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Question about Electricity and Water

  1. Nov 14, 2004 #1
    I had a question about electricty and being electrocuted. It is well known that if you drop a toaster that is plugged in into a bathtub you are sitting in, you will be electrocuted.

    My question is, how far reaching is the electrical energy in a body of water?

    For example, if I am in standing in one end of an olympic sized pool and someone drops a plugged in toaster in the other end, will I be electrocuted like I would in small body of water such as a bathtub? Or is there to much area and the electrical energy dissipates? Any help would be appreciated, thank you!

    Last edited: Nov 14, 2004
  2. jcsd
  3. Nov 14, 2004 #2


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    Possibly, If you happened to be positioned near a suitable current sink.
    OTOH, if you were in a fiberglass tub with plastic drain plumbing you might be able to simply pick the toaster up and drop it outside the tub with no harm to you.

    Don't try this at home. :surprised
  4. Nov 15, 2004 #3


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    Tap water is a conductor of electricity, in the same fashion as wire but with higher resistivity.

    If it were electrically insulated, the entire body of water would be at the same potential, and, if you were grounded, touching it anywhere could be adequate to kill you. On the other hand, if the circuit you make involves kilometers of water, the total resistance through that water (in series with you) will be quite high, and not much current would flow.

    The resistivity of water varies immensely with the amount of type of substances dissolved in it; I can probably provide you an exact answer to your question -- how much water is necessary to provide a large enough resistance to avoid electrocution -- but I'll need to find the resistivity of pool water first. I wasn't able to find it after a few minutes of googling, so I gave up.

    - Warren
  5. Nov 15, 2004 #4
    I was swimming in the West Pearl River when lightning hit a tree about fifty feet from the bank. Some of the current spread out through the water. I was lucky to notice it at all because it was so weak and fast but I recognized the waveform signature. Some kids who were closer to the shore felt a stronger current and were impressed. No one was hurt but we were all properly scared and took refuge beneath a train bridge.
    I had a long lasting interest in electrolysis as a kid and did many experiments with DC and salt water. I found that if you make the connection between two cans of salt water, 20 or 30 volts is about all you can stand. With fresh water the voltage is higher. With dry hands you will need about 70 volts to feel a shock. I compared my findings with some NASA documents while working on Space Station and they were in the same ball park. Intact skin on the fingers has a resistance near 30K ohms. You will have a hard time feeling less than 0.1 ma. A cut or puncture will conduct more and hurt more.
    Electrical currents through the eye will cause you to see white light and not colors.
  6. Nov 15, 2004 #5


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    You could make some simplifying assumptions.
    Parallel plates with water between them.
    Maximum current for the standard US household outlet 20 amps @ 110v.
    Divide the pool into a number of parallel resistors the size of a heart, the key problem area. A guess at 16 sq inches.
    Say its survivable it the current stays at or below 5ma through your heart.

    ((20a * 1000)/5ma)*16in= 64000 sq in/144 = 445 sq ft = aprox 22 X 22 plate.

    The internal resistance of your body is very low so you want the pool long enough so the percentage change of the resistance of a column is small.
    Knowing the actual water resistance here would be helpfull.

    In the above setup your position between the plates won't really matter.
    For point sources and sinks the problem becomes much more complicated.
    Your location matters.

    For an actual toaster the majority of the current will flow within the toaster and the pool itself will only get some minor residual currents.
    Unfortunately, a residual current of 10ma across your heart is quite sufficient to kill you.

    The above exercise really does not mean much in terms of the original question, since the outcome will be determined by the available current sinks and your position relative to them and the toaster. Current will always take the shortest path. Don't get in the way.
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