How can water be a medium to conduct current while its ionisation is so negligible that, in principle, no current should flow?
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AccidentalFourierTransform
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asked May 19, 2017 at 18:41
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2$\begingroup$ It only takes 20mA to kill a person while the power socket is in the 100-250V range. "Not a good conductor" is simple not good enough, you need very good insulator just to be safe. $\endgroup$– user3528438Commented May 21, 2017 at 20:00
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3$\begingroup$ "Good" and "bad" are not exact terms. "Good enough" or "too bad" are more exact: It's good enough to kill you but too bad for anything useful. $\endgroup$– Peter - Reinstate MonicaCommented May 22, 2017 at 19:52
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6 Answers
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"Pure" water is a very poor conductor (resistivity is actually used as a measure of purity).
"Real" water is not pure - it contains electrolytes and is quite conductive. Also - when your skin is wet, its resistivity is significantly lower.
For example - "pure" water has a resistivity of (about) 18.2 M$\Omega\cdot\rm{cm}$. With 10 ppm of dissolved NaCl ("very good quality tap water" would have less than 50 ppm), the resistivity drops to about $43~\rm{k\Omega\cdot cm}$
A lot of detail can be found in "Conduction of Electrical Current to and Through the Human Body: A Review" (Fish and Geddes, ePlasty 2009, 9, e44). Table 3 sums it up:
Table 3
Why immersion in water can be fatal with very low voltages
Immersion wets the skin very effectively and great lowers skin resistance per unit area
Contact area is a large percentage of the entire body surface area
Electric current may also enter the body through mucous membranes, such as the mouth and throat
The human body is very sensitive to electricity. Very small amounts of current can cause loss of ability to swim, respiratory arrest and cardiac arrest
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29$\begingroup$ Also, although water isn't a great conductor, it is a liquid, which means it can flow through small cracks and get into paces that solids obviously don't. This means that it can create short circuits and dangerous connections in hard to stop ways. With wet hands for example it is possible for excess water to seep into an electrical device and create a connection between you and the circuitry that wouldn't be remotely possible with dry hands $\endgroup$– PhillSCommented May 19, 2017 at 20:47
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32$\begingroup$ Even if water is a very poor conductor, it depends on what you compare to. Air insulates better. So if you and the electric device are submerged in air, that is safer than water. $\endgroup$ Commented May 20, 2017 at 1:17
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$\begingroup$ I thought water conducted well not because of water it self, but the chemicals dissolved in it. When salts dissolve in water, they form a ionized flow of positive and negative charge which conducts electricity very well. Pure water would be safe to swim in with electrical charge. Put some salt in it and see what happens. $\endgroup$ Commented May 23, 2017 at 14:38
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$\begingroup$ @DroidChris - that is pretty much exactly what my answer says. It takes a tiny amount of impurity to take water from "poor conductor" to "not poor enough to be safe with electricity". $\endgroup$– FlorisCommented May 23, 2017 at 14:50
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1$\begingroup$ @An_Elephant conductivity is normalized to volume. Sure if you have larger electrodes you can carry more current but there are few ions per unit volume and that’s the problem. $\endgroup$– FlorisCommented Aug 1, 2022 at 13:37
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The problem is in the definition what a "good" or "bad" conductor is. In school children are taught that everything is quite easy: there are "good" conductors like metals and "bad" conductors like plastic.
Short question before: What do you think is the difference in electrical conductivity between silver (the best non-exotic conductor under standard conditions) and stainless steel ?
Silver has a conductivity of 63 000 000 S/m, stainless steel has 1 450 000 S/m, silver conducts electricity 43 times better than stainless steel! (I am writing the numbers out because the difference is hidden for laymen if you use unfamiliar scientific notation).
S is an unit called siemens, and siemens per meter (S/m) is the SI unit of conductivity.
The thing is that electric conductivity is one of the physical quantities which has the largest observable differences between various materials. I give a short overview:
Semiconductors (They are still looking like metals!)
- Germanium: 30 000 000 ($3\times10^7$) times worse than silver ($2.17$ S/m)
- Silicon: 40 000 000 000 ($4\times10^{10}$) times worse than silver ($1.56\times10^{-3}$ S/m)
Bad conductors
- Seawater 13 000 000 ($1.3\times10^{7}$) times worse than silver ($4.80$ S/m)
- Damp wood: 100 000 000 000 ($1\times10^{11}$) times worse than silver (From $10^{-4}$ to $10^{-3}$ S/m)
- Glass/rubber: ~100 000 000 000 000 000 000 ($1\times10^{20}$) times worse than silver (From $10^{-15}$ to $10^{-11}$ S/m)
- Air: ~1 200 000 000 000 000 000 000 ($1.2\times10^{21}$) times worse than silver ($3-8\times 10^{-15}$ S/m)
No, it is not a joke. Let's imagine that the distance from you and the moon is comparable to a good conductor. The school definition of bad conductor would be then the height of an apple tree for sea water and the size of an atom for glass.
