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A toaster and a bath

  1. Feb 2, 2005 #1
    I've seen enough movies to know that tossing a plugged-in appliance like a toaster into a bath could be dangerous for the person in the tub. My question is, why, specifically, is it dangerous? Why would the electricity bother going through the person? I'm not going to try it out myself, but I am curious.

    My guess is that the current shorts out the toaster and trips the circuit breaker, but I can't see why the person in the tub would get a shock. I'd figure the hot and the neutral in the toaster, once exposed to the water, would close the circuit and trip the breaker. The person in the tub would not be part of the circuit.

    Thanks for any insight.
     
  2. jcsd
  3. Feb 2, 2005 #2
    any low resistance path to earth ground from mains live can kill, this is what happens
    when you drop a live toaster into water
    ---------------------------
    My guess is that the current shorts out the toaster and trips the circuit breaker
    ---------------------------
    there is also the path to earth ground
    ----------------------------
    The person in the tub would not be part of the circuit
    ----------------------------
    there is also the path to earth ground
     
  4. Feb 2, 2005 #3
    espear:
    and trips the circuit breaker
    ------------------------------
    at 50ma the person could die circuit breakers trip at 10A or more
    too late to save a life
     
  5. Feb 3, 2005 #4
    But where is the path to earth ground? That is my point, really. I don't see how a person is part of that circuit.
     
  6. Feb 3, 2005 #5

    russ_watters

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    I've always wondered this as well - the person really shouldn't be a part of the circuit - the wires inside the toaster are close together and there is an easy path from one to the other to short-out the toaster without any of the electricity going through the person. And even if electricity goes into the tub, it'll head for the drain (if its metal).

    I think its a myth, but I'd like confirmation if anyone really knows...
     
  7. Feb 3, 2005 #6

    DaveC426913

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    Doesn't the path spread? While the path does go to ground, not 100% of the current will take that path directly. The water itself acts partly as a small electron sink (no pun) does it not?


    The ratio of current shared betweeen all paths would be inversely proportional to the resistance met. So, if the water provides 1000x more resistance than the wire-to-ground, then 1/1000th of the current (.01 amps) would still pass through the water. (You would only get 0 if the toaster circuit were superconducting.)


    Think of turning on a faucet and pouring water down a drain. It will all go down the drain eventually, but if there's something in the way, or if the faucet is flowing too fast, it will not all go directly down the drain; it will begin filling the sink.

    To continue the analogy, the drain is not quite as large as the faucet - reflecting the fact that there is some resistance in the wires for the toaster.
     
  8. Feb 3, 2005 #7

    dduardo

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  9. Feb 3, 2005 #8

    russ_watters

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    I'm sure it would act as a bit of a sink, but that doesn't give you a shock: when you hold on to a Vandegraff (sp?) generator, it gives your entire body a charge (a high voltage one). But you don't feel a shock until you touch something else, and then not much of a shock because while the voltage was high, your body doesn't make a very good capacitor and can't hold much of a charge.

    For the charge spreading out - certainly. But how much? I'm really not sure...
     
  10. Feb 3, 2005 #9
    I know that most homes in the USA built in the last thirty years have plastic for drains, so I don't see the current going anywhere in particular in those cases.
     
  11. Feb 3, 2005 #10

    brewnog

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    Well they also have copper plumbing, which itself is earthed, so that is a moot point.
     
  12. Feb 3, 2005 #11
    I think the current would flow from one terminal to the other terminal of the power cord connecting the appliance to the electrical system of the house. This would also explain why the circuit breaker of the house's electrical system broke.

    From my last safety class I think I remember fresh water (I also remember salt water is a better conductor) having a resistance of something on the order of 100 ohms per meter and the human body having a resistance on the order of 100K from hand to hand (through the heart). I also remember this varied wildly from person to person (body fat plays a big part) and drops dramatically when a person was wet.

    So a circuit model having the victim in parallel with the water directly between the terminals of the house electrical system, assume the breaker goes at 10A, would have on the order of 10mA passing through them at the time of the open.

