Electricity in water=death question.

Factual Questions
1

Throw a hair dryer that’s plugged in into a bath tub and whomever is in it dies:eek:.

Throw a radio that’s plugged in into a small swimming pool, and the occupants die.:eek:

But how far will electricity travel across water?:confused:

What if I throw a plugged in hair dryer into one end of an Olympic size pool. Are the people at the other end affected?
How about a small lake? A large lake? The ocean? There must be a point where the water stops conducting the electricity.

2

A relevant link that will only be good until deleted.

3

There’s no clear answer to this one, because it depends on many factors including the geometry of the situation.

Mains voltage electrocution usually causes death by stopping the heart. The current required is typically rather small - a few milliamps will do it, but this current has to pass close to the heart itself.

Electricity transmission uses the Earth as one leg of the transmission cicuit, so in any scenario you have the “live” or “hot” terminal, and current flowing between it and ground. Just about your worst situation is to have a hot wire in one hand and your other hand grounded, causing the current to flow through your chest. (A nice way to do this to yourself is to firmly grasp your metal-bodied toaster in one hand and try to dislodge the stuck bread with a metal knife using the other. Get yourself a Darwin award!)

So let’s throw a single bar heater into an occupied plastic bathtub. Current will flow from the live end of the element to the grounded end. The easiest route is still through the heater bar itself, and this will take the majority of the current. However, a small fraction will flow through the water - you can visualise it as passing in a looped path out from one end of the bar and back into the other. And some small fraction of that will pass through your wet skin, the bag of salty water that is you, and back to the bar. The longer the loop, the smaller the current though, and you have a good chance of surviving this, especially if you have a ground fault trip on your power supply.
A nastier situation would be someone throwing a single live wire into the bathtub while you’re touching a faucet. Then pretty much all the current will flow through you to ground via the metal water pipes and your chances are poor.

Do the heater trick in a swimming pool and there is a definite range factor at work - someone quite close to the heater is going to get a bigger dose of current than someone at the far end of the pool. Chances are good that most or all of them would survive - that heater bar really does suck up a lot of the current.

It gets more interesting and dangerous if someone drops a live wire into one end of the pool and a grounded wire into the other. If the pool lining itself is an insulator, then you’re going to have all the current passing through the water from one end to the other. The current distribution will be complicated - further from the wires will be better than close, and corners of the pool better than the centre. Not a happy place to be in any event. For more democratic electrocution, have a live metal sheet lining one side of the pool and a grounded one lining the other, giving a uniform current distribution and frying everyone evenly.

If the pool lining isn’t an insulator you can dispense with the grounded wire or sheet. Current distribution will be altered and highly complicated - again, best to be furthest from the live wire.

For really large grounded bodies of water such as the sea, you can assume hemispherically symmetrical radial current distribution from a live wire dipping into the surface. The danger you’re in depends upon what fraction of the current you intercept. So if the total current flowing from the wire is 10 amps, at 1 metre from the wire you’re getting 2-3 amps through you which is not good (assuming intercept “area” of a human is 1.5-2 m[sup]2[/sup].) At 10 metres from the wire you’re getting 0.02 to 0.03 amps which may still give you trouble. At 100 metres you’re only getting a few tenths of milliamps and wondering why all those people near that cable are being so noisy. At great distances the hemispherical distribution is no longer valid but the current densities are so low it makes no odds anyway.

4

How I wish my electroshocking notes weren’t packed in a box at another house right now - I can give a better response tommorrow. Electroshocking is a way of collecting fish by stunning them with a current put through the water. It’s been a couple years since I’ve looked at my notes, so I can’t remember all the saftey ranges to stay away from.
Just quickly, the amount of current that travels how far through the water depends on what’s dissolved in the water, the amperage, voltage, etc. How much is dangerous to you depends on how much of you is in the water (contact surface area). When shocking for fish using… (dammit can’t even rember how much current - more tommorrow), I think your supposed to keep at least 50 meters away, but I’ll check. The current will “dissapate” for lack of a better word, but there are too many variables for me to remember off the top of my head right now. The purer the water, the less danger there is. Absolutly pure H2O won’t conduct a current at all, but you’re not likely to find anything that clean in the real world.

