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Q & A: conducting salt water

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Most recent answer: 10/22/2007
Q:
How far can electricity travel through salt water? We’re going to be visiting the ocean next week and thunderstorms are forecast. We know that we shouldn’t be in or near the water when we can see lightning or hear thunder, or just prior to or immediately after a storm. But now we’re wondering just how much of an area ocean lightning strikes effect. Obviously when lightning strikes the ocean, it’s not conducted through the entire body of water, or else no one would ever be able to go swimming!
- Trish (age 31)
Iowa City, Iowa
A:
When salt dissolves in water, it comes apart into ions with positive and negative electrical charge. That's actually what we mean by calling a material a salt. Ordinary table salt forms Na+ and Cl-. These charged particles are dragged opposite ways by an electrical field, making an electrical current. So that's how salt water conducts.

In a thunderstorm, strong vertical air currents carry charged water droplets up and down. Typically, the negative charges are on the bottom of the thundercloud, and the positive charges are on the top. The negatively-charged droplets on the bottom of the cloud create a strong electric field nearby, repelling negative charges nearby. This means the negative Cl- ions in the water get pushed away from under the thundercloud, and positive Na+ ions get attracted. The area in which the charge distribution of the surface of the water is affected can be quite large -- at least as big as the thunderstorm cloud. The polarity of the charge buildup should switch at some distance from the bottom of the cloud. Very far away from the thunderstorm, there should be very little charge buildup on the water.

When the lightning bolt hits, the electrical current should flow mostly on the surface of the water. Salt water is a very good conductor, and so the electrical potential within the bulk of the water is nearly constant (if it weren't constant current would flow until it was); the same is true with metals. I'd expect the current density to fall off the farther you get from the bolt in inverse proportion to the distance, since the current spreads out in all directions to neutralize the charge buildup in a large area under the cloud.

The current will take the path of least resistance, and that means through seawater. People are made up mostly of saltwater (the salinity of blood is remarkably close to that of the ocean) and conduct rather well too, but maybe not quite as well as seawater. I wouldn't risk it however. What might be a bigger risk is the fact that if you are standing up near the shore, you become a living lightning rod. The water is good at staying below your level, so you may well be the tallest thing around, electrically connected with the charged water surface, and therefore a good target for a lightning strike.

Mike W. and Tom J.

(published on 10/22/2007)

Follow-Up #1: desalting water

Q:
Can you add a chemical to saltwater to make it less conductive ?

Lennon
- Lennon (age 8)
Lakewood,Co USA
A:
Youíd want a chemical that would bind the salt ions (electrically charged parts that the salt forms in water) to molecules that donít move around easily- like big polymers. Thatís because the electrical current comes from electric fields dragging those ions around in the water. I think the polymers used in ion exchange resins would do the job.
One way to desalt water is to run it through ion exchange columns in which those polymers are bound up so they donít come out into the water. I bet if you instead just dissolved them in the water, the conductivity would still drop.

You might wonder what happens when a positive ion, Na+, for example, binds to one of these polymers. Usually, the polymer gives off a positively charged H+ (hydrogen) ion. Binding a negative ion like Cl- usually gives off a negative OH- ion. Although H+ and OH- also conduct, most of them bind together to make un-charged H2O, which doesnít conduct.

Mike W.

(published on 10/22/2007)

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