    According to a previous poster and this source, http://hypertextbook.com/facts/2000/JackHsu.shtml,
    that is plenty to kill a person.
     
  13. Feb 15, 2005 #12

    pervect

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    There's a mytybusters episode on the subject (that's a TV show). They fumbled around a bit on the topic (given that they can edit their show, I don't understand why they did that). But anyway, it's worth watching if you ever run across it, in spite of the fumbling around.

    Basically, the big issue is that the plumbing in the tub is grounded. So, you have current flowing from the live wire of the toaster through the body of the person in the tub to the drain / plumbing of the tub, which is grounded.

    (Not all of the current flows through the plumbing instead of through the return wire on the toaster, but some of it does).

    A very small current, on the order of a few milliamperes (I don't recall the exact figure) thorugh the heart can cause fibrilation.

    t
     
  14. Apr 10, 2005 #13
    Just a note here on Circuit Breakers a 15A breaker will not trip at 15 amps. There is a time curve involved the better the path back to the panel the faster the breaker will trip.

    The bathtub example if there is no direct path, FaultPath back to the breaker it may not trip. Typically to get a CB to trip in 0-5 seconds the fault current needs to be about 90-150 amps for a 15 amp breaker. A 15 amp breaker overloaded to say 20 amps may take up 90 seconds to trip. This would be too long to sit in a bathtub with a toaster in there with you.

    This is why GFCIs and Submersion devices are recommended for bathrooms, (actually required). I believe there are voltage gradients in a tub the closer the person get to ground the better contact he makes with the circuit. However today they make mostly figerglass tub which are insulated from ground also if the plumbing is plastic that would create another problem.

    The lower the body resistance the more severe the shock. As a matter of fact .016 amps is maximum let go as far as a shock is concerned. across 120 volts.
     
  15. Apr 11, 2005 #14
    I agree. The only possibility I can think of is that, since the water and the person immersed in it are in parallel, if a really large amount of current flowed, it might be possible for the ~15ma or so needed to kill a human to flow through the bather. But the current density in the water would decrease extremely rapidly as the distance from the toaster increased. I also wonder about electric eels...the same considerations apply.
     
  16. Apr 13, 2005 #15
    Pack rat2

    How would you be in paralle with the toaster, the circuitry of the toaster is confined to the toaster. Depending on the resistance/impedance of the water and would determine how much current would flow between the hot and neutral.

    Current will always try to get back to the source and will take all pathes to do that. If the tub is insulated and there are no pathes except for the circuit in question then it should be safe (???) or will the water be electrified just by virtue of the toaster being in the water with the bather ?

    Another thing, Now, Linemen working on 500Kv lines are doing what is called 'bare-handing' as long as they are insulated from ground they are safe, they evin use Heliocopters to access the lines, they use a bonding stick to ensure the 'chopper' is at the same potenial as the lines are, but agian they use only leather gloves to repair and check the lines.

    If there is no other return point for the current to return to the source there should not be a problem. Unless the bather becomes part of the circuit, i.e., unless somehow the bather gets in series with the neutral.
     
  17. Apr 13, 2005 #16
    "How would you be in paralle with the toaster, the circuitry of the toaster is confined to the toaster...."
    All of the water and everything in it that's a conductor is in parallel with the voltage source in the toaster. Here's a similar situation which illustrates the point. Suppose you have a 1 foot square sheet of copper, and you attach two electrodes with a voltage applied to it. Most of the current will flow in a straight line between the electrodes, but not all of it. As you get away from the electrodes, the current density decreases greatly. I think in the toaster scenario, the current density outside its case would be very, very low, and not constitute a hazard, which was my initial point.
     
  18. Apr 13, 2005 #17
    "If the tub is insulated and there are no pathes except for the circuit in question..."

    One side of the power line is grounded, and current will therefore flow from the "hot" side through the water, to earth ground. Try this...fill a bathtub or sink with water, and connect an ohmeter between the water and a known ground (like a cold-water pipe). It will show a low resistance.
     
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