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Coulumbs and Amps

  1. Oct 15, 2004 #1
    A coulumb is a massive amount of charge, almost impossible to place a charge of 1C onto an object. Yet, a current of 10A, 10C/s is quite reasonable. Explain the discrepancy?

    I know it has to do with the fact that current is measured as the net charge over time, but I can't fully explain it. Can anyone help?
     
  2. jcsd
  3. Oct 15, 2004 #2
    You have misunderstood it a little bit. 10A currents means that 10 C of charge is flowing through the given part or wire, it means that 10 C charge is entering and leaving the wire at the same time. You can compare it to a pipe with water flowing in it. But you are taking it as 10 C current appears on the wire in 1 sec.
     
  4. Oct 15, 2004 #3

    cepheid

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    I'm curious: what do you mean by quite reasonable? In terms of physiological effects of currents, currents in milliamps can be quite harmful, and a 1 A current can be deadly. So from a human point of view, 10 A is not something you want to mess with. I haven't seen 10 A anywhere before, so I'm wondering if there is a specific device you saw that draws such a large current.

    (10 C/s) * (1 e / 1.60 x 10-19 C) = 6.25 x 1019 e/s

    Is that a reasonable rate of flow of electrons? Anyone?
     
  5. Oct 15, 2004 #4

    ehild

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    Yes it is. It depends on the voltage applied. For example, an electric heater of 2 kW connected to a 230 V supply draws about 9 A. (The power is voltage times current.) Just look around in your flat and check the power (watts) of the light bulbs, computer, micro, tv, and so on.

    ehild
     
  6. Oct 15, 2004 #5

    My book defines amps as C/S...or coulumbs per unit time...but the definition of current is given as the amount of charge flowing through a certain region, so I guess that's where my confusion is placed.

    This is a discussion question in my book. What I'm thinking is that it's 10 C flowing through the wire, not 10C stored in it, and that's why it's reasonable. Is this right?
     
  7. Oct 15, 2004 #6
    I have an air compressor and a small welder that require 15A. 15A is the max rating on most household outlets.
     
  8. Oct 16, 2004 #7

    cepheid

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    Ok cool^^. I just wanted to know.
     
  9. Oct 16, 2004 #8

    ehild

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    Current is the amount of charge flowing through the cross-section of the conductor in unit time. You can imagine that C/s in the following way. You charge a body, say, a big sphere through a wire that is connected to a source. The charge flows onto the sphere through the wire. The increment of the charge on the sphere in unit time equals the charge flowing through the wire during the same time, that is, the current.

    This is right. There is no charge stored in the wire, it contains equal amount of positive and negative charges, positive metal ions and negative electrons which are the "charge carriers" in metals. The amount of both the positive and negative charge is quite high (try to estimate, how much is it in a piece of wire) but they cancel each other. The current is proportional to the density of the charge carriers N, their average "drift" velocity v, their charge q, and the cross section A. I=NqvA.

    ehild
     
  10. Oct 16, 2004 #9

    Integral

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    Many CPUs (the device which is at the heart of your computers) dissipate about 70W, running at about 2V. This means your computer CPU draws around 35A.
     
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