Drift velocity and measurement in wire

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In class today we talked about drift velocity and we measured it for a 14 gauge copper conductor wire. It came out to be 3.55 * 10^-3 cm/s. I was wondering if the speed is so slow, then how can light come on instanteneously when you flip the switch over long distances? Is it because there might be something in the wire to push it foreward?
thanks
nertil
 

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  • #2
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Say I poke you with a stick. Does the drift velocity of stick make much differences to how quickly I get a reaction?
 
  • #3
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So does that mean that it depends on the type of wire used? I want to know in terms of physics.
 
  • #4
Dr Transport
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Remember that Ohm's law is really stated [tex] J = \sigma E [/tex], the electric field moves at nearly the speed of light in a material, so that is why the light comes on pretty quickly after you turn on the switch.
 
  • #5
Yea, once the circuit is cut the electrons are still in every part of the wire. Once the connection is made they move continously.

What I don't understand is how this would work in long wires that carry data. Since it is not a continuous stream of electricity. Wouldn't the drift velocity apply to data in a wire?
 
  • #6
Gokul43201
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No, it's the same deal with data. You are merely transmitting a voltage from one point to another. You do that at the speed with which the electric field propagates.
 
  • #7
marcusl
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So does that mean that it depends on the type of wire used? I want to know in terms of physics.
Yes, the drift velocity depends on the type of wire used. Electrons are accelerated by the applied electric field, but scatter off of thermal vibrations ("phonons"). Accordingly, wire resistance drops, and drift velocity rises, with temperature, until reaching constant values due to frequent collisions with impurities and imperfections in the metal crystal. This occurs near absolute zero for pure metals, at higher temperatures for less pure metals.

If the metal is impure enough, then impurity scattering and collisions dominate. Pure annealed copper is a terrific conductor (annealing relieves stresses and produces good crystal structure), quenched pure copper has poorer crystal structure and worse conductivity, and brass (which is copper with zinc added) is worse yet.

Finally, drift velocity is directly proportional to the applied electric field.

Here's a web page that may be useful:
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/ohmmic.html" [Broken]
 
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