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Sending strong electric currents through seawater

  1. Dec 28, 2011 #1
    Hi everyone,

    I'm writing a novel in which a character "electrifies" water. He sends electric currents through the water to fend off a monster. So obviously the current must be high in volts and amps.

    My question is, what would be the easiest way to do this? So far, the best thing I can come up with is that he has a live wire connected to one of the turbine engines (which helps power the ship). He holds the wire with something like an electrode holder, the type used in welding, and sticks the live wire in the water. This makes shocking currents, to ward off the monster.

    I'm pretty sure something like this is possible, since I have found many news articles about kids dying in swimming pools from tragic shocks. I just need to figure out the logistics. I entertained the idea that he would use an actual welding "stinger," but I'm not sure if that would work to send currents through the sea.

    Again, it is fiction, and I just want to figure out the best way of sending huge amounts of electricity through the ocean water.

    Thank you so much for your help! It's much appreciated.

    Taylor
     
    Last edited: Dec 28, 2011
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  3. Dec 29, 2011 #2

    NascentOxygen

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    For anything but short duration, I think it will have to be AC power. DC will cause the water to break down into its component gases around the electrodes, and this will severely hinder the current flow from the electrodes. If it's a metal hull then that can be used as one electrode, but you'll still need to trail one (or more) other electrodes some distance from the hull.
     
  4. Dec 29, 2011 #3
    Thanks for your reply! I'm not sure about using the hull as one electrode, though.

    Can you explain why you suggested that?

    If he attached two electrodes to the turbine, via really long wires, and then dipped them both in the water, using an electrode holder, do you think that would work?
     
  5. Dec 29, 2011 #4

    NascentOxygen

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    That would electrify the water, but only in the vicinity of the electrodes. If you used the whole hull as one electrode, it would create a field around a large part of the hull--that region near to the trailing electrode.

    Optimal would be something like 2 or 3 exposed electrodes running the entire length of the hull, but insulated from it. It depends on whether you wish to surround the hull with a uniform field, or want to localize it to the direction you see the creature approaching from.
     
  6. Dec 29, 2011 #5

    Bobbywhy

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    taybot, we humans can learn a lot by observation of other creatures in nature. Since you want to apply an electric shock to fend off a monster, why not check out how this is already done. Think "defensive" shocks of the electrical eel. Google and Wiki are your friends! This is from Wikipedia:

    "The electric eel has three abdominal pairs of organs that produce electricity: the Main organ, the Hunter's organ, and the Sachs organ. These organs make up four-fifths of its body, and are what give the electric eel the ability to generate two types of electric organ discharges (EODs), low voltage and high voltage. These organs are made of electrocytes, lined up so that the current flows through them and produces an electrical charge. When the eel locates its prey, the brain sends a signal through the nervous system to the electric cells. This opens the ion channel, allowing positively-charged sodium to flow through, reversing the charges momentarily. By causing a sudden difference in voltage, it generates a current."

    "The electric eel generates its characteristic electrical pulse in a manner similar to a battery, in which stacked plates produce an electrical charge. In the electric eel, some 5,000 to 6,000 stacked electroplaques are capable of producing a shock at up to 500 volts and 1 ampere of current (500 watts). Such a shock could be deadly for an adult human. (Electrocution death is due to current flow; the level of current that is fatal in humans depends on the path that the electric current takes through the human body and can range between 0.07-0.7A.)"

    Now, you can read more about how this fish defends himself at:

    http://en.wikipedia.org/wiki/Electric_eel
     
  7. Dec 29, 2011 #6
    I've not checked this but I think all the fish that use this defence mechanism are freshwater fish. I suspect that the conductivity of seawater is so high that it would be impossible to electrocute anything even with a lightning strike!

    Edit. I have checked now and I was wrong. Torpedo rays are saltwater fish that use electricity to stun prey. I would still bet that the prey has to be practically touching them.
     
    Last edited: Dec 29, 2011
  8. Dec 29, 2011 #7
    Salt water can conduct electricity, but it is a relatively poor conductor compared to metals. The electrolytes in the water give it its conductivity.

    Also, EM propagation in salt water is very poor because of the conductivity. This is why SONAR dominates in underwater applications. The energy is dissipated into the water quickly. However, there are companies who are using EM to survey for oil in the sea at low frequencies.
     
  9. Dec 29, 2011 #8
    "That would electrify the water, but only in the vicinity of the electrodes. If you used the whole hull as one electrode, it would create a field around a large part of the hull--that region near to the trailing electrode.

