What does +ve charge in conductor means, do they move

  1. Hello pf,

    Electrons are the charge carriers in metal conductor. while explaining about the fact the fact that electric field inside a conductor is zero, there is a depiction showing charge moves to surface (+ve and -ve charges to extremes of metal conductor surface) in emt books of sadiku and griffiths etc. ok -ve charge refers to electrons , but what about +ve charges? are they immobile ions. please help..
  2. jcsd
  3. sophiecentaur

    sophiecentaur 14,714
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    Positive ions in a metal (except mercury, of course) are not mobile. I imagine that a bowl of mercury would slosh over towards the negative terminal as the positive ions all flowed until the gravitational force limited their movement.

    You just made me think. If you could suspend a metal ring and induce a current around it, would there be a measurable force / rotational motion is the direction of the conventional current?
  4. Sir I did not understand what you told. It did not cleared my doubt.
  5. Drakkith

    Staff: Mentor

    If electrons in the metal move to one side they leave behind an area that is depleted in negative charge. Each electron can be thought of as leaving one "hole" behind that has a + charge. The nuclei of the metal atoms are immobile. They do not move.
  6. rollingstein

    rollingstein 636
    Gold Member

    Isn't that the Hall Effect Experiment?

  7. I thought that 'hole' is a concept applicable to semiconductors only. Still in doubt.
  8. Drakkith

    Staff: Mentor

    That's fine. In that case just realize that when a bunch of electrons pile up on one side of the conductor it leads to charge imbalance. So one side has more negative charges than positive, while the other has more positive charges than negative. This is simply the result of electrons moving around, the ions don't go anywhere.
  9. sophiecentaur

    sophiecentaur 14,714
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    It's certainly based on the same idea, only in this case, I think the material itself would move. There is a problem of 'detail' here though. For conservation of momentum, the drift electron momentum must be equal to the negative of the momentum of the ring (normally we would be talking about the momentum change of the Earth, which really would be silly). The atomic mass of copper is about 60 so the relative velocities of electrons and copper ions would be 120,000 : 1. The drift speed of electrons is around 1mm/s so we are talking of a very low speed for the copper but one which could just be detected using optical interferometry, being around 1nm/s. The suspension would need some ingenuity. Perhaps a very long carbon fibre thread?
  10. This is not true. The charge carriers in a real metal are not always conduction-electrons. Sometimes, the charge carrier in a metal is a valence-hole.

    My favorite reference on solid state is:
    “Introduction to Solid State Physics 7th Edition” by Charles Kittel (Wiley, 1996).

    On page 167 (Chapter 6, Table 4), there is a list of metals, their Hall-coefficients and their free-carrier types. Most metals have conduction-electrons as carriers. Aluminum and indium have valence-holes as carriers. Both aluminum and indium are true metals and good conductors.

    Valence-holes carry electric charges in some trivalent metals. However, the carrier-type of beryllium was left out of Table 4 of Kittel. I don’t know why. Beryllium has a positive Hall-coefficient, so one would think that it is a hole-metal.

    Note that in all these cases of “true metal”, electrons really carry the electric charge. Both conduction-electrons and valence-holes are examples of quasiparticles. Both conduction-electrons and valence-holes are composite systems made of true electrons.

    Ions don’t usually carry electric charge in solids. I will not discuss those anomalous examples where ions do move in solids. You will recognize them when you read about them. Moving ions do carry electric charge in electrolytes and in plasmas.

    These quasiparticles behave differently than the electrons would behave in a vacuum. For example, both conduction-electrons and valence-holes have an effective-mass different from the real mass of an electron. Their “particle-like” behavior breaks down under some circumstances.

    You shouldn’t think of the conduction-electron as being any more like a real-electron than a valence-hole. Furthermore, you shouldn’t think of the valence-hole as being any less “real” than a conduction-electron. This is why I always prefer to use an adjective to qualify the word electron or hole.

    Kittel describes the measurement of the Hall-coefficient. If the Hall coefficient is negative, the free-carrier is a conduction-electron. If the Hall-coefficient is positive, the free-carrier is a valence-hole.

    Some other group had this discussion. However, I particularly like the point that this poster made.
    “There are two essential facts that make a hole a hole: Fact (1) The valence band is almost full of electrons (unlike the conduction band which is almost empty); Fact (2) The dispersion relation near the valence band maximum curves in the opposite direction to a normal electron or a conduction-band electron. Fact (2) is often omitted in simplistic explanations, but it's crucial, so I'll elaborate.”
  11. sophiecentaur

    sophiecentaur 14,714
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    I know that. in many ways, holes 'act' like particles but there is no actual movement involved of anything other than electrons. Holes are useful constructs but much less 'real' than electrons.
    You put electrons in one end and you get electrons out of the other.
  12. You be careful. Neither conduction-band electrons nor valence-holes are fundamental particles. They can both be considered quasiparticles that emerge from multielectron-systems in the material.

