Will touching an unpainted, grounded metal surface clear your entire body of excess static, or just the part you touch with the surface? If I hold a grounded surface with my right hand, and touch electronics with my left, will I effectively be static free?
It will discharge your entire body. Your body is an excellent conductor, sitting on top of a good insulator (soles of your shoes). All you need to do is ground one part of your body, and your whole body will be at ground.
Watch out! If you were highly charged, then whenever you “zap” yourself upon a grounded object, you create a pulse of intense EM fields in the space around your body.
It’s possible that you can damage some sensitive electronics by zapping yourself, especially if you’re close to long I/O cables, speaker wires, etc. The cure? Use a grounded wrist-strap. That will prevent any net-charge from building up on your body in the first place.
Also, if the humidity is very low, grounding your body does not remove the net charge from your clothing and hair. The voltage fields from your clothing and hair will be partially shielded if your body is grounded, but for hyper-sensitive devices (such as microwave detector diodes), sometimes this is not enough. In that case, wear one of those blue conductive smocks and a hairnet.
Along the same lines, may I ask if anyone knows what the tiny spark is when you discharge static electricity? Is it akin to a tiny lightning bolt? Is the small pop the equivalent of a thunderclap?
If so, does that mean that a small volume of air is briefly heated to a plasma state, causing a sudden expansion of air? 'Cause that’s just wiggy.
Yes. At least, I don’t see what else it could be.
Yup…lightning is exactly like a static discharge albeit considerably more powerful. Consider that lightning is jumping through several miles of air and that air is a pretty good insulator. Your static discharge only happens when you are a fraction of an inch from some whatever you’re about to touch.
In order for a static discharge to happen there has to be a fairly good voltage differential between you and the grounded object. At a minimum you need about a 500 volt differential although you’d find most static discharges you experience are on the order of a few thousand volts.
Why aren’t you hurt by a few thousand volts? In this case the amperage (current) is exceedingly low. Remember that in general voltage isn’t what kills people…it’s amps you have to watch out for. Hold two 9-volt batteries in your hand for a total of 18-volts and what happens? Nothing you can notice. Now grab the poles of a 12-volt car battery and you’ll find a MUCH different result. [I shouldn’t have to say this but **DO NOT GRAB THE POLES OF A CAR BATTERY!** It can kill you and at the very least hurt a lot!] The difference between the two is the available current (amps) of the two batteries.
It is for this reason it is important to ground yourself when working with electronics (computer chips in particular). A CPU runs on around 3 volts of electricity. A 2,000 volt zap from your finger is more than sufficient to wipe out the CPU.
>> DO NOT GRAB THE POLES OF A CAR BATTERY! It can kill you
Now I know I am Superman as I have done this many times and nothing happened to me.
Did you grab both at the same time? Was the battery fully charged?
As it was explained to me by my teacher in high school grabbing the poles (one with each hand) causes an electric current to flow through your body which happens to pass your heart. Muscles run on electricity so this would constitute a Bad Thing.
Of course, teachers aren’t necessarily always right and maybe he was merely being cautious. Now thinking on it I wonder if the current would flow through your body or over it (i.e. on the outside of your skin). I would suppose running on the outside wouldn’t be a problem but I don’t really know. Still, I’m not going to be testing this myself.
The human skin has a fairly high resistance, at least when dry. 12 volts isn’t enough to pass a lethal current - or even a noticeable current - through the skin. Wet hands have much lower resistance - maybe not low enough that a 12-volt battery can kill you, but I would advise against trying it.
As I recall - ie: vague memory - Dry air requires 30,000 volts/cm to break down and be conductive. That’s why our lightning bolts happen only at close distances.
12 volts isn’t enough to be RELIABLY deadly, but it can still kill you. It’s generally considered to be a “safe” voltage, meaning that the risk of death is fairly low. There are a lot of factors that go into whether or not you die, including skin resistance (which varies with moisture, as scr4 pointed out), your particular physical structure and the exact path that the electricty takes through your body, and where your heart is in its cycle when the shock occurs. If you get hit just right, you throw your heart out of whack, and you better hope that there just happens to be someone standing next to you with a defib machine in his hands, otherwise there’s a good chance you are going to be worm food.
The current at DC is going to be pretty much internal. It’s not so much what the electricity does to the muscles as it is a danger to interrupting the way your heart works. The muscle bit is just what makes you jump 5 feet into the air and do all kinds of interesting acrobatics. And you are exactly right about the current being the deadly part, not the voltage. Any electrical engineer has probably heard the phrase “it’s the volts that jolts but the mils that kills” (mils being milliamps). A static discharge from your finger can be several thousand volts, and if you can actually see the spark it could be up in the 40,000 to 80,000 volt range. An electric chair runs at about 4,000 volts, a lot lower voltage, but it has a lot more current behind it. Electric chairs kill you by (1) interrupting your body’s electrical system and (2) cooking you to death (and quite often it’s the latter part that actually finishes you off). Lightning is up in the range of a few million volts, and also has enough current behind it that quite often its mechanism of death is cooking.
You can cook a hot dog by taking an electrical cord, attaching a nail to either wire, put the nails in either end of the hot dog, and plug the cord into an outlet. Electrical cords with bare wires like that are called “suicide cords” by us electrical folks because they are so dangerous, so kiddies, don’t try this at home unless you have someone around who knows what they are doing.
s, it is the current that gets you but it’s the voltage that drives the current through your skin. I just shorted a 9V battery and measured 725 mA. That’s three times the current needed to cause fribulation of your heart.
http://www.prl.res.in/~bobra/EARTH/html1/chapt3.htm
Given a condition of dry, healthy hands and the choice to grab the 2 9V batteries (18V) or the car battery, I’ll take the car battery. Given the choice about dropping a wrench accross the batteries though, of course I choose the little 9V batteries.
