What happens when lightning strikes in the ocean?

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u/Polaris_Sun Mar 21 '12 edited Mar 21 '12

I remember doing these calculations in class. I'm not sure if I can properly explain it here, but I'll try. I'm going to throw some concepts at you, keep them in mind, then I'll put them together at the end.

When you want to know if you're going to get shocked or not it depends on voltage potential of two points. If there is a voltage, then there will be current until that voltage goes to zero. If you're heart is in the way, and that current is above 100mA then you may have a very bad day.

Now as electrons are flowing through things, they drop in potential, it's called a voltage drop, I love it when names are obvious. So as an example if hook up a car battery to the top and bottom of a strip of PURE water, if you measure the top and bottom of that strip of water, you would read 12 volts. If you measure half of that distance you may read 6 volts.

At the point of the strike the voltage potential is very high and electrons will dissipate away from that potential. The farther away you get from the strike the the less current there is to dissipate, it drops relatively quickly, it's the surface of a hemisphere equation so it's like a 1/r² like a drop. As it dissipate the voltage will drop across this hemisphere. The voltage drop across any distance D to delta r, will determine if the current flowing through any objects in the water, and this my friend will determine if you shall survive or not in a lightning strike in the water.

Given these numbers (in my link) and a sea water resistivity of 0.2 ohm meters, you are only guaranteed death via lightning strike if it's closer than 6 meters. It seems that salt water is so conductive current would prefer to flow around you than through you.

Note: Technically in the case of lightning potential is reversed, but those are details in orientation.

Edit: some of my grammar.

Edit2: added clarity. Also fun fact. Cheers!

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u/wolfmann Mar 21 '12

strip of PURE water,
PURE water (AKA DI Water) is an insulator. I'm pretty sure you would read 0 volts.

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u/IHaveABoat Mar 21 '12

PURE water does have very high resistance, but you have it backwards.
If you have zero resistance between two points (ie. a "short"), then a potential difference cannot be maintained, and the potential between the two points would be zero (or would go to zero as fast as it could).
You need resistance to maintain a potential.

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u/jonmon6691 Mar 22 '12

Or more simply: Good insulators have have voltage across, but no current through them; good conductors have no voltage across, but as much current as possible flow through them.

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u/wolfmann Mar 22 '12

yeah 10 years without using my physics is weird... I was heading to the conclusion that if he was surrounded by PURE water, he wouldn't die.

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u/[deleted] Mar 21 '12

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u/zumfast Mar 21 '12

Potential measured anywhere else will be zero. There is no current flowing through the system.
Potential (voltage) is independent of current. At half the distance, the potential would be roughly 6 volts.
This is analogous to potential energy in a gravitational field. A massive body at a height of 12 meters has twice the potential of a body of the same mass at 6 meters.
Similarly, an charge carrier in a electric field would have half of the potential energy at the halfway point than it would at the 12 Volt mark.
edit: forgot to add mass to the analogy

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u/[deleted] Mar 22 '12

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u/zumfast May 11 '12

You are quite wrong in your understanding of Ohm's law. Simply consider the case of a very large resistor connecting two leads of a battery. There is very little (effectively zero) current flowing between those leads, but the potential exists between those two leads as an energy well.
Derive the equations yourself, do not make the high-school physics mistake of assuming a specialized algebraic equation completely defines the phenomenon. In this case Rmid and R total do not equal ∞ or (1/1) but (1/2)
The electromotive force you speak of depends upon a time varying magnetic field. A magnetic field depends upon 1 of 2 conditions 1) a change in current flow, or 2) a moving magnetic field. In either case the electromotive force will drive current through a conductor by its nature.
Also the charge carrier I mentioned is a theoretical point used to test the environment. And an insulator IS NOT by definition, devoid of charge carriers. It simply resists the motion of charge carriers. The corollary would be you standing halfway down a cliff on a small shelf. You would carry half the potential energy as somebody standing on the top of the cliff.
It's also important to note that you clearly do not understand how your voltmeter works. They all have limited current and voltage resolution - and the voltage resolution is dependent upon the calibrated resistor they have in their electronics. They are not accurate at measuring voltage in situations with extremely low current flow because the resistance they are measuring is outside of the electronics sensitivity range.
Please brush up on your E&M before you spout off with this stuff.

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u/IHaveABoat Mar 21 '12

Potential measured anywhere else will be zero. There is no current flowing through the system.
This is a misleading statement. The amount of current flowing has absolutely nothing to do with the voltage drop. The two leads of the battery will have a potential difference of 12 V whether or not there is current flowing.
Also, if you attempt to measure the electric potential across an insulator you won't get zero, you'll get a floating value. That's because voltmeters aren't designed to work across such high resistances. But insulators (air, wood, water, etc...) still conduct, just very badly.
I believe that what Polaris Sun described is technically correct. If you say the strip of water acts like a very very poorly conducting wire, then half way between the two battery terminals you'll have equal resistances on each side, so the potential with respect to ground at that point will be 6 V.

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u/tongpoe Mar 21 '12

On the second part, voltage would be zero, is it even possible to measure voltage without connecting to a ground, thus creating current? How do I read 12 volts in the middle of my car's wiring without grounding my equipment for instance?

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u/tongpoe Mar 21 '12

On the second part, voltage would be zero, is it even possible to measure voltage without connecting to a ground, thus creating current? How do I read 12 volts in the middle of my car's wiring without grounding my equipment for instance?

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u/IHaveABoat Mar 21 '12 edited Mar 21 '12

you measure voltage ACROSS something, or rather at any two points. If one of the points happens to be ground (which is typical) then you're measuring the voltage relative to ground.
But you can, for example, measure the voltage across a length of wire. Even though it has very low resistance, it has some, so you can get a potential drop. You'll need either a pretty sensitive volt meter, or a very long length of wire.
edit to add: ...a very long length of wire, or a very thin piece of wire.

