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B Battery and capacitors

  1. Jun 30, 2016 #1
    (Look at the level above)

    If you have a battery which is made by two plates having opposite charges ( constant E-Field), But outsidethe electric field doesnt cancel out. (The same applies to capacitors) not infinite

    So the question is why doesn't an electron get affected by a really huge force when it gets so close form the positive or the negative terminal from the outside?
    Last edited: Jun 30, 2016
  2. jcsd
  3. Jun 30, 2016 #2


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    Have you tried calculating the force?
  4. Jun 30, 2016 #3
    Yep I did on really small scales it was huge around 10^-14 m from the center of an electron for example (Using coulomb's law) from just a single electron. The electric field from other electrons will add up too (not directly but by a component at least).

    Still haven't learned how to integrate (This year I hope). What should be the work done by it?
  5. Jul 1, 2016 #4
    Can you explain your reasoning why it should be affected by a large force ?
    The effect could AT MOST be as strong as if it was inside the capacitor.
    You don't actually push any electrons that close together..
    The other electrons get repelled and leave some room where the next electron can "sit".

    I also think that it is more useful to think of charge as a continuum rather than discrete charges in this case since there are so ridiculously many electrons that they essentially behave like a continuum.

    Oh and if you think "but the force between two bits of charge is steadily increasing as you move closer, how can the force remain almost constant"
    The answer is : While the force from one bit of charge increases the angle of the force of the other charges becomes "worse" these effects cancel each other out.
  6. Jul 1, 2016 #5
    For example, Assume that one plates is connected to the negative terminal of the battery. On that plate, it will gather a negative charge. How can the force of this electron or (charge)on a plate remove an electrons from its atom on the other plate where the force is so strong?
    Last edited: Jul 1, 2016
  7. Jul 1, 2016 #6
    I still don't get it. Why would an electron be "removed from its atom". We are in a conductor and probably a metal.
    The electrons aren't bound to a single atom...
    Could you please formulate an easily readable sentence ? Maybe consider making a diagram.
    This really doesn't make much sense to me.
  8. Jul 1, 2016 #7


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    I think what Biker means is...

    Assume that one plate of a capacitor is connected to the negative terminal of a battery and the other plate is connected to the positive terminal of the battery. The negative plate will gather a negative charge. How can an electron (E1) on the negative plate repel a second electron (E2) from the positive plate? Surely the force holding E2 to it's atom is stronger than the force between E1 and E2 because it's very much closer to it's atom than it is to E1?
  9. Jul 1, 2016 #8


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    The answer is that the atom of the positive plate have some fixed electrons as well as protons.



  10. Jul 1, 2016 #9


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    You can't simply use coulomb's law to calculate the force when you're dealing with extended objects like plates of a capacitor or terminals of a battery. As tazerfish said in post #4, the charge on each plate is spread out all over the the plate. As the electron approaches, different parts of the plate are at different distances from the electron, so the force exerted by the charge at each section of the plate on the electron is different and will be much, much less than assuming the charge on each plate is at a single point. Also, 10-14 m is about a thousand times smaller than even an atom, which is around 10-10 m, so you wouldn't use distances of this magnitude to calculate the force on an electron in a circuit. In addition, at this scale even coulomb's law begins to fail and you would need to use quantum physics to accurately calculate things.

    In a conductor, one or more electrons from each atom are "free", meaning that they are not bound to a single atom. Instead, they move around the entire conductor and are able to respond to electric and magnetic fields, which is why a conductor has very low electrical resistance and other properties. It takes very little force to move an electron off of a conductor. It's only when you remove a great many that the force needed becomes large.
  11. Jul 1, 2016 #10
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