Realistically though, neither is likely to harm a healthy adult as long as the skin is not broken. Most international electrical safety standards allow for up to 60VDC to be touched without protection when in a dry environment.
I can assure our dear readers that a 12 VDC automobile battery provides enough current to provide a painful reminder to watch what you are doing under the hood. Slight perspiration on skin (not unusual when working on your car, mind you) and contact with the grounded chassis and the positive terminal – you’ll take care to avoid that.
It is inadvisable to wear metallic jewelry, rings, watches etc in the vicinity of a car battery as well – the high current in the event of a short would near instantly turn the metal to cherry red, and may “spot weld” itself to the positive-ground/earth connection, prolonging the agony. That sounds awful, and I’d bet 100 dollars it’s happened at least once.
High current, low voltage electricity is a definite hazard, and sometimes more so than it might be because it isn’t “respected” as the high voltage, high current we all know and love. Be careful out there.
Yes, but so can the smallest 12V battery. To cause pain you only need a few milliamps.
I’ve never noticed this. Now, if you should have any cuts on your hand, THEN it certainly is painful to get wet hands across the 12v terminals.
As far as electrocution is concerned, the high amps of a 12V car battery is meaninless. Your body CANNOT draw high amperes, its resistance is just too high. However, it only takes a few thousandths of an ampere to cause pain. 18V from two 9v batteries is more dangerous than 12v from a car battery. The main difference is that the 12V car battery is connected to large metal objects, so it’s easy to make good contact with wet hands.
UL/CSA ratings are based on insurance company reports, and the UL/CSA regards 40V as the lower threshold of “shock hazard.” If you put three car batteries in series for 36V, you might have a very mild shock hazard. But at 12V, the worst you can do is make small wounds in your wet hands sting. (But the battery acid can do that all on its own!)
Given all that’s been said above, wouldn’t it then be most accurate to say that power (or wattage, if you prefer) is what constitutes the hazard? If I read you all correctly, you need the presence of both adequate current to propagate the energy and sufficient potential to overcome the resistance inherent in the given situation.
Our school had synthetics carpeting, dry winter air, and metal partitioning walls. Zap.
Quite harmless, if it wasn’t the ‘girl with the generator-shoes’ who liked to sneak up on you. (bless her heart :))
On a class that was was so boring I don’t even remember what it was, I played with the sparks on the walls.
Shuffle foot, zap finger, shuffle foot, zap finger, shuffle foot, zap finger. Hypnotic.
I wondered if you can feel the spark when the current has to flow through the lead of a pencil.
Loud ZAP!, I almost passed out and felt groggy for a few seconds. What happened?
That’s simplifying it too far. Current is definitely what kills. But in order to get that current flowing, you need voltage. So it’s a high-voltage source capable of delivering high current which constitutes a hazard. That’s not the same as “high power.” A 12-volt car batter can deliver a huge amount of power but will normally not kill you, as already discussed.
FWIW, I too have touched the poles of a car battery simultaneously more then once
and I felt nothing, however, I have also
had the misfortune (stupidity?) of touching
said poles while the car was running and -yes- we are talking ZAP city. Just an antidote about dry skin Vs. wet skin conductivity
take that 9 volt battery and cross the
poles with your tongue, this is how we
used to test our walkie/talkie batteries as kids.:eek:
This means the internal impedance of the battery is 9 / 0.725 = 12.4 Ohms. For two batteries together in series, the internal impedance would be 24.8 Ohms. The most power a load (e.g., you) could receive from two of these batteries occurs would occur if the load was also 24.8 Ohms. In that case, you’ve got 9 volts across the load (the other 9 volts being across the internal impedance), and the load is receiving V^2/R = 81/24.8 = 3.3 Watts. Not very much, and that’s the most the load could receive.
I do not know the internal impedance of a car battery, but I imagine it is very small; it takes a lot of power to start a car. I’ll assume an internal impedance of 1 ohms, but I suspect it’s less than that. In this case, a 24.8 ohm load would have a voltage across it of 12 * 24.8 / (24.8 + 1) = 11.5 volts, and the power into the load would be 11.5^2/24.8 = 5.3 watts. Still not much, but more than from the two 9 volt batteries.
So just because the two 9-Volt batteries have a higher voltage, they won’t necessarily shock you worse.
I should emphasize that the power transferred into a person depends significantly on the person’s resistance, which I don’t know. For a car battery internal resistance of 1 ohm, I get that you’d get the same shock from the two sources if your resistance were 46.6 ohms. Less than that, the car battery shocks you more, and more than that the two 9-volt batteries shock you more.
If I remember, I’ll try measuring my resistance tonight with an ohmmeter,
It’ll be up in the hundreds of Kohms, but you can get as low as a few tens of Kohms if you have damp fingers and grab the leads firmly.
ZenBeam hit the DiHard on the head with the mention of power transfer. And scr4 makes an important point in the current/voltage relationship. But the demand of the load is what is going to determine how much current is drawn from the source voltage. Just because a battery can supply 450 cold-cranking amps does not mean that any load connected will draw 450 amps. It means that the battery can theoreticaly supply 450 amps, if the load so demands.
Assuming a good, clean contact with the terminals, a 12 volt source connected to a load that is in the low hundreds of Kohms range will result in anywhere from 20 to 100µA (amps×10[sup]-6[/sup]). The threshold of perception is around 1 mA (amps×10[sup]-3[/sup]). At higher levels:
5mA is considered the maximum harmless intensity.
10-20mA is considered the “let go” value.
50-100mA crosses into the threshold of pain.
100mA+ may cause ventricular fiberlation.