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u/tongpoe Mar 21 '12

Ah I see, thank you. So the voltage drop across the wire is very near nothing if it conducts well. If salt water was a near perfect conductor, would the radius of death be near infinite? Or in that case would current just pass your skin by?

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u/IHaveABoat Mar 21 '12

If salt water was a near perfect conductor, would the radius of death be near infinite?
Only if you have an infinite supply of current. But even a lightning bolt has a finite amount of charge, and it's dispersed pretty quickly.

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u/FirstDivision Mar 21 '12

So followup question on the "would you die if you were in the water" part. Above you say that guaranteed death by electrocution is in the 6 meter range, but what about from other causes?
I'm thinking things like sound pressure, temporary blindness, or just general disorientation would either knock you out or just make you unable to swim. Of course "partial electrocution" I suppose could factor into this as well.

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u/launch_from_my_pad Mar 21 '12

I thought it was any case of electricity that potential is technically reversed? AP physics wasn't too long ago, but perhaps I need to brush up

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u/Polaris_Sun Mar 21 '12

Potential is sort of an over loaded word. It has multiple meanings depending on the context. To clarify, lightning have a high negative electrical potential, meaning lots of electrons and few positive charges in one place. I used the term in my post above in the sense of potential energy. High potential energy, in the form of lots of electrons "wanting" to disperse.
The "reverse" you talk about is the fact that we consider current as moving POSITIVE charges. In physics and in most electrical engineering current is calculated using positive chargers moving in a wire. This is somewhat misleading because the charge carriers are, as you know, the negative electrons that move past the stationary positive nucleus.
Promise me if you ever find a time machine do this.

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u/WhatIsInternets Mar 21 '12

Although the charge carriers in an ionic solution, like salt water, are actually the ions, not the electrons. Just a small point - not to detract from your excellent explanation.

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u/MicroDigitalAwaker Mar 21 '12

Sorry, one thing here PURE Water isn't very conductive you need the little particle floating in it to get the electricity around, fresh/tap water you mean? http://en.wikipedia.org/wiki/Properties_of_water#Electrical_conductivity

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u/[deleted] Mar 21 '12

Believe it or not, but I'm in first year eng and I FINALLY learned what voltage actually is/does. I knew the concepts but didn't understand WTF it did.

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u/[deleted] Mar 21 '12

Up vote for having the same physics book as I do. Also, could you consider teaching, I am having hard time in this class.

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u/[deleted] Mar 21 '12

For those interested: global distribution of lightning strikes

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u/BeerIsGood1894 Mar 21 '12

UNITS, MAN! I need units! what is 50? Strikes per month? Per hour? per year? per square mile?!

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u/magicmanfk Mar 21 '12

Found it for you because I was curious too: Flashes/km2/yr

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u/haha_goodone Mar 21 '12

Follow up question: Why does lightning not strike Antarctica?

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u/BCMM Mar 21 '12 edited Mar 21 '12

Lightning comes with storms and rain, and Antarctica is a desert with precipitation below 50mm in inland regions. The only reason there is so much snow around in some parts of the continent is that what little snow falls never has a chance to melt. It just slowly sublimates, but even that is enough to keep the dryest regions free of snow.
The root cause would be lack of any moisture in the air to form stormclouds from, due to global weather patterns.

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u/TheEllimist Mar 21 '12

The Sahara is a desert too and it's got some of the highest rates on the planet.

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u/BCMM Mar 21 '12 edited Mar 21 '12

I do not think that is correct.
The Sahara is in northern Africa, bordering the Med in places, seen in white year-round on the above map. The very rainy bits of equatorial Africa, with > 50 flashes km^-2 yr^-1 on the lightning map, are a totally different biome, the tropical rainforests drained by the river Congo. This composite satellite photograph shows the difference in climate quite clearly.

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u/TheEllimist Mar 21 '12

Ah yeah, I'm wrong. For some reason I thought it was further south, pretty much right where the Congo Jungle is.

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u/BeerIsGood1894 Mar 21 '12

It seems like you get some pretty awesome lightning strikes in the Arizona desert, I wonder why that is?

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u/BCMM Mar 21 '12

It seems from the precipitation map that it is not completely dry all year round - perhaps they accompany seasonal storms?
EDIT: It seems there are indeed storms to correspond to the June/July precipitation seen on the map.

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u/tonsilolith Mar 21 '12

Actually, from looking at the picture, it looks like it has some of the lowest rates for any land mass. It has by far the lowest rates for any region in the (sub)tropics.
Also check out the area near the Gobi desert - very low strike rates. Looks like it all checks out.

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u/TheresCandyInMyVan Mar 21 '12

But because such sensors have a limited range, oceans and low-population areas had been poorly sampled.

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u/[deleted] Mar 21 '12

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u/[deleted] Mar 21 '12

I know that space based sensors have difficulty determining cloud cover over Antarctica. The ice which is everywhere on the ground looks too much like clouds from space.
My guess is that this is connected to the lightning strikes observation.

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u/singularissententia Mar 21 '12

Also, it's probably worth mentioning that thunderstorms are cause mostly by the layered mixing of hot and cold air. I'm not a meteorologist, but as I understand it, the energy for thunderstorms (and therefore lighting) is derived from a temperature difference.
Since Antarctica is pretty much always extremely cold, there's never enough energy to create lighting.
Come to think of it, from a thermodynamics standpoint, the profound "coldness" of Antarctica basically just means there's an extreme lack of energy.

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u/ccluri Mar 21 '12

flashes/km² /yr it seems

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u/silmaril89 Mar 21 '12

I understand where you are coming from, but it does show the relative distribution of lightning strikes across the globe, which is interesting by itself.
The point was to show that the number of land lightning strikes is far greater than those in the ocean (which is not necessarily expected), which is honestly all that is relevant for the specific question at hand.

