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Conceptual Q about charge.

  1. Oct 8, 2013 #1
    If we have charge in an isolated conductor, is there any way to get it off without physical contact or arcing?
     
  2. jcsd
  3. Oct 8, 2013 #2

    UltrafastPED

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    The presence of a free charge indicates that there are extra electrons, or an electron deficit.

    If the charge is negative (electrons) the photo-electric effect plus a nearby anode would lead to the emission of electrons ... you would need to know when to stop the process!
     
  4. Oct 9, 2013 #3
    Hmm, I suppose this counts. It's not quite what I was thinking, though. In the photoelectric effect, we're sending energy to the conductor, right?

    I suppose in my original question, I was thinking in terms of fields. If an electric field is caused by charge, then could a charge experiencing an external field be drained away? But as you reminded me, charge is the result of mass carrying particles. I guess a chunk of mass just can't up and disappear... unless it obtains some energy and jumps off.

    ...it can also convert to energy? Are electrons allowed to convert into energy under any normal circumstances? Is anything?

    When I look at the definitions of Charge, Magnetic Field, Photon, Electric Field, Photoelectron, etc.. there are clear and obvious differences. But conceptually, these concepts keep bleeding together in my head.

    Sorry for the unclear questions, and thanks for your time!
     
  5. Oct 9, 2013 #4

    UltrafastPED

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    Electrons can be annihilated by positrons ... but this is a particle bombardment. Other than that electrons don't just disappear except in some nuclear reactions. But charge is absolutely conserved.

    If you impose a negative electric field the unbound charges of the isolated conductor will quickly be rearranged so as to cancel the external field on the interior of the conductor. There is a vast "sea of free electrons" within any conductor, and they can be rearranged within a few femtoseconds (10^-15 s). This is why charge appears to act as a fluid (currents, voltage "pressure") at the macroscopic level.
     
  6. Oct 9, 2013 #5

    UltrafastPED

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  7. Oct 9, 2013 #6
    When you say that 'charge' is absolutely conserved, can I replace 'charge' with 'electrons and protons are...' and keep the entirety of your meaning? I've been thinking of the subject in terms of fields and flow, but it can also be thought of in terms of individual mass caring particles?
     
  8. Oct 9, 2013 #7

    UltrafastPED

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    No ... protons are not conserved (they can be smashed, or modified by nuclear reactions, or annihilated), nor are electrons (which can be annihilated). But the net charge is always conserved.

    Thus positron (+1) and electron (-1) annihilate to generate two or three photons (zero charge). +1 + -1 = zero.

    There are other conserved quantities.
     
  9. Oct 9, 2013 #8
    A little more thought.. I guess a better way to ask my question would be, Can charge be thought of strictly as a property of mass? Further, strictly as a property of electrons and protons? (treating the components of the proton as a single entity, I'm not quite there yet)

    edit: Or positrons/antiparticles, apologies for leaving those out. haven't quite countered the anti particles yet, is there anything unique to the theory of charge that they add? Just opposites with all the same laws of physics?
     
  10. Oct 9, 2013 #9
    There are lots of different particles that carry electric charge. Charge is not a property of mass. It is a separate property on its own right.
     
  11. Oct 9, 2013 #10
    Hm, maybe if I open my question up a little more. How many different ways can an isolated conductor lose charge?
     
  12. Oct 12, 2013 #11

    Philip Wood

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    Thermionic emission (heat the conductor to high temperature), photoelectric effect (bombard conductor with photons of high enough frequency), secondary emission (bombard conductor with electrons), cold field emission (place the conductor in a very high electric field in a vacuum, by putting a high p.d. between it and a nearby electrode). That's all I can think of, though bombardment by particles other than electrons will no doubt also cause electrons to be emitted.
     
