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Absence of an electron at a point creates a positive charge

  1. Jul 3, 2010 #1
    I can't understand how does the absence of an electron at a point creates a positive charge there. In my opinion, it is wrong on account of conservation of charge. Because if we consider an electron(at a point A) as a system, and if the electron get displaced to another point B(in the system), then if we apply the concept of holes -> firstly, the charge of the system was -e and now the charge is +e-e = 0 !!. Which shows that charge can be destroyed.
    can any one help???
  2. jcsd
  3. Jul 3, 2010 #2
    Re: holes

    You need to distinguish between electrical neutrality at a particular location,(=local neutrality) and electrical neutrality for the whole body or system (=global neutrality).

    Atoms, for instance are globally electrically neutral.

    But they have regions of local charge, positive in the nucleus and negative in the surrounding electron cloud.
    So the sum of all the local non-neutralities balance out to global neutrality overall.

    Holes and electrons work similarly. At the site where the electron is displaced from there is a local region of negative deficiency and where is is displaced to there is a corresponding region of local negative excess, but the conductor remains in global neutrality.

    I would suggest you don't pursue this concept too far as the model involving electrons as little balls of charge moving around is seriously deficient in solids.
    It becomes easier to match more closely to reality when you study band theory.
  4. Jul 3, 2010 #3
    Re: holes

    It's OK for this case - just needs a little thought. The same mechanism occurs with landslip along faults and lattice movement at dislocations.

    Think about the electrons as being stuck to their atom like peas on a wire grill. Every pea sits in a hole.
    If you try to push the peas to one side, they won't move easily because each one resists and the total resistance is huge.

    If however, one pea is missing, what happens is that the pea upstream of it can slip into empty space. then the one behind it can slip into the space it vacated - and so on.

    The result is that the 'hole' moves in the opposite direction to the push. On a grand scale, you get lots of 'holes' rushing back upstream and looking to all the world like positive charge moving in the opposite direction to the actual electron movement.

    Some people like to treat it as if the 'holes' were actual positive charges - in a way they are and when doing calculations it's a lot easier to do the math that way. - but I prefer to keep in mind what's really going on when I think about the actual physics.
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