5

Matt, I am not so sure about that with the pool, the reason is that the heavier chlorination of the pool makes the water a far better conductor than the human body, such that the water should act as a ‘short’ in comparison. So the one live wire on one end and the ground on the other probably will not electrocute quite as well as you are implying. (Given enough current of course, and all bets are off.)

6
      • Uh, no.
  • I don’t suppose that some people would believe it unless they saw it, but standing in a tub of water when someone drops a plugged-in whatever into it doesn’t equal death. I’d do it in front of you if I had the chance, just to show it’s true (like the putting-a-cigarette-out-in-gasoline trick). The electricity has to be forced through your body; electricity just traveling through the water past your body ain’t enough to do much. The danger of tubside appliances is not that you will be in the water while the appliance falls in, the danger comes when you are still sitting in the water but try to lift the appliance out, forcing the electricity through your hand->arm->chest->body. Elsewise, hair dryer falls into water, electricity travels through bath water to metal drain pipe, electricity gets grounded, you are still alive.
  • To shock fish, you need two rods (one for each battery contact) and you have to maneuver the rods so that the fish have to travel or be located between them, where some of the electricity will actually travel through their little heads. The electrical current would disperse somewhat between the rods, but not that much: shocking one spot doesn’t stun every fish in the lake, and lightning striking the ocean doesn’t shock everyone in the ocean everywhere in the world. - MC
7

I still haven’t been able to find those electroshocking notes, but I can give a bit more info in the meantime about electrocuting things in the water. You carry a small generator or battery pack on your back, and have a metal cable trailing behind you in the stream (the cathode) and a metal ring on a pole (the anode). You zig-zag the anode on the pole through the water in front of you, while the cathode trails maybe 8 or 10 feet behind you. Depending on the size and species of fish, pretty much anything within 2 yards of that zig-zagging anode with be drawn in, while everything else feels a current and tries to hide. It’s kind of cool how they are drawn in - it’s not to do with a current running through their heads, but through their muscles. You actually take over control of the fishes muscles and cause them to contract with the pulse from the shocker. Given the shape of a fish causses it to move forward when it flexes, they end up uncontrolably swimming right to the anode; it’s known as galvanotaxis I believe.

Electroshocking is very dangerous and highly illegal unless you have the proper training and permits from the government. Regulations do vary however; yesterday while trying a quick search I came across a photo from an American environmental group showing three guys shocking… all three would have been hauled into court if they were shocking in BC - none were wearing gloves, and two had rubber boots and obviously wet pant legs on. They could all have been killed due to an obvious lack of saftey regulations wherever they were.

8

Anyone else remember the old National Lampoon cartoon where a young girl executes her baby sister in a wading pool by tossing a toaster in… the line “hmmm, smells like soup” figures prominently.

9

Narile - Fair point, and clearly it will be impossible to electrocute someone immersed in, say, mercury, by passing a current through it. Then again, the total current for a given voltage will increase due to the greater conductivity. So you can approximate the situation with a resistor network - the human body in parallel with a “bypass” resistance due to the water, BUT the whole lot is in series with a limiting resistance, again due to the water. There must be an optimum conductivity for electrocution for any given situation!

In truth, working out the lethality of a given situation will be difficult. No pool will be lined with a perfect insulator - there’ll be grounded metal fixtures, rails, filter ducts etc. which will make determining the current distribution a job for a computer. Then there’s the charge-transfer resistances at the wires in the water to consider, and the skin resistance of the people in the pool is probably a function of how long they’ve been in it. I don’t have any hard data - my posts on this subject are informed (I hope!) speculation.

mmmiiikkkeee - interesting stuff, thanks! Beats dynamiting any day.

Do you recall, if you wire up your electroshocking rig the wrong way, do all the fish swim away backwards? (That’s a joke, but your post is quite specific about the polarity. I’m wondering if it makes any difference.)