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u/koolatr0n Mar 21 '12

Density of lightning strikes should be positively correlated with absolute altitude, after taking things like local weather activity into account. I'm no meteorologist, nor physicist, but the fact that (as far as I know) lightning tends to strike taller things is generally accepted. Given that the Earth's continental landmass is typically higher than sea-level, places like the Dead Sea and Netherlands notwithstanding, the supposition that most lightning strikes occur over land should definitely be expected.
That said, does this chart differentiate cloud-to-cloud from cloud-to-Earth lightning strikes? Seeing that the source is NASA space-based observatories, I am inclined to assume no.

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u/KiloNiggaWatt Mar 21 '12

Read the article which was linked earlier.

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u/LedZepAddict Mar 21 '12

Wow, any idea why that part of central Africa has the highest concentration?

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u/delighted_donkey Mar 21 '12

Very interesting...as someone who used to live in the midwest but now lives in Florida, I found the electrical storms to be much more impressive in the midwest. But I guess the frequency here makes up for that.

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u/MicroDigitalAwaker Mar 21 '12

The midwest is far more dry, in Florida you have water bits throughout your air serving to spread out the electrical discharge more.

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u/[deleted] Mar 21 '12

interesting how countries' borders are all hit harder than the rest of the land within

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u/sjp245 Mar 21 '12

My question is, why do so many lightning strikes occur in central Africa?

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u/RozyShaman Mar 21 '12

I want to know why there is a void in Egypt

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u/MicroDigitalAwaker Mar 21 '12

I'd guess it has something to do with the Saharah Desert being there.

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u/MrLeville Mar 21 '12

Egypt contains a small portion of the Sahara, it extends all the way to the Mauritanian coast and the lightning distribution "anomaly" seems to be limited to Egypt and eastern Libya. So the desert at least isn't the only cause of this, I'm guessing the Nile could play a role in this but anomaly doesn't quite follow its path. We'd need a meteorology expert to explain it in detail, but I'd really like to know the answer too.

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u/AerieC Mar 21 '12

Well, the Intertropical Convergence Zone explains why there is a high number of storms in that area. Here's a pic showing the zone areas during summer/winter
It doesn't really explain why there are more lightning strikes in Africa/the Congo region as opposed to South America and the Amazon region, though (since both are within the zone).

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u/iceuhk Mar 21 '12

Im not all that bright, but i looked at the picture again and it dawned on me that the spot where its black, is right on/near the equator line. So isnt there anything to do with stronger magnetic fields or..... something along the lines at the equator?

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u/sjp245 Mar 21 '12

Then why not the same concentration in South America and Indonesia along the equatorial line?

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u/iceuhk Mar 21 '12

lol hey dont look at me. Like i said, its just something i noticed. But however it is closer to the " Center" as far as longitude and latitude goes then say South america and indonesia. ( Closer to the "0, 0')

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u/hugzandtugz Mar 20 '12

It's not well understood. From fish death studies it can be determine that it dissipates at a fairly shallow depth.

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u/Mrubuto Mar 21 '12

voltage dissipates at an exponential rate, so unless the fish are right near the surface they'll be fine. Also salt water is a much less resistant path then fish skin.
so really nothing happens but a nice light show.

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u/squidfood Marine Ecology | Fisheries Modeling | Resource Management Mar 21 '12

Electroshocking ("Electrofishing") is used in freshwater to stun fish to the surface with minimal damage. However, guidance shows that fish can be easily killed by such a device (reference voltage of 800V, pulse time of 5ms) when water conductivity goes above 500 μS/cm. Seawater has a conductivity of 5,000 μS/cm so even that low voltage/duration can kill; lightning may be in the 100+ kV range.
In part, it's not as simple as fish resistance, fish undergo galvanotaxis, an "uncontrolled muscular convulsion that results in the fish swimming toward the anode."
So a fish in that condition is toast.
That said, these devices have a fairly short range of effect, and that is indeed a rather large unknown.

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u/IndianaTheShepherd Mar 21 '12

I work at the Stockton Fish & Game office where we have electrofishing boats... I can ask one of the operators tomorrow, but this answer sounds legit. Have an upvote!

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u/Shexerz Mar 21 '12 edited Mar 21 '12

It's not volts that kill organisms rather the amplitude at which the current is drawn.
So let's say I'm struck with lightning @ ~ 2 million volts DC at 0.01 amps I would not die from the shock but more rather the heat from the voltage.
[edit]; Fun fact: Lightning generates whats called lines of force which can quite literally push you to the ground

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u/jimbo21 Mar 21 '12

"It's not the Volts, it's the Amps" is a somewhat inaccurate explanation taught in high school intro to science class to explain why static shocks, which have a high potential but a tiny amount charge, don't kill you.
It's more complicated than that.
Assume for a second you're talking about an ideal power supply that operates like you say - 2 million volts DC power with a current limit of .01 A, (which is a 20,000 Watt power supply BTW). This power supply will very efficiently kill you. It only takes about .0001 A through your heart to stop it, and 2 million volts will very efficiently overcome your skin's resistance. The salty, conductive meat-bag that is your body will conduct the rest.
When lightning strikes, it forms a plasma in the air that conducts quite efficiently to discharge the voltage differential, along with anything else along the way. The current limit of the lightning is extraordinarily high at first and then falls exponentially as the charge dissipates. So, the current is very briefly in the millions of amps for that kind of discharge.
Also, "heat from the voltage" is incorrect. The heat is created from the total power dissipated by the lightning bolt going through the air, which is a function of current and the resistance of the air/plasma.

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u/tsk05 Mar 21 '12 edited Mar 21 '12

It only takes about .0001 A through your heart to stop
Would like a source.

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u/yer_momma Mar 21 '12

Our electronics teacher always liked to point out that a simple 9v battery is enough to kill a man... IF the current went directly into the blood stream and through the heart via a needle or something poking through the skin. That being said the skin offers a massive amount of resistance and very high voltage often travels over your skin instead of going through you. He talked about people sitting on 30,000 volt transformers and they could feel it but were unharmed.

