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Conduction of electricity

  1. Jul 7, 2011 #1
    air, glass, mica, rubber, water, etc all are insulators because they don't have free charges to carry the current.... I don't understand what do we mean by "carry"?? i mean, i don't really understand what happens at the micro level during electrical conduction through a medium...How do the free(mobile) charges "carry" the current..
     
  2. jcsd
  3. Jul 7, 2011 #2
    Charge, like mass, is a property of matter.
    It is more complicated than mass because simple experiments show that there are two types or polarities.
    All matter possesses mass, but not all matter possesses charge.

    Experiments show that uncharged (neutral) matter exerts a force of attraction between two masses and that this force is governed by the inverse square law. We call this gravitational attraction.

    Work is therefore done on the mass of any matter moved against this force.

    Further experiments show that an additional force exists between charged matter, over and above that exerted by gravity. It is further observed that the direction of this force depends upon the relative polarities of the participating charges. We call this electrostatic attraction or repulsion.

    Matter of any size, from sub atomic particles to astronomical objects may posses charge. In general we are concerned with charges attributed to two particular sub atomic particles the proton (+ve) and the electron (-ve). The electron in particular is often shortened to 'charge carrier' or just 'charge', but it should be remembered that it also possesses all the other properties of matter.

    go well
     
  4. Jul 7, 2011 #3
    @studiot.. thanks for the answer.. but i already know all of it... i am asking something else....

    HOW is charge carried by mobile electrons??? Is it like when you apply potential difference, then the mobile electrons present in the material are attracted towards the positive plate and move towards it... If this happens, then won't all of the elctrons get accomodated on the positive plate and no more mobile electrons will be left in the material???
     
  5. Jul 7, 2011 #4
    Charged particles of matter = charge carriers = electrons in this case are able to move around the crystal structure (lattice) of a solid conductor.
    That is why they are called mobile electrons.

    They are able to do this because, unless the crystal is at absolute zero, some of the thermal energy is distributed amongst the electrons in general. The hotter the cystal the more the available energy.
    This energy is not distributed in an even way. Some electrons have more, some have less. Of those that have more some have enough to become mobile.

    These mobile electrons move (drift) around the solid in a random fashion with no net direction, until we apply an electric field (voltage).

    When we apply a voltage it does indeed cause the electrons to move towards the positive, on average.

    This 'piling up' of electrons at the applied positive end and consequential depletion of electrons at the applied negative end constitute a voltage in the opposite direction to the applied voltage.
    The piling up continues until this internal voltage equals the applied voltage.
    If we remove electrons at the positive end, by completing the circuit, then current will flow, according to Ohm's law.

    I called it a 'piling up' but that is rather a grand term. In actual fact it is really an increased drift in one direction over another.
     
  6. Jul 7, 2011 #5

    Drakkith

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    Staff: Mentor

    Current is simply the flow of electric charges through a medium. If electrons, which are considered charges, move through a conductor we refer to them as "carrying" the current. In a similar way water molecules "carry" the current in a river. In reality both of them ARE the current. The difference is that in a conductor you also have electrons which DON'T move around because they are not valence electrons. They are charges, but don't "carry" any current.
     
  7. Jul 8, 2011 #6
    Hello Drakkith, it is sometimes really difficult to avoid giving the wrong impression when making general statements at a more elementary level.

    All electrons in a normal substance are originally 'valence' electrons.

    If there were additional non valence electrons the substance would not be electrically neutral!

    The truth is very complicated but essentially it works like this.

    When neutral atoms join together to form solids their individual electron orbitals also join up to form what are effectively giant orbitals that encompass the whole solid,
    We call these bands.
    Each band incorporates many quantum levels, (there must be many because there are many electrons to be accommodate in a mass solid and you can only have one electron per 'slot').

    Just like in isolated atoms the levels vary in energy and have gaps between in an energy diagram.

    I have shown this diagrammatically in Fig1

    In both the isolated atom and the multi-atom band we electrons naturally start at the bottom of our diagram and work up until there are enough to balance the positive charges of the nuclei that are present.
    Additional thermal energy or energy from an electric field allows electrons to jump up to higher levels as already described.

    This is not a question of spatial distribution ie distance form the nucleus both Figs 1 and 2 are energy level diagrams.

    The lowest (=least energy) band is usually filled or nearly filled and called the valence band because the electrons here are tied to their positions.
    The next band is usually empty, apart from the thermally promoted electrons which jump up and fall back in dynamic equilibrium. This band is called the conduction band, because the electrons here have the mobility to move about and transfer charge, as previously described.

    It is the different structure of these bands and gaps that determines whether a substance is a conductor, a semiconductor or an insulator.

    In a conductor the conduction band is close to or touching the valence band so electron exchange from one to the other costs little energy and is easy and frequent.

    In semi-conductors and insulators the bands are separated by a gap which reduces the exchange as the gap increases.

    I have shown this diagramatically in Fig2

    Go well
     

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  8. Jul 8, 2011 #7
    There seems to be a bit of a disagreement here and it's mainly to do with a definition.Not all electrons are valence electrons.Valence electrons are those electrons which participate in chemical reactions and which carry current,they are to be found in the outermost shell(s).
     
  9. Jul 8, 2011 #8
    Yes you are right, I was guilty of my own canon. Thank you for the correction.

    :blushing:

    However there idea of an 'outer' shell is suspect as all shells have values right to the nucleus.
     
  10. Jul 8, 2011 #9
    @studiot Thank you so much... it really helped, your second comment
    just one more thing to ask...
    figuratively speaking, if you move the accomodated electrons from the positive end through the battery and back from where the cycle started??? is that what you meant??
     
  11. Jul 8, 2011 #10
    Yes that's exactly it. Although perhaps I should have said "allow the electrons to..." because you don't have to do anything once the circuit is complete - it will happen naturally.

    Incidentally, in case you ask any more questions, it would help to know if you are studying what we call conventional current that is from positive to negative or electron current that is from negative to positve.

    go well
     
  12. Jul 8, 2011 #11
    I was talking about the electron current... thanks for all the help... I was just revising some of my concepts.... thanks
     
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