    Optimal would be something like 2 or 3 exposed electrodes running the entire length of the hull, but insulated from it. It depends on whether you wish to surround the hull with a uniform field, or want to localize it to the direction you see the creature approaching from. "


    This has been very helpful. Thanks.

    And Bobbywhy, thanks for your suggestion about the eel. I am looking to do something like what the eel does, but on a larger scale obviously. So thanks to you too.
     
  10. Dec 29, 2011 #9

    jim hardy

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    an old timer at electric company related to me an occasion where they needed an electrical load to test a generator they'd just rebuilt. they hooked BIG cables to it and threw them into a saltwater canal adjacent the plant.

    so your premise is plausible.



    i was once swimming in a lake when lightning hit the ground about a mile away.
    the shock was short duration but fairly intense.
    so your premise is again plausible.



    fwiw..

    old jim
     
  11. Dec 30, 2011 #10
    Just make sure to obey the laws of physics. Are you familiar with the basics? Here are a few points just in case.

    1) A higher voltage will give you a higher current if you assume a fixed resistance.
    2) Resistance is proportional to the length of the conductor. So the closer your character can get the positive and negative conductors to the animal the better. It will result in a higher current. Although it would be nice if you could make sure the current actually conducted through the animal and not around it. Salt water is a pretty good conductor.
    3) Current will always choose the path of least resistance. If there is more than one path the current will be divided up such that more current flows in the lower resistance paths.

    IMHO this device is a bit farfetched. You'd really need to elaborate on it. It would have to be some kind of unusual scientific setting where very high voltages were available. I'd suggest something like 15 kV.

    I think your biggest problem may be that seawater is a better conductor than any animal. If you put a ground wire on one side of him and a hot 15 kV wire on the other it would probably just go around him. You'd probably have to actually touch him with the wires.

    The only way I can think that this could work in the real world would be if you shot the creature with a harpoon or arrow or something attached to a hot wire. And then another arrow or harpoon attached to a ground wire. Obviously the harpoon/arrow would also have to be a conductor. And ideally you'd first shoot the animal one one side and then on the other so that the current flows through his vital organs and doesn't just burn a patch of skin off. That might just get him mad.

    Electricity isn't really all that convenient of a weapon. It might be better just to shoot him in the head with a gun or some other projectile weapon in the vicinity. Or maybe actually let the monster win for a change. He probably deserves a break. Everyone's always trying to kill him. Maybe you should make him a more 3 dimensional flawed character and not some 2 dimensional automaton who only lives to kill, kill, kill. Even great white sharks don't just swim around killing everything they see.
     
  12. Dec 31, 2011 #11

    NascentOxygen

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    OP didn't say he wants to slay the monster; he said he wants to "fend it off". So he's not needing to par boil its brain, or char its heart with electro heating. (In any case, being a monster, it probably has multiple brains and hearts.)

    All he needs to do is interfere with its muscle coordination, at the most. It might even be like sharks, where a relatively tiny field repels them because they have ultra-sensitive sense organs used for detecting the E field around a moving fish.

    I'd like to see a human swimmer attempting try to swim through an AC field of say, 100v/m. At some relatively safe level they probably would be practically unable to make any coordinated movements. Just beef this up a bit for marine creatures and see how they like it!
     
  13. Dec 31, 2011 #12
    I want to thank you all for taking my question seriously. It has been a huge help.

    Metiman, you make some good points. Your list of physics rules was very helpful.

    "I think your biggest problem may be that seawater is a better conductor than any animal."

    But still, this defense works for electric eels and other such animals, so I think it is possible.

    "OP didn't say he wants to slay the monster; he said he wants to "fend it off"."

    Right, this technique doesn't kill the monster.

    In my research, I came across the practice of using of Electric Fish Barrier and Guidance Systems.

    I think this is close to what I'm attempting. The literature in the above link reads:

    "Smith-Root barrier and guidance systems use from two to six pulsators (pulse generators) to provide ascending levels of field intensity. The pulsators have their outputs connected to an array of evenly spaced electrodes placed across a stream bottom. Each pulsator can be adjusted to provide an increasing voltage between successive electrode pairs. This creates a gradually increasing electric field along the array."