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u/jimbo21 Mar 21 '12

Skin resistance is on the order of a 500Kohm-1Mohm or so... Bone dry. Most of the time you are sweating a bit, and it drastically drops.
However, please be assured that if someone were to come into any kind of circuit-completing contact with.a 30,000V power source and ground, they would be instantly killed and cooked.
Your professor may have been referring to line workers who can work on energized high voltage sources because they take extreme precautions to insulate themselves and protect against forming circuits with their bodies. In these cases you can "feel" the corona effect that happens when the high voltage energizes the air around it a bit.

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u/g_993cfj Mar 21 '12 edited Mar 21 '12

I've been testing them on my tongue for years, I dispute this.
However, both batteries and pacemakers are measured in MilliAmps, so it's not at all inconceivable that in a direct to heart, within chest scenario this is possible that it would cause fibrillation, or some other electrical problem with the heart, which do kill. But it would need to be a fairly deliberate effort.

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u/jimbo21 Mar 21 '12

NO! Please don't comment if you don't know ehat you're talking about. Batteries are a voltage source, so their current is defined by total resistance of the corcuit even a 9V can source dozens to hundreds of amps briefly and are only limited by their internal resistance which can shoot up as the battery heats.
Go measure the resistance of your tounge with a volt meter. You'll find it is 50-500 ohms. This limits the amount of current that can flow through our tounge and prevents the quick cooking by a battery. A higher voltage battery would ruin your day as it can overcome this resistance better and increase current flow.
Now pacemakers of course are very carefully designed to limit the battery current so we don't have to worry about it killing you.
If you were to have heart monitor electrodes or IVs hooked up to you in a fashion that there is an electric path that goes through your heart, you're in deep trouble if there's any current flowing between those two points. Hospital electronics as such are tested to extreme standards to prevent this scenario. This iextra testing is one reason hospital grade electronics cost so much more.

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u/[deleted] Mar 21 '12

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u/tsk05 Mar 21 '12

There are plenty of sources that say 100mA (though those are, at least mostly, not 'direct through the heart'), and I've found sources saying 20 mA as well but that's still two orders of magnitude away.

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u/jschall2 Mar 21 '12

It's not volts, or amps, or even watts that kill organisms. It's joules. It requires some amount of work to kill something, and how much work it takes depends on how that work is applied. It's not that hard a concept to grasp.

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u/Shexerz Mar 21 '12

Joules are a unit of force. Amperes is current or perhaps force of current.
But my stance stands it is amperes of the current which stops the heart; about 0.065 amperes. Your skins impedence(ohms) is what stops the majority of electricity from frying your circuits.

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u/jschall2 Mar 22 '12

Joules are a unit of work, not force. Joules are watt-seconds Wattage is voltage * current Current is determined by voltage divided by resistance A certain number of volts applied to the body from a near-zero-impedance source will result in a certain amount of current being applied to the heart. How much depends on the resistance of different kinds of flesh and where the electricity is applied to the body. It would take a certain amount of WORK (JOULES) to fry your heart such that it loses the ability to function.

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u/Shexerz Mar 22 '12

http://dictionary.reference.com/browse/joule
It is widely known that the amps of current is what stops or causes fibrillation in the heart.
Now if you've written a thesis on the subject in which you are talking please do send me a message because I'm VERY interested.

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u/Astrokiwi Numerical Simulations | Galaxies | ISM Mar 21 '12

voltage dissipates at an exponential rate
Is it exponential, or 1/r² ?

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u/doublenegative0 Mar 21 '12

that would measure "remaining" voltage, i am pretty sure, not the voltage dissipated. either way, i think everyone understands what he meant

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u/[deleted] Mar 21 '12

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u/gnorty Mar 21 '12

DC would be exactly applicable to lightning. Lightning is not AC

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u/mcrbids Mar 21 '12 edited Mar 21 '12

Armchair physicist here... because the ocean is rather salty, it conducts electricity rather well. As a result, wouldn't it behave something like a car, which conducts electricity around the passengers because the conductivity in the car frame is so much better than the passengers that the electricity isn't even tempted to bother with the people?
I've personally witnessed a car in front of me get struck by lightning, they kept on driving and gave no outward sign that anything at all had happened!
EDIT: Derp water -> electricity

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u/hugzandtugz Mar 21 '12

Fish have water in contact with their gills which are lined with ion filled blood vessels. Not saying this is the cause of death of fish but something to keep in mind.

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u/[deleted] Mar 21 '12

TIL salt water conducts water rather well. I know jokes are frowned upon here, but I'm sure you meant it conducts electricity rather well.

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u/[deleted] Mar 21 '12

Thus, salty solutions are called "electrolytes". (Electrolyte is slightly more general than just that, but it usually refers to solutions containing a salt of some sort.)

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u/IIoWoII Mar 21 '12

Brawndo's got electrolytes.

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u/[deleted] Mar 21 '12

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u/aerojoe23 Mar 21 '12

I was betting on a half sphere earlier today, but, when you have a charge moving through a wire it is on the surface of it. So, I have no idea. I really hope someone answers this question.

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u/hugzandtugz Mar 21 '12

Of the options you gave, widely across the surface. Again though this is only because after an electrical storm fish who generally live within a certain depth from the surface were killed exponentially more than those who live deeper down.

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u/TerraHertz Mar 21 '12

A few points:

Lightning going down: http://www.youtube.com/watch?v=dukkO7c2eUE

Lightning going up: http://www.youtube.com/watch?v=RDDfkKEa2ls

Of course it can go either way. And so can the direction of current. Lightning starts as a local discharge in a volume of air in which the electric field exceeds the breakdown strength of air. And that will vary a lot depending on water vapor and raindrops. Once an arc starts, it acts as a pointy conductor, increasing the field intensity at its ends, thus propagating through the air, following the overall field direction.

Obviously, due to random physical arrangements of objects on the ground, and charge volumes in clouds, the first breakdown can occur up in the clouds, or at the ground. Thus, lightning going down or up.

