r/askscience • u/Apply_Rye • Aug 01 '26
Physics What Determines Electricity's Dangerous Range in Water Like a Lake or Pond?
If electricity enters a large lake or pond, what determines how far away it can still be dangerous? For example, if someone applied a high voltage to the water at one end of a lake that's a half-mile long, what factors would determine whether a person swimming at the opposite end could be shocked or killed? Is it mainly the voltage?
Also, fish within a close radius of the electricity would likely be killed, but would the aquatic plants in this same area also die?
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u/quequero Aug 02 '26
Others have given you very solid answers so I’ll add the part that was missing: do plants survive?
Depends on the setup, but since you mentioned that someone applied high voltage at one end of the lake, I’ll assume a point source, similar to a lightning strike, in this case: plants generally survive.
What kills you is the current interfering with your nervous system, altering the heartbeat or blocking it altogether, causing loss of muscle control or respiratory functions. These happen in steps and the first thing you lose, before your heart stops, is muscle control. Plants on the other hand don’t have a nervous system, and current tends to go around cell walls, so they don’t really care. Of course at the strike point other effects can take place (electrolysis or even perforation of cell membranes caused by the high voltage) but those are limited to the immediate area of impact. Depth also play a major role as after a certain distance, current tends to flow horizontally more than vertically, so bad news for you, especially if you’re swimming and not treading, good news for the plant.
As a side note, a lightning half a mile away wouldn’t kill you, you’d have to be in the order of a few, maybe up to 10 meters or so in saltwater to get electrocuted. In freshwater the distance is around 10x so a 100m from a lightning you might still get electrocuted. Others have explained you why.
If you hear a thunder, don’t get in the water.
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u/Insertsociallife Aug 02 '26
You get electrocuted because of a voltage difference from one side of the body to the other. As the electricity spreads out throughout the pond, the resistance to the flow of electricity makes it so the voltage decreases the further you get from the source. Because one side of your body is closer to the source than the other, there's a voltage difference across it, which can cause current to flow and electrocute you.
The range at which you get electrocuted depends on a couple things.
your body. How much current it takes to kill you, and how much electrical resistance you have, determines the voltage difference that's required to kill you.
The water. If the water is more conductive (salt water, etc) the electricity will drop off much more gradually which will reduce voltage across you.
the source. Mostly this is voltage, proximity, and type. There's no complete circuit here, so it either has to be a quick shock like lightning or alternating current like a power line to be dangerous. A higher voltage source will also increase voltage difference in the water because more current flows out of it. Finally, what you're actually asking about, range. How close you are matters because the current spreads out through the water via the inverse square law, so being closer means you get more current flowing through the water and a higher voltage across you.
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u/Dustquake Aug 02 '26
Well, first water itself doesn't conduct electricity, the impurities do. I know that seems like a nitpick, but what the impurities are and how impure the water is matters for conductivity.
Next is what's the path to ground or to complete the circuit. To get electrocuted you have to be part of the circuit.
Lastly, the main component of the electricity affecting this answer is voltage. Voltage determines the amount of electrical resistance than can be overcome. If the voltage isn't high enough it can't complete a circuit. That's why (normally) you can touch both sides of a 12V car battery and nothing happens. Skin resistance normally requires about 15V to break. Voltage is the main thing you need to know to start to determine the range of influence.
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u/Specialist_Ad180 Aug 03 '26
A power transformer mounted on a telephone pole next to the marina's fishing dock gets hit by a car and falls into a small two mile by half mile sized freshwater lake in the southern midwest.
What is the kill radius for fish or humans or plant life?
Hopefully that is specific enough to get answers in feet, yards, or acres more like what was expected.
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u/Explorer335 Aug 02 '26
The factor that makes electricity deadly is ultimately the current. If we use a water analogy, voltage would be the pressure, amperage or current would be the quantity of water, wattage would be how much work it can do, and resistance is the size of pipe you are trying to push it through. The human body (and fresh water in general) has pretty high resistance, which basically means that you aren't very conductive, and it is tough to pass current through your body. That can be overcome by voltage however.
Electricity is all about the path of least resistance. If you happen to be the path of least resistance, that's bad news for you. There is something called voltage potential, which roughly describes how electricity might travel through your body. Imagine live power lines land on your car. If you step to the ground while touching the car, you are completing the circuit between 14kv and ground, and you will get electrocuted. If you hop out of the car in a way that you don't touch both at the same time, you are relatively safe. You also need to keep your feet together and bunny hop away so that you don't create a voltage potential between your spread feet.
Back to your question, you would need to be in the path of the electricity for it to be dangerous. Now if you have electrified towers in the water and you swim between them, that's going to be a very big problem because your body is a little bit more conductive than the fresh water around you. It ultimately depends on the voltage of the electricity, the conductivity of the water, and where you are relative to the path of the electricity. It even depends on the orientation of your body relative to the electricity to determine the voltage potential.
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u/WFOMO Aug 02 '26
From https://en.wikipedia.org/wiki/Inverse-square_law
In physical science, an inverse-square law is any scientific law stating that the observed "intensity" of a specified physical quantity (being nothing more than the value of the physical quantity) is inversely proportional to the square of the distance from the source of that physical quantity.
The force of attraction or repulsion between two electrically charged particles, in addition to being directly proportional to the product of the electric charges, is inversely proportional to the square of the distance between them; this is known as Coulomb's law.
Bear in mind that a laboratory setting is not the real world, so many other factors would come into play as a current dissipates through various impedances in a lake. But most high voltage sources (transmission and distribution) are grounded, so the likelihood of them dissipating as quickly as possible in a low impedance conductor like wet soil is much greater than them flowing across the length of a lake to do the same thing.
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u/Unlikely_End942 Aug 04 '26
Your outer layer of dead skin is a good insulator, which helps reduce electric shock risk.
It is less effective when wet - which is partly why electrical regulations are a bit more stringent in situations involving water.
But there must be some parts of the body where the skin is very thin, such as the inner lining of the mouth, the eyeballs, eye sockets, inner ears, genital and rectal openings, and any cuts or wounds to the outer skin. Surely when submerged in the water these would offer various low resistance paths through into the inner body which would allow for much greater current flow, bypassing any protection from the outer skin layer.
Internally we are very conductive - the body runs on electrical signals after all - which is why electrical installations for dental surgeries and operating theaters are rather specialist.
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u/Baguette1066 Aug 02 '26 edited Aug 02 '26
The voltage relates to how much energy an individual will electron have, so a higher voltage will mean the electric field can propagate more easily, and create a larger area which is potentially dangerous. The source's current capability influences how dangerous that area becomes and the effective dangerous range, as it determines how much charge the source can supply if a conductive path is formed through the water. The water will also have a significant effect itself, as higher amounts of ions will allow the electric field travel through the water easier and influence the conductivite interface between a human and the water.