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Positive and negative charges(mainly relating to copy machines)

  1. Apr 20, 2003 #1
    Why is it that black colors create a positive elctrical charge, while white colors send a negative charge? Does it have to do with different wave lengths?

    When light (in a copy machine) is focused on to the photoconductive drum, why and how does the black color become positively charged? I know that the ink that is applied is negatively charged, and through the priciple of opposite attraction, the ink is attracted to the colors (black or whatever, as long as it isn't white). But why can't the ink get negatively charged? The black colors are already positively charged.

    How do things get positively or negatively charged? I know its when an ionic bond takes place, and an atom has more electrons than protons, but how is this induced to occur? Why does this sort of reaction create a charge?
  2. jcsd
  3. Apr 20, 2003 #2
    OK, the situation in copy machines doesn't have anything to do with the wavelength. It's actually arbitrary -- here's what happens. First, we charge up the drum, which I'm going to draw as a flat plate:
    Code (Text):

    + + + + + + + +     positive charge
    ===============     photoconductive plate (in the dark)
    - - - - - - - -     negative charge
    Now, the photoconductive plate is a special type of semiconductor which only conducts electricity when you shine light on it. Light from a lamp is bounced off the paper; the white reflects the light, the black doesn't. This is basically the definition of white/black -- we see something as black when there's no light coming from it, and white when it's an even mix of colors.

    So the light only gets reflected off the white parts, and hits the corresponding parts of the plate. Those parts of the plate become conductive, and the opposite charges flow across it and cancel.
    Code (Text):

    + +       + +
    - -       - -
    ^^^Where the white area is in the middle.

    To create the original charges, we basically create a really strong voltage that can pull electrons off the top of the plate, into some wire. The voltage is got from the electric power lines in the wall.... that power is originally generated usually using some sort of magnets. See "dynamos."

    Ionic bonding is something that charged particles can do, but it's not relevant to electricity creation.
  4. Apr 20, 2003 #3
    What exactly is a semiconductor. I just read a definition that said

    "Semi-conductors such as Silicon (see p.58) fall into this category."
    'This category' being "...the conductivity of some substances varies significantly with certain conditions..."

    Not a very good definition, so can you explain it to me?

    That's really interesting. But how do you get the electrons into the wire? Why do you need electrons in the first place? They're negatively charged. Or do you put an extra electron in a negatively charged atom?
  5. Apr 21, 2003 #4
    Umm ... i thought duplicates of the same topic in more than a single forum are not allowed in PF ... anyway ...
    You may know more here , the page also include some information about how light will affect the photoconductive plate.
  6. Apr 21, 2003 #5
    Staii, was this thread that you speak of, by any chance, started by me? I started a thread on this very subject, but the problem is I can't find it.
  7. Apr 23, 2003 #6
    Did you check the "How Stuff Works" forum ? (Well, I did !)
  8. Apr 23, 2003 #7
    Oh, I never new that forum ever existed! I'll go find it....
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