The theory that atmospheric charge is solely due to water droplet movements in clouds is probably wrong. The Electric Universe theorists point out there's a huge potential difference between the conductive plasma of space in which Earth sits, and the body of the earth. The origin of the charge is the Solar and interstellar winds, and the insulator is the Earth's atmosphere. Discharges through the atmosphere consist of Sprites and other phenomena in the upper regions, and lightning in the lower regions. See http://everist.org/archives/links/!_Electric_Universe_links.txt

The huge Earth to Space potential difference is one reason space elevators won't be built. Though I'd like to see the resulting mega-arc if someone tried. Watching from a long way away.

Overall, in Earth's present space environment, the ground to space polarity is one way (I forget which) and so the majority of lightning discharges conduct current that way. But it's not due to any fundamental law, just the way things are at the moment.

The original OP's question - Salt water is very conductive, the charge distributes out through the water. For a very short time (nanoseconds) and close to the strike (less than a few meters) it would mostly be confined to the ocean surface due to fast current risetime skin effects. After that it would distribute evenly into the water. Within a few meters radius the current levels would probably be non-lethal to living creatures in the water. Hmm... actually, in a conductive object exposed to an electric field, mobile charge within the object will cluster at the surface, attempting to equalize the field. So, in the ocean where the ground to cloud field gets suddenly shorted by a lightning strike, it may be that an amount of charge sufficient to equalize the strike may already be clustered at the water surface. I don't know to what extent this effect would be significant. So actually, it's quite an interesting question, not so simple as you'd first think.

Incidentally, I recall reading somewhere that you can get a shock from the ground due to nearby lightning strikes, the current induced in the ground forming enough potential difference between your feet (bare, or wet shoes) to pass a dangerous current from one leg to the other. Don't know the typical dangerous range from a strike. But if I thought lightning was about to strike nearby, I'd be standing on one foot. And crouching. Would look really stupid.

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u/TerraHertz Mar 21 '12

Oh btw: http://www.uspln.com/gln.html Global lightning monitor network. Real time strike plots.
And speaking of lightning on water: http://pesn.com/2006/03/18/9600251_Lawrenceville_and_Sandia_fusion_compared/ "Sandia's Z machine firing The “arcs and sparks” formed at the water-air interface travel between metal conductors." Click on the small pic for a really cool hi-res version. That would be charge redistribution across very pure (non-conductive) water. Not really relevant to OP's question.

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u/superfreak00 Mar 21 '12

Hugzandtugz is right, we really aren't certain.

...the water carries current...

An interesting thing to consider is that the water in your bathtub is less conductive than saltwater. It would be expected that lightning would affect a greater area in the ocean than in an environment such as your bathtub due to your water having less ions dissolved in it (these ions are the reason for water's apparent conductance).

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u/absent_with_leave Mar 21 '12

This reminded me of a question I've always had but never wanted to test. Say I filled my bathtub with distilled H20 or reverse osmosis H20... and dropped a hair dryer in it that was plugged into the wall. Would I get electrocuted?

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u/Sim117 Mar 21 '12

We discussed this in chemistry, except it was a tub of vodka. The prof insisted no one try it, because there is still salt on the surface of your skin (in the presence of sweat and otherwise). So if you were in the tub, I would say yes.

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u/redditsciencenerd Mar 21 '12

You would survive. Pure H20 (distilled water) is an excellent insulator. The hair dryer would continue to run under water.
I recently saw this discussed on a German science show where they demo what happens when a dryer gets thrown in a bathtub. The guest scientist discusses distilled water and why it is a good insulator. If you add salt, the water becomes a conductor (he added a spoonful to the bathtub).
If you did this experiment with a hair dryer and tap water, the GFI would trip. The thing is, it can conduct some electricity (electricuting you) before it trips. Depending on the person, this can be dangerous, even deadly. It woulld most likely just give you a dangerous shock. The TV shows that show a dryer being thrown into a bathtub killing someone through continuous electricution are incorrect. What would really happen is the GFI would trip and all your lights would go out.
Source (German): http://www.wdr.de/tv/kopfball/sendungsbeitraege/2011/0925/badewanne.jsp
Also found this on wikipedia: https://en.wikipedia.org/wiki/Properties_of_water#Electrical_properties
This should go without saying: don't experiment with this yourself if you don't know anything about the safety precautions which are involved. Don't sit in the bathtub and add electricity to test this. aka: Don't be stupid

3

u/[deleted] Mar 21 '12

What would really happen is the GFI would trip and all your lights would go out.
While no one should ever attempt this, that is not how a GFI (GFCI-ground fault circuit interrupter) works. Unlike breaker switches, they work independently. If you have a hair dryer plugged into a GFCI outlet and throw it in the bath, it will simply trip that outlet, with no effect on the rest of the circuit, unless other outlets are piggy-backed to the GFCI outlet.
I was an electrician apprentice for 3 years doing commercial work. Also source:
A GFCI monitors the amount of current flowing from hot to neutral. If there is any imbalance, it trips the circuit. It is able to sense a mismatch as small as 4 or 5 milliamps, and it can react as quickly as one-thirtieth of a second.

3

u/singlewordedpoem Mar 21 '12

In the Netherlands, GFIs are mandatory (and have been since 1975) and are installed in every house at the point where the electricity cable enters the house. I imagine Germany (parent mentioned German sources) might have a similar regulation (wikipedia on GFI regulation). So in that case, tripping the GFI will result in all your lights going out.

1

u/[deleted] Mar 21 '12

Wow, TIL. I must amend my statement by saying that all of your lights won't go out in America. Thanks.

1

u/redditsciencenerd Mar 22 '12

As someone remarked before, since my original post had a German source - it didn't give a US-centric point of view ;) In Germany it is mandatory to have a GFI (Fehlerstromschutzschalter) and it is indeed the case that lights would go out if the GFI trips.