    Another noteworthy paragraph:

    "Smith-Root barrier and behavioral guidance systems are designed to be non-lethal and to use only low-frequency pulsed DC to create electric fields. Humans are three times more likely to be harmed by alternating current (AC) than by DC current, and it has been shown repeatedly in the scientific literature that use of AC can injure fish. Pulse frequency (especially) and duration and current can all contribute to potential damage, thus Smith-Root typically sets these values well below the electrocution threshold of a typical ground fault interrupter. Pulse frequencies for barriers are much lower than those used in traditional electrofishing. Our interest for most barriers is in changing fish behavior, not achieving galvanotaxis or anesthesia."

    I was thinking I would have them use AC, and turn up the voltage and current, to lethal levels.


    This parallels what NascentOxygen described at the beginning of the thread, when he wrote:

    "If you used the whole hull as one electrode, it would create a field around a large part of the hull--that region near to the trailing electrode.

    Optimal would be something like 2 or 3 exposed electrodes running the entire length of the hull, but insulated from it. It depends on whether you wish to surround the hull with a uniform field, or want to localize it to the direction you see the creature approaching from."


    I'm happy that I can have these electrodes submerged and therefore out of sight.


    Your thoughts are appreciated, especially regarding the voltage I should use, and anything else that comes to mind. Thank you very much!
     
  14. Jan 3, 2012 #13
    I'm trying to figure out the equation to produce 300 volts in sea water.

    I learned that the resistivity of sea water is .2. So I think that means to produce 300 volts it would take a current of 1500 amps. Is that right?

    Thanks!
     
  15. Jan 3, 2012 #14

    jim hardy

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  16. Jan 3, 2012 #15
    Wow. You really don't know the basics. It doesn't work that way. Voltage is a potential difference. Think of rolling a boulder up to the top of a hill. When the boulder gets to the top it has a certain amount of (potential) energy. When you let it go and it starts accelerating down the hill due to the force of Earth's gravity it is the equivalent of a voltage drop across a load. Voltage is sort of like a measurement of electric potential energy. That analogy doesn't quite work though. The boulder at the top of the hill is more analogous to electrical energy measured in Joules (watt seconds). Maybe the steepness of the hill would be a better analogy. Or the size of the boulder perhaps.

    Another analogy would be water pressure. Voltage is sort of like water pressure. If you have a water pump capable of producing 500 psi or pressure you will get more water flowing through a given pipe or hose than one that can only produce 5 psi.

    In some parts of the world people get water for their homes or apartments from a rooftop tank. Sometimes it may only be one or a few 50 gallon drums. When you take a shower in such a home you don't get a very strong spray. But if you add a powerful water pump, which is like adding a high voltage battery to an electrical circuit, the water will spray out of the shower nozzle with great force and more water molecules per unit time (current) will be transferred.

    So current comes from voltage, not vice versa. To produce a voltage you need a device that produces electrical potential difference. Basically either a generator, which creates an electrical potential difference or electrical pressure by essentially moving a magnet across some wires very quickly via some form of mechanical energy, or a battery which creates the potential difference or pressure chemically. To produce a current you just need a voltage and some kind of conductor connected across the two terminals. In this case the animal would be your conductor. You need to know how your voltage is being produced.

    Also resistance is not a unitless number like a ratio. Pay attention to the units. They are important. You need them to do the math. Perhaps you discovered that sea water has a typical resistance of 0.2 ohms per meter or something. Without units 0.2 is useless.

    Don't forget that you also need to know the resistance and size and distance between your two conductors near or in direct contact with your imaginary creature. I'd suggest measuring your own resistance to get some idea of what kind of resistance animals tend to have. You can also measure the resistance of salt water directly as well. Just get yourself a voltmeter. You can measure the resistance from your foot to your neck and from one side of your abdomen to the other. Then multiply that times the relative distance that the voltmeter terminals are on you vs what they would be on your creature, assume linearity, and multiply the resistance by that factor.

    If the seawater path (around the creature) has an equal resistance to the path through the creature then half of your current will go into heating up the seawater around the monster. If the seawater has 1/10 the resistance then 10 times more current will flow through the water than through the creature. Etc. You have to consider both voltage division and current division. If you place the conductors, say, 1 meter away from each side of the animal. That will give you a voltage drop between the conductor and the animal which will depend on the resistance of seawater and maybe also on the size of the conductor. So your 300 volts would no longer be 300 volts when it reached the creature's skin.

    I'd highly suggest you do a little reading on the basics of electricity before you place such a device in your story. You're never going to get it right unless you understand it.
     
