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What resistance is in terms of electrons

  1. May 8, 2005 #1
    please can someone explain to me what resistance is in terms of electrons please please im so stuck!!!! :confused:

    i understand what affects it the width of wire and length etc but i don't understand why it is that the electrons find it harder or easier to pass through!
     
    Last edited: May 8, 2005
  2. jcsd
  3. May 8, 2005 #2
    Well imagine two pieces of copper wire. One is 20mm thick and the other is 20m thick (just exmaples remember).

    Let us take the diameter of the electron to be 5.64 x 10-15 m. The number of electrons that can pass along the wire next to each other is:

    - for the 20mm thick wire = [tex]\frac{20 \times 10^-3 \ m}{5.64 x 10^{-15} \ m}[/tex] = 3.55 x 1012 electrons.

    - for the 20mm thick wire = [tex]\frac{20 \ m}{5.64 x 10^{-15} \ m}[/tex] = 3.55 x 1015 electrons.

    More electrons can pass in a 20m thick wire than in a 20mm thick wire. This means the resistance of the 20m thick wire is less than that of the 20mm thick wire.


    Now imagine two pieces of copper wire. Both are 20mm thick but one is 20m thick and the other is 200m thick (just exmaples remember). It will take longer for an electron to travel 200m than 20m.

    This means the resistance of the 20m long wire is less than that of the 200m long wire.

    Does this make more sense now or do I need to do some more exampling??? :smile:

    The Bob (2004 ©)
     
  4. May 8, 2005 #3
    The Bob,

    You just made all that up, right?
     
  5. May 8, 2005 #4
    By the way you are sounding serious, yes. :frown: It is just one way to imagine it, or am I being stupid?

    The Bob (2004 ©)

    P.S. Nuts I feel bad now.
     
  6. May 8, 2005 #5
    The Bob,

    You're not being stupid, you're just uninformed.

    The question by huh! was why electrons find it "harder" or "easier" to pass through a thin vs thick wire or a long vs short wire. The quick answer is, they don't. The shape of a wire has nothing to do with how easy or hard it is for the conduction electrons to get through it. Your answers gave the impression that it does.

    Ohm's law doesn't say that it's harder for the electrons to get through a higher resistance. It just says that for a given voltage, fewer electrons will enter or leave the resistor (in general, fewer will pass some arbitrary point) in a given amount of time. Thin wires have fewer electrons per unit length, so for a given voltage, you get fewer of them passing in a given time. So the resistance is greater. In a long wire the voltage drop is spread over a longer distance, so the field that makes the electrons move is weaker, so the electrons don't go as fast. So for a given voltage you get less electrons passing in a given time. Again, greater resistance.

    At extermely high voltages, this simple model breaks down. But as long as V = IR, you don't have to worry about the electrons crowding in a thin wire, or that they have a greater distance to go in a long wire.
     
  7. May 8, 2005 #6
    This is where a hole in my knowledge has now been filled.

    :smile: :biggrin: Thank you very, very much. :biggrin: :smile:

    The Bob (2004 ©)
     
  8. May 9, 2005 #7
    Thank you that makes a lot more sense than that of the books i was using and sites i was using to research it!
     
  9. May 9, 2005 #8

    DaveC426913

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    Gold Member

    Resistance ... is ... useless.
     
  10. May 9, 2005 #9
    I thought it was futile. Oh well.......
     
  11. May 9, 2005 #10

    Gokul43201

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    Make sure you understand jdavel's post completely. This is an important fundamental concept that is often misunderstood. If there's even some trivial detail that does not seem completely specified, do not hesitate to clarify.
     
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