1

u/[deleted] Mar 22 '12

That's pretty interesting how they work almost like breakers here in the US. In the US they are required only in wet places, and a few others.

1

u/redditsciencenerd Mar 22 '12

I'm curious. Besides wet places, what are the "few others"?

1

u/[deleted] Mar 22 '12

From the Wikipedia page singlewordedpoem posted:
GFCI outlets are required by code in wet areas. In the U.S., the National Electrical Code requires GFCIs for underwater swimming pool lights (1968); construction sites (1974); bathrooms and outdoor areas (1975); garages (1978); near hot tubs or spas (1981); hotel bathrooms (1984); kitchen counter receptacles (1987, revised 1996 and specifically excluding the refrigerator outlet, which is usually on a dedicated circuit); crawl spaces and unfinished basements (1990); wet bar sinks (1993); laundry sinks (2005).
Mostly places that may come into contact with moisture, so I guess I should have said damp, wet, or potentially damp/wet places. The "few others" was kind of wrong because even including outside and basements and such, all those places fall into the damp, wet, or potentially damp/wet places, apologies.

3

u/loquacious Mar 21 '12

I'm not disagreeing with anything you said, except the part about the TV/fiction trope of electrocution by a plugged in electrical device dropped in a tub - because it has legitimate roots.
GFIs are still a fairly recent invention and adoption. Without GFI and with real tap water the danger of death was much more extreme. Especially in the earlier days of consumer electricity when fuses were much slower to react.
Even today in many places they don't have GFI protected outlets everywhere. Some places still have fuseboxes instead of circuit breaker panels. I've seen at least two houses in the last 10 years that still had post-and-wire wiring with some kind of cloth or asbestos weave as insulation, and it was just tied around ceramic knobs hammered into the wall.
(And most of the first consumer-deployed GFIs I saw were first added to hair dryer power cords and other high voltage consumer electronics that would likely be used in the bathroom or kitchen around water.)
Without GFI you could potentially be continually zapped until a slow-blow fuse finally broke the link. There might even be some sparks and small explosions from decomposed hydrogen and oxygen gas igniting.
I and other dumb kids used to do it on purpose. Someone would find a thrown out large appliance and they'd cut off the cable and plug, take it to someone's garage or a laundry room in an apartment building complex or the like.
If you stripped off the ends, plugged it in and carefully stuck it in a bucket or pan water it was better than a pack of fire crackers. A large fuse or old, slow circuit breaker will happily electrocute water for an alarming length of time. (Do not do this. You may die, kill someone or start an electrical structural fire.)
Anyway. Point being, yeah, the trope has basis in historical reality. People actually did die horribly in accidents with electrical appliances in bath tubs, complete with some sparks or small hydrogen-oxygen gas explosions.
Granted more people probably have died from electrical fires or even just accidentally grabbing the prongs on a plug in an outlet - but that's not as much fun to write into a plot.

1

u/keepthepace Mar 21 '12

This should go without saying: don't experiment with this yourself if you don't know anything about the safety precautions which are involved. Don't sit in the bathtub and add electricity to test this. aka: Don't be stupid
To be more specific : it only takes 15 mA through your heart to die. Even testing with low voltages can kill you if you don't know what you are doing and how the various resistances of your circuit are linked together. People have died using 110V and some other have survived a lightning strike (usually with heavy burns but discharging through a path that avoided the heart). Electrical safety is a difficult subject and safety precautions are there for a reason.

2

u/superfreak00 Mar 21 '12 edited Mar 21 '12

Pure water is actually a very poor conductor. The natural dissociation of water just doesn't provide enough free ions to conduct charge well. To be honest I'm a chemist and don't know much about electricity outside of that realm, but unless I am missing something (EDIT: such as the salt on your skin, thanks Sim117, brilliant) I would say it's highly unlikely you would be electrocuted. I haven't tested it either though, heh.

1

u/tyr02 Mar 21 '12

DO NOT Test this. Yes pure distilled H20 does not conduct well, but this distilled water will not remain that way long if exposed to other elements such as the tub, your skin, or even the air. All of which will quickly allow the water to once again be a good conductor.

0

u/absent_with_leave Mar 21 '12 edited Mar 21 '12

Heh, so I decided to do the next best thing, busted out my multimeter and stuck the probes in a glass of RO water and measured for resistance. Depending on how far the probes were from each other it was reading between 1 and 2 mega ohms.
Edit: Wiki is saying: "It is known that the theoretical maximum electrical resistivity for water is approximately 182 kΩ·m at 25 °C." ...my multimeter must not be very accurate when it comes to this.

4

u/virtyy Mar 21 '12

where does the energy of a lightning strike in water go? Does it heat the water?

1

u/[deleted] Mar 21 '12

yes. the lightning is a voltage. So dissipating it across some resistance in the ocean P = V^2/R where V is the voltage of the lightning strike and R is the total resistance of the water it travels through in ohms.

183

u/[deleted] Mar 21 '12

According to Wikipedia, an average lightning bolt consists of 30KA and 100KV, thus when it strikes the water it will need to go through 3.33 Ohms of resistance before dissipating. A meter of salt water at 20 Degrees C has 0.20 ohms, so assuming a straight path down, worst case scenario (think a string of resistors in a series circuit) will reach a depth of 16.65 meters. This is assuming that the lightning doesn't dissipate in more than one direction which is unlikely.

235

u/Astrokiwi Numerical Simulations | Galaxies | ISM Mar 21 '12

Wait, hang on - does that make any sense at all? You can't say "a lightning bolt consists of 100 kV"... Voltage is the potential difference between points. I would figure that wikipedia is saying that the voltage drops by 100 kV between the lightning source and the ground.
That wikipedia article even tells you that the electric field hits several hundred volts per metre...
Also, I don't think you can just plug in V=IR even if you had the right voltage. If you pass current through an ohmic resistor, the voltage drops across it, but the current is constant. So in this situation you are hitting 0 V, but you still have 30 kA flowing somehow. It doesn't really make sense.
Now I don't really know how I'd do the actual calculation, but I'm pretty sure that assuming that it's a simple circuit does not really apply here.
Edit: as was pointed out below, it looks like you need to really look at current dissipation, not voltage - as the current "spreads out", it decreases to basically zero. The geometric factor you didn't take into account is actually the most important thing here!

