If I grab some batteries and hold them by the ends, why don’t I get a shock? That happens often when I build up static electricity on a carpet, or sometimes when I get out of the car and touch the metal part of the door. Does this mean that a battery has much less charge than I can build up with some socks and a carpet?
It means that the battery has much less voltage, but doesn’t say anything about the charge. The static shock you get may be thousands of volts, but it’s over in a tiny fraction of a second, while the battery can keep cranking for much longer.
I seem to recall that people have a high resistance (in the MegaOhm realm).
Since I= V/R we’re talking 1.5V/10000000 ohm = not a helluva lot of current.
Stick your tongue on both poles of a nine volt. (Really - it won’t damage you or anything, but you’ll know it’s a battery.)
That, or you will learn that the nine volt “battery” you’ve kept in a litle used drawer for the last decade, contains no mobile electrons.
Whatever you do, don’t touch the positive terminal of the 9 volt battery to your upper braces, and the negative terminal to your bottom braces.
Just don’t.
Si
The human body has a rather non-linear response to electricity. If you touch a relatively low voltage, very little current flows, and it’s almost like your body has a very high resistance value. If you measure your resistance with a multimeter, it will be up in the megaohm range. Once you get above 50 volts or so, though, you get much more current flowing, and it’s like the resistance of your body has dropped quite a bit. To household current, your body has a resistance of only about a thousand ohms.
Static electricity, if you can feel it, is up in the tens of thousands of volts range. Static electricity jumps about 1 cm for every 30,000 volts, so if you see a half a centimeter long spark you know you were up around 15,000 volts.
A battery stores a lot more energy, but it has a much lower voltage. Static has a very high voltage, but stores very little energy, which is why the static shock dissipates so quickly and doesn’t do you any harm.
Ouch.
People with dry skin have a high resistance.
IIRC, the resistance of dry skin is about 100,000 ohms. The resistance of wet skin is only about 1,000 ohms. The resistance drops further if the skin is punctured.
So for a 9V battery, the current through dry skin would be about 0.09 mA, which is below the threshold of perception. The current through wet skin would be about 9 mA, which is perceptible, but not painful. This corresponds to the feeling you get in your tongue. If you connect a 9V battery to a couple of sharp points and penetrate the skin, it is conceivable that the resistance could easily drop another order of magnitude from that of wet skin (say 100 ohms), which could result in a current of 90 mA. This is getting into the danger range–it is high enough for severe pain and involuntary muscle contraction.
Even without puncturing the skin, you can lower the resistance through your body if your skin is wet with a conductive solution. This includes sweat!
The general industry thumbrule is that a potentially lethal current can result from voltages as low as 30 V, assuming worst-case conditions.
When I was a wedding photographer, about 25 years ago, I often used Honeywell Strobonar flash units. These were powered by a 510 Volt dry cell battery. I assure you that if you touch both terminals at once you will know you’ve gotten a hold of something serious!:smack: