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Need help with voltage and current~

  1. Jun 28, 2004 #1
    i m studying cathode ray currently, but i was confised with the

    idea of voltage and current flow

    i recalled that voltage is a potential difference in a circuit, which allow current flow. but as i reseach on the interent there is some site talking about negative or positve volatge???????

    2. i remmber that when a voltage applied, a current will flow from positive to negative

    but how come cathode ray is the opposite?

  2. jcsd
  3. Jun 28, 2004 #2
    Consider two plates, one charged positive and the other charged negative. The difference in potential between the two plates is defined as the voltage. The potential is defined to indicate how much potential energy a charged particle would possess if placed at the point where the potential is defined.

    So a point having a potential of 200 volts indicates that a +2 coulomb charge would have a potential energy of 200*2 = 400 joules when placed at that point.

    At another point the potential could be (say) -200 volts. In that case the +2 coulomb charge would have a potential energy of -400 joules.

    So if the charge moved from the first point to the second point, it would move through a potential difference of 400 volts and lose 800 joules of potential energy in the process. (If there are no forces acting on the charge, it would then gain 800 joules of kinetic energy during the process.)

    So we say that 400 volts of voltage exists across the two points.

    (Yes, two coulombs of charge is ridiculously large, but I like to keep the math simple until the concepts are understood.)

    A positive voltage (I am guessing, because the term is not well-accepted) would indicate movement from a point of low potential to a point of high potential. I would stay away from the concept of positive and negative voltage completely.
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