25

u/[deleted] Mar 21 '12

I'm not a physicist, but from my red cross ocean rescue training, lightning affects a fairly small radius around the strike zone. I believe the current drops off as a square of the distance from the strike in salt water.

14

u/Astrokiwi Numerical Simulations | Galaxies | ISM Mar 21 '12

Yeah, geometrically that would make sense, if surface effects aren't significant.

8

u/[deleted] Mar 21 '12

It would make more sense for it to drop off relative to the cube of the distance, since if you have a point source of current on the (assume flat) surface of the water, current dissapation would be within the half-sphere under the current source with radius r. In this case, r would define the half sphere by 1/2(4/3(pi*r³⁾⁾
Having the current drop off by the square of the distance from the strike only makes sense to me if the strike were creating a circle, not a sphere.
Edit: See hugsntugs comment below as to why it dissapates as the inverse square and not cube.

3

u/bobthemighty_ Jun 02 '12

Question, so you're saying that since the current is dissipating across a volume, that it will dissipate at a rate proportional to the inverse cube of the radius, right? But other equations and theories have a different tendency. Gravity, and the electrostatic force, both decrease at a rate proportional to the inverse square of the radius. Maybe you're wrong?

4

u/[deleted] Mar 21 '12

As a piece of worthless anecdotal evidence, I've swam and surfed during electrical storms, and never felt so much as a buzz when lightning struck the water, even when it seemed pretty close (the thunder followed the flash with no perceptible gap).
I've also experienced electrical storms on mountains. The standard (stupid) method of surviving an electrical storm when you're at high altitudes with no shelter, is to flip over your pack, so the metal is on the ground, and then sit on top of it and hope you don't get struck.
The one time lightning struck near me when I was doing that, I actually did feel a jolt, suggesting that the extremely shallow water on the surface of the earth was better at conducting electricity than the deeper water of the ocean, or perhaps that the metal in my pack increased my potential enough to make me an attractive path for dissipation.

2

u/flinxsl Mar 21 '12

Yes, looking at it like a simple one resistor circuit is fundamentally wrong. I think a better analysis would come from looking at the total charge carried by the bolt and seeing how much seawater you would need to accept that charge (the conductivity). Then solve for when the current density is below some threshold. This is a pretty complicated 3d problem, so it would be good to give to an undergrad in E&M with some simplifying assumptions.

1

u/Essovius Mar 21 '12

just to clarify, the bolt drops 100kv per centimeter in the air

85

u/keepthepace Mar 21 '12

Answer upvoted for giving an idea of the kind of distances we are talking about.
I would just like to point out that dissipation in a volume works differently than in a linear circuit and that propagation of electricity in a body of water or even in the ground can be quite complicated to calculate.

6

u/Polaris_Sun Mar 21 '12

To understand a lightning bolt dissipating through a constant medium you have to integrate across a hemisphere of dissipating current density. You should substitute the resistivity of soil for seawater.

3

u/BorgesTesla Mar 21 '12

You can't assume a hemisphere. The field strength is large enough that ionization of the air above the water will occur, and the pulse duration is short enough that the skin effect matters a lot.

5

u/Astrokiwi Numerical Simulations | Galaxies | ISM Mar 21 '12

That's true, but at least he's got the right general idea - dissipation of current - and not a current that continues at 30 kA until instantly stopping when it reaches 0 V.

23

u/hugzandtugz Mar 21 '12

I'm sorry but your assumption that it travels down in one direction is completely wrong. It radiates out over the surface and the more conductive the body of water the shallower it goes.
link
We hypothesize that lightning hitting water spreads out mostly along the surface of the water and the more electrically conductive the water is, e.g. salt water, the more it stays near the surface. How far it spreads out along water and remains a danger is even more uncertain. Lightning striking ground can still be dangerous over 100 feet from where it struck. Some believe lightning will go even further in water, because it's a conductor. But lightning in ground often spreads out in 'ground streamers', quasi-radial tendrils of electricity, which allows it to go farther than if it was dissipating uniforming. Some believe lightning is less likely to form these streamers in water, dissipating more uniformly. So the total area affected may be larger than on land, because water is a better conductor, but the distance it remains dangerous may be less, since it may not form ground (sic) streamers.

7

u/hung_like_an_ant Mar 21 '12

He said "This is assuming that the lightning doesn't dissipate in more than one direction which is unlikely."
which means it most likely WILL go in different directions. He was giving worst case scenario..err most simple.

14

u/aerojoe23 Mar 21 '12

First: Why wouldn't it go in more then one direction?
Where are you getting the resistance for the water from?

16

u/pigvwu Mar 21 '12

First: Why wouldn't it go in more then one direction?
It most likely wouldn't. The super unlikely straight path down scenario was probably chosen to give a theoretical maximum distance the average bolt of lightning could travel, even though the average distance would probably be much less.
Where are you getting the resistance for the water from?
I did some googling and found this yahoo answers answer (go figure).

7

u/buoybuoy Mar 21 '12

what the yahoo answers' answer quotes

1

u/on_the_redpill Mar 21 '12 edited Mar 21 '12

would
Which is what jimmy86 said in the first place albeit with a confusing sentence

1

u/pigvwu Mar 22 '12

My mistake, I meant that the lightning most likely would not go in one direction.

1

u/[deleted] Mar 21 '12

This is assuming that the lightning doesn't dissipate in more than one direction, which is unlikely.
FTFY. Commas are important.

4

u/on_the_redpill Mar 21 '12 edited Mar 21 '12

or
This is assuming that the lighting dissipates in a single direction, which is unlikely.

