I teach science in an elementary school. I think I know the answers to the following questions, but I’m not sure enough to bet the bank on them (and mine, as a public school employee, is a very small place anyway…)
Why exactly do people conduct electricity so well? (I’m thinking it’s because of the large amount of water in our bodies, along with other conductive materials like salt and iron.)
If we have to get out of the water when there’s lightening because we can get electrocuted even a distance away from the strike, what happens to the aquatic life nearby a strike? (My guess is that anything unlucky enough to be close enough to the strike will be toasted too.)
Do any metals NOT conduct electricity? (Titanium, perhaps?)
Any enlightenment…or illumination would be appreciated.
You’re right about the minerals in the water that is us. That’s why.
The big hazard about being in the water or, worse, being in a boat, waving a graphite fishing rod around, is that you are the highest protruding conductor for yards around. Lightning usually hits the highest grounded item. Lightning is capricious stuff, though, and being the second highest item may not keep you from being struck.
Nitpick: Pure water is not a good conductor (I think it will break apart into H and O ions before it will conduct, but I’m not positive). In nature, and in the human body, water has a lot of ions already suspended in it (Na+, Ca-, K+), which flow easily when an electric field is applied.
I think a metal is defined by having a lot of loose electrons in the valence shell, so I assume they all conduct. Not positive (wait for laughter) though.
I wouldn’t call humans good conductors or electricity. Hand-to-hand contact resistance is anywhere from a few hundred to a few thousand k[symbol]W[/symbol]. This can result in anywhere from a few [symbol]m[/symbol]A to a few hundred mA. ot what I would really call a lot of current. The significance is that it only takes but a few mA to induce arrhythmia.
The main factors that influence human resistance are the contact area and pressure, along with the chemical make-up of whatever oils and sweat are on the skin. When I grab tightly onto the test leads of my ohm meter, I can get a low as 80k[symbol]W[/symbol] if I dampen my fingers with salt water.
Actually, that should be 25uA – micro-amps, not milli-amps.
I actually taught this subject as ‘Electrical Safety in Hospitals’ as part of my job for several years. Hospitals are very dangerous places, especially for sick people.
It’s true that the skin has amazing resistance to electricity, however when you are sick, that resistance rapidly decreases. So what do we do? We take you out of your bed at home, and put you in an electric bed, with an electric nurse-call button, and all kinds of electrical outlets and lights on the headboard.
Then, if you’re really sick, we attach many electrical devices to you, some of them piercing the skin. The sicker you are, the more devices we attach.
It’s no wonder that an entire profession grew up around making sure this equipment is safe: Bio-medical Equipment Technician. Every hospital has a few of us, usually in the basement, near the morgue.
Anyway, I did, and it seems to be one of several alloys of iron, nickel and chromium. Some sort of resistance wire, I believe, since iron and nickel are both poor conductors, as metals go.
Come to think of it, that might be transformer core material. I never came across that one at EWC, though. I thought I knew most of the commonly used materials. Meh.
Guys, thanks for the info but…ya lost me. Did I mention I was an ELEMENTARY school science teacher? I work best with information in short, easy to digest, math and formula free bites (much like my students—not that we eat them, of course. Well, not with any great frequency.)
I got the the part about sea water. I think I get the gist of the idea about human conductivity and the resistance of skin. But the metal part is still fuzzy. Are there any metals that don’t conduct electricity? I heard once that lightening doesn’t kill airplane passengers because the the charge is carried around the body of the plane by the metal skin. However, I was told, if the metal of the body is changed to a lightweight metal like titanium in order to increase fuel efficiency, that would no longer be the case. True, false, bunkum?
V = IR is not very useful when trying to calculate current through the human body.
Yea, yea, I know what some of you are thinking… “Ohm’s Law is a law. It’s always true!” Well, yes and no. Read on…
By definition, R = V/I. For any material or substance. So if you know the voltage, and you know the current at that voltage, then you can easily calculate the resistance at that voltage (or equivalently, the resistance at that current).
We then decree the resistance to be constant for all voltages (or equivalently, the resistance to be constant for all currents). If the resistance is constant (or nearly constant) over a broad range of voltages / currents, then Ohm’s Law is a very handy tool for predicting voltage based on current (V = IR) or current based on voltage (I = V/R). Such is the case for all metals. But if the resistance is not constant over a broad range of voltages / currents, then Ohm’s Law is pretty much useless, i.e. voltage can’t be predicted based on current and vice-versa. In this case, we say that the resistance is “nonlinear.” Semiconductors fall under this category. So does the human body.
The human body has a lot of water in it. It also contains a lot of ions, salts, and acids. As it turns out, the resistivity of this mixture is nonlinear, and thus Ohm’s Law should not be used. (Though I suppose it can be argued that Ohm’s Law can be used for a very rough / first-order approximation.) And it gets even more complicated, since different locations in the human body can have very different “resistance profiles.”
If you want to get a handle on how complex it can be, spend a couple hours googling “resistivity of water” and “conductivity of water” and you will learn the following:
Scientists have spent over 100 years trying to get a handle on the resistivity of water.
The DC resistivity of water is different than the AC resistivity.
Resistivity of water is a function of frequency.
Resistivity of water is a function of voltage (or equivalently, a function of current).
Resistivity of water is a function of temperature.
Resistivity of water is a function of ion/mineral content.
Water purity meters use a low-voltage, AC signal in an attempt to measure the resistivity of water.
The upshot of all of this is that any discussions on current flow through the human body can only be generalist in nature, and that calculations using Ohm’s Law are only very rough approximations at best.
I did a quick google search. If this page is accurate, we learn that silver has the lowest volume resistivity (15.9 nΩ•m) of any metal while manganese has the highest (1440 nΩ•m). But a resistivity of 1440 nΩ•m is still fairly conductive. So I think it’s safe to say that all metals (and probably all metal alloys) are conductors.
Of course, there are metal oxides that are insulators…
>Why exactly do people conduct electricity so well?
-They don’t conduct very well, per other posts
>other conductive materials like salt and iron
Salt isn’t conductive by itself. When it’s dissolved in water, it separates into positive and negative parts that can migrate around - that system is therefore conductive. But nowhere near as conductive as metals are.
>Do any metals NOT conduct electricity? (Titanium, perhaps?)
They all conduct. So do all metal alloys, though generally alloys are not as good at conducting as the best ingredient metal is by itself.
There is also a class of elements that are semi-conductive, called semiconductors. These include silicon, gallium, germanium, and others. They conduct better than most other things but not as well as metals.
would be appreciated.
