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Electron volt

  1. Nov 15, 2006 #1
    got a question about the definition of an electron volt. The net says that it is the kinetic energy gain when an electron moves thru a potential diference of 1 volt. But the distance between the positive and negative source of that 1 volt... does it not matter or affect the definition? Because this distance is never mentioned in the definition.

    An electron that is stationary but then suddently exposed to a + and - charge source 1 meter apart and the - source positions it self right on the electron so the electron flies to the + source and at that instant when it reaches the + charge source has a total kinetic energy x which we all think is 1 electron volt right? Would the result be diferent if instead of 1 meters the distance between the + and - charge sources was 2 meters or 100 meters?

    Nothing is mentioned also about how long the electron has to be exposed to the 1 volt to gain 1 electron volt.

    Dave
     
    Last edited: Nov 15, 2006
  2. jcsd
  3. Nov 15, 2006 #2

    ZapperZ

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    It does not matter because the electrostatic FORCE also changes as you vary the distance between the two fixed potential Thus, the accelerating force differs with different distances between the two potential.

    Think of a ball rolling down an inclined plane. The potential difference is defined as the height. However, you can make the ball roll down an inclined of various angles. This will then dictate how far the ball actually moved. Yet. no matter what this distance is, the KE gained is still the same, since it is going in between two fixed potential in all cases.

    Zz.
     
  4. Nov 15, 2006 #3
    how come then dc current in a wire experience the same voltage no matter where along the wire u measure it's voltage ?

    also the dc current along the wire has the same amp no matter where along the wire u measure it.

    lastly, if a bunch of electrons and protons adding up to 1 volt between them started moving toward each other, at the moment they hit each other, will they both gail 1 electron volt worth of kinetic energy ?
     
    Last edited: Nov 15, 2006
  5. Nov 15, 2006 #4

    ZapperZ

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    The charge carriers in a wire, on average, do not experience an "acceleration". The drift velocity is a constant value, not increasing. So you cannot make the same comparision here with free charges. Why? Because you are ignoring a whole buch of scattering effects that occurs in a conductor that you do not consider in a free particle.

    Zz.
     
  6. Nov 15, 2006 #5
    oh ok i just thought maybe the wire extends the voltage strength at the source .

    last question , if a bunch of electrons and protons adding up to 1 volt between them started moving toward each other, at the moment they hit each other, will they both gain 1 electron volt worth of kinetic energy ?
     
  7. Nov 15, 2006 #6

    ZapperZ

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    I don't understand that question. Electrons and protons adding up to "1 V"? Are they "moving" due to each other's field? If they are, then you should consider that each of them is not seeing a constant, static field as they get closer.

    <scratching head>

    Zz.
     
  8. Nov 15, 2006 #7
    yeah their in space and moving toward each other. 2 clumps of oppositely charged plasma clouds speeding toward each other in 0 g vacuum. im guessing maybe 0.5 ev is attained prior to the formation of hydrogen gas cloud?
     
  9. Nov 15, 2006 #8
    they started off with 0 speed ofcoarse.
     
  10. Nov 15, 2006 #9

    ZapperZ

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    But there is no symmetry here. A proton is humongously heavier than an electron. Take an electron and a proton, separate them by a distance and let them go. Where do you think the electron will be when they collide? Where do you think the collision point will be when compared to the original position of the proton? Why do you think this is similar to the OP when the field that the electron sees isn't a constant as when it is in a fixed potential?

    Zz.
     
  11. Nov 15, 2006 #10
    oh, replace the protons with positrons then.
     
  12. Nov 15, 2006 #11

    ZapperZ

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    But that still doesn't change the fact that the charges are not moving in a uniform field. Try this:

    Put -q at x=-l. Put +q at x=+L, where L>l.

    Now bring +q from L to l. What is the work done here? This is essentially similar to your original question.

    Now look at another scenario. Put -q at x=-L, and put +q at x=+L as before. Now let them together move towards each other until -q reaches -l and +q reaches +l. Again, calculate the work done here.

    You'll see that those two cases will not give you the same answer EVEN when they both end up at the same location from each other.

    Zz.
     
  13. Nov 15, 2006 #12
    yes i agree, i said that the 2nd experiment would probably give 0.5 ev for each electron. but i suspect that's wrong too. how many ev will the electrons and positrons gain prior to annihilation?
     
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