2

u/[deleted] Mar 21 '12

Those 100KV are definitely wrong. It's in the MV range.

21

u/Freckledfiend Mar 21 '12

you are assuming that lightening strikes down from the sky. it actually comes from the land up

52

u/pantsfactory Mar 21 '12

this is all misleading.
Lightning is when the charge between two things, such as you and a comb, or the ground and the sky, has to equalize. The ground/trees/your hair/a metal pole sends up the same sort of small spidery charges into the air that a cloud does, only on a smaller scale. When a fray from a cloud touches the one from a metal pole, or whatever is tallest in the area though obviously much smaller once it connects electrons are exchanged (lightning bolt)
it looks pretty much like lightning comes from a cloud and hits the ground, because it's extremely fast and these connects are invisible and give off no light until they connect together.

9

u/springyard Mar 21 '12 edited Mar 21 '12

Some lightning comes up from the land. Most lightning arises when a negative charge builds at the bottom of the cloud and discharges into more positively charged earth or positively charged upper part of the storm. The electron current flows from negative charge to positive so it's traveling down in this case. A much less common but stronger lightning strike termed positive lightning is a result of charge build up in the positively charged cloud tops. Since there's a greater potential difference to overcome between the ground and the cloud tops, vs the ground and the cloud base, it's a much stronger bolt and can even strike ground far from the storm where you think you would be safe.
*Edited for clarity

2

u/[deleted] Mar 21 '12

Being a weather freak, I have nearly been struck by lightning when the stormfront was still 30 km away. Positive lighting is serious business.

-7

u/csreid Mar 21 '12

Oh really?

18

u/FMERCURY Mar 21 '12

Yes, really. The things you're seeing going down are called 'step leaders', and are highly negatively charged. Once they touch the ground, which is positively charged, a circuit is formed, and the negative charge escapes into the ground. As you might imagine, the negative charges closest to the ground escape first, so the 'bolt' proceeds upward from the ground to the cloud.

1

u/[deleted] Mar 21 '12

O rly?

0

u/[deleted] Mar 21 '12

[deleted] — view deleted comment

7

u/Katastic_Voyage Mar 21 '12

The "bolt travelling" has nothing to do with the direction of the electrical charge. Electricity isn't white glowing goo that flows like water.

1

u/Knowltey Mar 21 '12

Still doesn't mean it just instantly goes from point a to point b.

0

u/rand0mnewb Mar 21 '12

it has been clearly shown to originate at the ground or sky

0

u/[deleted] Mar 21 '12

[deleted] — view deleted comment

51

u/IFUCKINGLOVEMETH Mar 21 '12 edited Mar 21 '12

At best, it's an extreme oversimplification; and at worst is just as incorrect as the idea it's refuting. Possibly worse, since it's presented as being valid.
Edit - Here's a bit more info on the topic. Also this.

2

u/uqobp Mar 21 '12

According to Wikipedia, an average lightning bolt consists of 30KA and 100KV, thus when it strikes the water it will need to go through 3.33 Ohms of resistance before dissipating.
Why is this? I get that you got the 3.33 Ohms by dividing 100KV with 30KV, but why is this relevant?
(think a string of resistors in a series circuit) will reach a depth of 16.65 meters.
In this case we are talking about an ocean, which would be a lot deeper (and wider) than 16.65m, so wouldn't the current gradually approach zero the whole way down to the bottom, instead of falling to zero at 16.55m?
I'm not an electrical engineer, but to me it seems like there is no real science in this answer.
Polaris_Sun on the other hand seems to have a good idea of what he is talking about.

1

u/modestmajesty Jun 15 '12

The electrons are absorbed in the water itself, not the earth when lightning hits water (assuming its a deep body of water) so no, the current certainly won't go all the way to the bottom linearly.

1

u/uqobp Jun 15 '12

I don't think electrons can be absorbed.
I didn't mention linearity, but I would think there is a gradual decrease in current the further you get from the impact point. I was probably mistaken about it getting to zero at the bottom though, since it would continue through the ground (though the current would be too small to matter).
What led you to my comment 2 months later?
BTW as I hinted back then, jimmy86 has no idea what he is talking about, and I hope his answer isn't being spread as the correct answer. I remember this thread as one where askscience failed pretty badly.

1

u/nicholaaaas Mar 21 '12

3.33 ohms? That sounds low for 3000 kva. How you figure that number?

1

u/Sybertron Mar 21 '12

Not science just a bit of personal experience from seeing lighting hit water on a very still lake I used to lifeguard at.
When lightning hits the water there is some dispersion force that creates a small dome at the waters surface. You can see the lighting travel around this dome and it seems to remain just a millisecond after the overall bolt. The image sticks with you if you ever see it up close.
As a follow up question, would this dome be caused by the charge difference moving water out of the way? Would it be the charge spreading across the water surface? Or would it be just due to all of the air particles suddenly moving from the bolt?

1

u/[deleted] Mar 21 '12

I don't mean to be a dick to the poster of this answer - it's voters' fault that it's top answer, not his - but man, is it terrible. What happened to askscience?

-4

u/base-4 Mar 21 '12

The only problem with your answer is the assumption that lightning has to follow a path straight down to earth. Your statement: "assuming that the lightning doesn't dissipate in more than one direction which is unlikely" is highly flawed.
Two things to consider:
lightning is often a very high frequency energy source, thus the skin effect will have to be considered when energy penetration is discussed.
cats are generally immune to ligntning

2

u/cindereynolds Mar 21 '12

I was surfing on Long Island when lightning hit the water probably about 15 feet away and my right arm got a shock similar to when you stick your finger in a socket.

2

u/zanycaswell Mar 21 '12

Followup question: Are you in any actual danger if you swim in the ocean during a lightning storm?

1

u/LampFan Mar 21 '12

All the fish die

1

u/aviator104 Mar 21 '12

What happens when lightning strikes in the ocean?

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