So why are we not aware about this staggering amount of difference? The reason is that we are quite fragile about electricity, even very small currents could cause pain or death. So water has still enough conductivity to cause problems despite being a comparatively bad conductor, metals are simply extremely effective conductors allowing to transmit electricity over vast distances with relatively small losses. The other materials as non-conductors like glass or air are effectively non-conductors even shielding someone from very high voltages (The problem is once a connection has been built, an arc of ionized molecules is forming which has much, much better conductivity than the unchanged material. Normally you need approx. 5 000 V per centimeter in air to build a connection so you would be safe from 1 000 - 10 000 V of transformer stations, so this is the reason they put up the danger signs).
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9$\begingroup$ Lay people are quite familiar with the words "million", "billion", and so on. Even as someone familiar with scientific notation, I would find it easier to read "1.45 million S/m" than "1 450 000 S/m". $\endgroup$– user300Commented May 21, 2017 at 1:53
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2$\begingroup$ @Rahul Not "and so on". "Trillion" is the last progression I know is actually used, "quadrillion" is practically unknown. There is also the problem that many countries (including my home country) still use long scale meaning that a billion is 10^12, not 10^9 and a trillion is 10^18, not 10^12 which has lead to innumerable errors. I really thought over the problem how to bring over the amount of scale without confusion and errors, so I used ratios to prevent switching the scale (from million to quadrillionth). $\endgroup$ Commented May 23, 2017 at 20:46
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1$\begingroup$ Spot on, Thorsten. Big number names are just plain silly, second only to roman numerals as an expression of innumeracy. I tell my children not to worry too much about them, that they will soon learn a much better way of dealing with big numbers (scientific notation). The only thing one needs learn is the names million, billion and trillion (and milliard for when they read the European news). Not because one would want to use these names, but rather so one will know how big the lies are that one is being told when politicians use these names to bedazzle audiences with economic "prowess". $\endgroup$ Commented Mar 14, 2018 at 14:03
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$\begingroup$ BTW I agree about the long/short confusion; a couple of names might actually be useful were it not for this disaster. In my lifetime this has even completely changed here in Australia. When I was little (1970s), the long system prevailed and "milliard" was a useful and current English word. "Billion" for $10^9$ was never heard. Now the short system is exclusively used in general parlance, but surveys show that many older people don't even know that they are meant to understand $10^9$ for "billion" now. $\endgroup$ Commented Mar 14, 2018 at 14:07
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In high school my chemistry teacher, Mr Stratton, set up an experiment. He had a light bulb fixture on a wooden paddle, with a power cord attached and two metal prongs projecting at right angles, such that the paddle could be set over a beaker of water with the prongs dangling into the water.
He first filled the beaker with distilled water and screwed a 40w incandescent bulb into the fixture -- no light. Then he tried something like a 10w bulb -- still no light. Then he tried a neon light -- very dim glow.
Next he poured in a small amount of salt (NaCl) and swished it around -- the neon light began to glow brightly. Tried the 10w bulb -- glowed at nearly full brightness. Tried the 40w bulb -- not fully bright but at least halfway bright.
Water is a good polar solvent, and combine that with any sort of chemical which ionizes when dissolved and you have a solution which can carry a significant current with relatively low resistance. (And salt-like chemicals are everywhere.)
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Water is indeed NOT a good conductor UNLESS impurities exist. And if you could keep the impurities from contaminating it, you could put a radio in it and it would still work. You can find out yourself. Take distilled water (about a cup) and check the resistance with an ohm meter with probes fixed distance approximately 1/2 inch apart. It will show infinite resistance for all practical senses.
Then dissolve about 2 grams of salt in a tablespoon of water. Keep the probes in the water and add the saline to the water and watch. The ohms dive from being an insulator to a fairly good conductor... You can even do this by just putting your hand in the distilled water as the salts from your skin dissolve in the water.
The argument that water is a conductor because "water seldom is pure enough not to contain some dissolved mineral contaminants" is like saying that it isn't clear either because look at the oceans or the lakes.
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1$\begingroup$ Even carbon dioxide from the air dissolving in the water will take you a long way from the perfect "pure" water as far as conductivity goes. $\endgroup$– FlorisCommented May 21, 2017 at 1:30
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Water is a poor conductor compared to wires and metal, but is an excellent conductor compared to air, glass, ceramic tile, and other objects in the bathroom or kitchen, particularly as mentioned in other answers if there are certain impurities dissolved in it.
When our bodies and feet are wet and we are walking across a wet floor, and we have somehow touched a wet electrical outlet or wire attachment; if the outlet is not ground fault protected, the electricity from the hot (live) side of the outlet will try to find a shortcut to ground through your wet skin and your wet feet to the floor to ground. As mentioned earlier, even a very small current through your body can stop your heart (cardiac arrest).
That is why GFI (ground fault interrupter) protection is required building code on all electrical outlets in kitchens and bathrooms. This type of protection senses when there is a short circuit bypass from the breaker ground and cancels the hot line on the outlet.
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Water is a conductor! Because water seldom is pure enough not to contain some dissolved mineral contaminants (salts / electrolytes). Standing water in a lake or pond or puddle is not almost never pure but never pure. Dirty water qualifies as impure water, by the way. Understand then that the contaminant is in the form of ions which make water a perfect conductor as it has another great quality: adhesion to solid or supple surfaces (human skin for example). Hence creating the superb if not the perfect condition of making water an ideal conductor of electricity to shock your backside into oblivion when there are raw high voltage wires from downed transmission lines/poles due to a storm or random car accident in or near a body of water as in puddle in the street.
