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Electricity Question!

  1. Mar 11, 2008 #1
    Hi I read that in an encyclopedia but I don't understand it.

    "Because the surface of the material is now electrically charged, either negatively or positively, any contact with an uncharged conductive object or with an object having substantially different charge may cause an electrical discharge of the built-up static electricity; a spark. A person simply walking across a carpet may build up a charge of many thousands of volts, enough to cause a spark one centimeter long or more. This type of discharge is usually harmless because the energy ((V2 * C)/2) of the spark is very small, being typically several tens of micro joules in cold dry weather, and much less than that in damp conditions."

    THE RED PART iS confusing to me.
    How come one can creat thousands of volts by rubbing their feet on a carpet while walking? And if that's true the energy is not small at all... E = (V^2xC)/2.... if u have 5000 volts... ?///

  2. jcsd
  3. Mar 11, 2008 #2


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    Hi zinedine!

    Think of yourself as a 5,000-volt battery.

    An ordinary 1.5-volt battery has exactly 1.5 volts, no matter whether it's fully charged or almost expired. Even when it doesn't have enough power to light a light bulb, the potential difference between its plates is still 1.5 volts.

    You are a 5,000-volt battery which is almost expired. You have hardly any charge, but what little charge you do have is 5,000-volts different from the earth's potential.

    To put it another way:
    Electric potential is like gravitational potential.

    Imagine that a lot of electrons were about to fall on you from a height of 5,000 feet (ignore air resistance).

    If there were billions of them, and they fell in a lump, they'd have a huge amount of kinetic energy, and you'd be crushed.

    But if only a few electrons fell, they'd have hardly any kinetic energy, and they wouldn't do much harm!

    The spark contains electrons "falling" through 5,000 volts. But so few of them fall that the total energy is so small that it does you no harm! :smile:
  4. Mar 11, 2008 #3
    Thanks, I think I got it!

    So what kills you in real life is the amount of electrons passing thru ur body, not the voltage. The higher the amperage the more dangerous is ? Am I right
  5. Mar 11, 2008 #4


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    :smile: Your fear of carpets is cured! :smile:
  6. Mar 12, 2008 #5


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    Why you have hardy any charge ? there are so many electrons in your body.
  7. Mar 12, 2008 #6


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    Yes, but theyr'e almost all balanced out by the opposite charge of the protons in your body.

    Only the electrons from the carpet have no protons to balance them, and so your charge comes only from them.

    And there's very few, so your charge is extremely small. :smile:
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