  13. Oct 12, 2013 #12
    Cold field emissions sounds like what I'm looking for! Could you explain it to me like I'm 5?
     
  14. Oct 12, 2013 #13

    Philip Wood

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    You could use a needle-like electrode,N, and place another electrode, A, of any shape very close to the needle point in a vacuum. If you put a high enough voltage between the two electrodes, with N made negative and A positive, then a very high electric field will exist near the needle point and electrons will be sucked out of the metal N and across the vacuum gap to A.

    This happens, but, according to pre-quantum Physics, it shouldn't. This is because the electron needs energy (the work function before it can get out of the metal. The fact that afterwards it can pick up a bonanza of energy from the electric field won't help it to escape in the first place. If a marble is just below the top of a rounded hill, it can't by itself start rolling towards the top and then right down the other side, however far it could roll down on the other side. But Quantum Physics, which came properly on stream in the 1920s, solved the problem by predicting the phenomenon of tunnelling through the hill. The chances of this happening for the marble are utterly negligible, but for the electron coming out of the metal they are not negligible (largely because the distance an electron near the metal's surface has to travel in order to escape is very small).
     
  15. Oct 12, 2013 #14
    Alright, these feel like stupid questions, but I can't really form an answer to them on my own.

    Why can't an electron pick up energy from an electric field? I guess I'm wanting to ask, what is an electric field?
     
  16. Oct 12, 2013 #15

    Philip Wood

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    It can - once it's outside the metal. The electric field is outside the metal. The electron has first to get out of the metal in order to experience the field. [An electric field is a region in which a charged particle experiences a force proportional to its charge. We create an electric field in the region between N and A by placing the voltage between them.]
     
  17. Oct 12, 2013 #16
    Oh wow, alright. And inside the conductor, the electron is still experiencing that force, right? This is the reasoning behind electrostatic pressure?

    If that force becomes strong enough, what happens? Will the conductor itself rip apart, or will the electron be pulled out of the conductor?
     
  18. Oct 12, 2013 #17

    Philip Wood

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    Your first point. No: not the force from the applied external field. The external field doesn't penetrate inside. That's why tunnelling is required.
     
  19. Oct 12, 2013 #18
    A free electron in an electric field will experience a force. But when that electron goes into a conductor, what happens to the force? Wouldn't it still have to be acting on the electron, just spread over the conductor?

    What's going with electrostatic pressure?

    [itex]P = \frac{e_0}{2}E^2[/itex]
     
  20. Oct 13, 2013 #19

    Philip Wood

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    "Wouldn't it still have to be acting on the electron, just spread over the conductor?"

    I don't think that that is at all the right picture....

    The force on the electron (charge –e) is –eE, in which E is the local electric field strength, that is the electric field strength at the point where the electron is. A simple large-scale picture, good enough for most purposes, is that E abruptly changes at the surface of the conductor, from being very high outside the surface (due, in this case to the high voltage between A and N) to being zero inside the metal. This has been discussed in Physics Forum in the past.

    When it comes to the details of electron emission, the picture I've just given isn't good enough. In particular we can't just say that E inside the metal is zero. We need to take account of the array of ions which gives rise to a periodically varying potential, to a band structure of energy levels, and to the existence of a work function. We're now into a quantum-mechanical picture. I believe that cold field emission (or, as, I think, it's usually called these days, field emission) is hard to model in detail, even using the quantum mechanical picture. Too hard for me, anyway. So let's hope someone more knowledgeable takes up the thread...
     
    Last edited: Oct 13, 2013
  21. Oct 13, 2013 #20
    We can leave the field emission thing alone if you'd like to. I'm still confused with the classical rules.

    I'm still confused with the force.

    An electron experiences F=qE in any field because it is a point charge, and a sphere experiences the same F=QE in a uniform field because it might as well be a point charge? Since the surface of a conductor is an equipotential, should I consider a conductor to be a "charge" with extra properties, as opposed to an electron being a charge with no properties (at this level at least). I'm not sure how to relate polarization to this, though.

    What do I do with this concept of an electron being ripped away from the conductor via an E field? Is this concept completely meaningless in the classical sense?
     
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