seems like it might take significant time.

    What is it that causes the electrons to drift ?
     
  4. Jun 30, 2015 #3
    Electrons drift due to their collision with atoms of a conductor.
     
  5. Jun 30, 2015 #4

    SammyS

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    That just gives them some random motion. They do that even if the switch is off.

    Why do they drift in some preferred direction.
     
  6. Jul 1, 2015 #5
    Due to the potential difference by any active element in the circuit.
     
  7. Jul 1, 2015 #6

    SammyS

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    Yes. That's one way to look at it.

    Your initial idea can also be fixed-up with only a little added explanation.
    Rather than collisions causing the subsequent motion, think about what happens when a few electrons are induced to drift into a region which was previously neutral. Those electrons set-up and electric field which will repel other electrons farther ahead in the wire. Similarly, those electrons do then repel others farther along the wire. This all happens very quickly far ahead of that initial drifting electron. At the other end of the wire, electrons drift away from neutral atoms, leaving excess positive charge, thus attracting electrons from farther along the wire to replace the drifted electrons, etc.

    In fact, by flipping the switch ON, you setup a small electric field field in the wire nearly instantaneously. When you think about it, this all has to happen very quickly. Your circuit is likely to be of the alternating current variety, with polarity changing direction 100 to 120 times per second. That electron does not drift very far in 1/100 of a second, but the bulb lights up just the same.
     
  8. Jul 4, 2015 #7
    Thank you for your help.
     
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