    Conduction-electrons are not any more real than holes. The electrons of the conduction-band are electronic excitations whose properties are determined by the potentials of the electrons in the crystal. Conduction-band electrons are not fundamental particles, any more than holes are fundamental particles.

    For example, the effective-mass of a conduction-band electron can be much less than that of the free-electron. Furthermore, the effective-mass of the conduction-band electron can be anisotropic, which isn't possible for a free-hole.

    Consider the case of a semiconductor or even a semimetal with an indirect bandgap. The electrons at the Brilluoin edge can dominate the transport of electric charge. The electron of minimum energy can have maximum pseudomomentum There are also effects like the Gunn-effect.

    You are correct that electrons come out onto the wire. Unless the wire is made of a trivalent metal like aluminum. The majority carrier in aluminum is the valence-hole, not the conduction-electron.

    There seems to be this common misunderstanding that conduction-electrons are "real" electrons and valence-holes are "unreal" positrons. Conduction-electrons and valence-holes are both single-particle approximations of multielectron bands.
  13. sophiecentaur

    sophiecentaur 14,714
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    I was asking for that, wasn't I?
    A soon as I pressed the 'post quick reply' button, I realised there would be a reply like yours. :wink:
    I am the last person to insist that anything is 'real' and I take your points wholeheartedly. Could I suggest that you might make an effort to reply in a similar vein to posts that insist on the mechanistic view of things like 'electricity' and demand explanations that they 'can get their head round'? Too few of PF's contributors appear to accept that Physics isn't as simple as that.
  14. jim hardy

    jim hardy 5,460
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    ahhh - one more piece in that fifty year puzzle i'm working. Thanks, guys!

    I was taught conduction in semiconductors is like chinese checkers - marbles move and holes stay in the board.
    Particles are imagined more easily that fields. I guess that's because we start physics with studies of Newton's laws.
    Perhaps educators will learn to start us with Maxwell's instead. My exposure to them was brief, and only in in third semester physics.

    Mortals like me use simplified models to get through life, to earn a living by fixing machines and teach others to do the same.
    I have always envied those of you you who understand more deeply.

    thanks again,
    old jim
  15. sophiecentaur

    sophiecentaur 14,714
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    That sort of model works fine and it's only when some clown asks about what's "really" going on that there's a contention. It makes an even bigger mockery of the 'electrons flowing like water round the circuit' idea and further justifies my contention that people should just use Current and Charge and get on with it. (or Over It, perhaps).
  16. jim hardy

    jim hardy 5,460
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    methinks the college textbooks and teaching methods are moving in that direction.
    Even in 60's we used 'conventional current',
    but i worked with so many Navy Nukes it was imperative to be conversant in both electron and charge approach.
    The 1960's Navy training produced exremely competent, practical people who, in an industrial setting, could run circles around many university undergrads.
    I dont know what is current Navy approach.

    You may notice i've pulled away from the water analogy in my explanations.

    old jim
  17. I do try. You are not the first person that I have addressed on such points. However, I may not be able to do so much longer.

    I almost didn't reply to your link because I was afraid of being dinged by the moderators, again. I appreciate your telling me that I was right, or at least well-read on the subject. Most teachers don't present these points in introductory courses, right? However, I hope that you also agree that they are part of "mainstream science". Saying something in a novel way doesn't make it wild speculation.

    For some reason the moderators haven't been appreciative of all my physical insight. I have been reprimanded many times that I don't know what I am talking about, that I have been willfully misleading the PF public and that I would be banned outright unless I started to take their criticisms seriously.

    I would certainly comply if I understood what they were talking about. They just say that I haven't read much about the subject. I have posted what I thought were appropriate links. However, they often accused me of making the stuff up anyway.

    I don't get a clear explanation of what I said wrong. I wouldn't mind getting corrected even in a vicious way if my sin were just pointed out. <e.g., Now listen, Turing, haven't you read...>

    I have this feeling that my membership on this forum is doomed no matter what I do. However, I have been sparing with my replies so as to delay my exclusion.

    Anyway, I am glad that you thought my reply appropriate. It does make me feel better.
  18. rollingstein

    rollingstein 636
    Gold Member

    Which posts? Have a link?
  19. I don't have a link. Worse, the forum search engine doesn't find it! The record of it seems to have been expunged from the forum.

    I was told that my comment post would be expunged. However, why hide the thread just because I am an idiot ?-)

    I thought Cragar asked a good question for someone very new to relativity.

    I do have a copy of the infamous exchange. A moderator sent me a message with a copy of the whole sorry exchange. He simply accused me of misleading the public and warned me not to do it again.

    I'll copy just part of the dialog, skipping both moderator comments.

    Quote by cragar
    "If I had 2 charged slabs that were moving together at relativistic speeds would both slabs
    emit cherenkov radiation. In each of the rest frames of the slabs the other slab is approaching it at a speed that is faster than the group velocity of light in its material."
    Quote by Darwin123
    "According to special relativity, that is not true.
    If in the center of mass frame each slab is moving at a speed, u, then the relative velocity as measured in the rest frame of either frame is w, where,
    One can show that if u<c, then w<c."
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