  17. Jan 5, 2012 #16

    NascentOxygen

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    Current calculations are probably best avoided, as it's too problematical. But producing 300 volts is easy: you use a 300v alternator! :smile: But a transformer is better, then you can tap all levels of voltage off it, in steps, if it has a multi-tapped winding. Or an inverter to give an electronically variable output voltage.

    The current varies as the size of the electrodes' surface area, and their distance apart. Just take it as read that your generators can deliver the current necessary, and skirt around the actual value.

    What you probably want to aim for is, say, 300 volts per metre of distance between the electrodes. So if the electrodes are spaced 5 metres apart, then you need 1500 volts to produce that same field in the space between them of 300 volts per metre.

    No. Skip it. Then no one can poke holes in fine details of your narrative. :smile:

    You can say things such as the gentle hum of the dynamos (or transformer, whatever), leapt to an angry scream as the machinery laboured to deliver the extra current when the monster contacted one of the electrodes, or the ship's interior lighting dimmed as the monster shorted out the electrical path and convulsed from the burns down its side, etc. ..... :wink: :wink:
     
  18. Jan 6, 2012 #17

    jim hardy

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    we need to talk with somebody from merchant marine.

    many medium size ships use diesel driven generators to power electric propulsion engines.

    i dont know what voltage but it'll be "medium" - hundreds of volts 480 600 or so with hundreds of amps capacity.

    your hero could tap into that if he can find some big wire , and hook it to some anchor chain or steel net...

    i dont know where you are . if in New York ,there's a Merchant Marine museum down near that marina by Brooklyn Bridge where the square riggers are parked. Friendly folks in there who'd be pleased to help.
     
  19. Jan 7, 2012 #18

    The ship has four 17 MW diesel engines and two gas turbine engines. Lately I've been thinking I would have submerged electrodes across the hull, with pulse generators pushing out the current, as is described here: http://www.wbez.org/frontandcenter/2011-06-21/electric-barrier-last-line-against-invasive-species-88123#

    But your post makes me think I could just use one or two of the diesel engines, instead of those pulse generators. My supposition is I just need to have cables connected to the electrodes. He might not have as much control to make a pulsing current, but I don't even want it to pulse. I want to make it a steady stream of AC (AC because apparently it is more deadly for fish).

    Please respond if you can. Thanks!
     
  20. Jan 7, 2012 #19

    jim hardy

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    i'd think it can be pretty crude.

    two wires in the water will have between them some uncomfortable real estate.

    i recall as a kid reading in encyclopedia about commercial "electrofishing".
    they used mild direct current because fish swim toward one polarity, i dont recall which.
    i think it was opposite ones in fresh vs salt water.
    and i may have confirmed that in Dad's aquarium with a flashlight battery and the guppies, but details have long since slipped away. i might've got a whippin'.

    so if you can get your creature between two wires, or between the hull and a wire or net, you can make him mighty unhappy or stun him.
    is he smart enough to avoid something that "bites" ?

    17 megawatts is a LOT of mechanical power , check your particular boat to see if they used reduction gear instead of electric drive .
    anything with that much propulsion would, i would think, have quite substantial electric generators for house power....

    i thought you were in an island hopping type boat maybe a hundred feet.
    Alaska Ferry i rode was 400 ft long, had two 6000 hp diesels for propulsion and several small diesel generators (on order of 150 hp) for house electricity. one of those i'd think would do what you propose.




    if your ship has large electric equipment there's likely some low kv (2400 or 4160 volt) available.
    for sizing, one KW is 1/0.746hp
    and of course watts = volts X amps, if three phase it's volts X amps X 1.732.

    toss a couple bare cables over the stern ...anchor chain might sink better..
    lure him in and and close the breaker on him

    i don't really know much about ships, just read a lot of sea stories in my youth.

    old jim
     
  21. Jan 11, 2012 #20

    I think I'll go the more crude route, as you suggested, and have the cables connected to one of the diesel/electric engines. But I would still like to have some semblance of an idea of how many volts or amps would come out of this. Again, this is a 17 MW engine.

    I know you can't figure out volts or the current based on the watts, but can anyone please lead me in the right direction, toward how I can get a vague idea of what these numbers would be?

    I looked up the watts and amps of some of these electrofishers. The most powerful one I could find produces up to 990V and only has a peak output power of 36,600W. That is obviously far less than my 17 MW engine I want to use, but I was wondering if anyone can help me figure out what the volts or amps would be around.

    Thanks to everyone for their help thus far!
     
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