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#1
Old 10-22-2015, 12:36 PM
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How many volts and amps does it take to kill a human?

My microwave kept on tripping the fuse so i decided to open it. I naively took out some parts trying to figure out what was wrong when someone in my office told my that's one of the most deadliest things in the house. He told me the HV capacitor can deliver 1000s of volts and 100s of amps to the body. So I decided not to touch and get an electrician

But seriously, how dangerous is it? If you were to touch the terminals accidentally would you feel it and would you have enough time to save yourself

Last edited by devineathiest; 10-22-2015 at 12:37 PM.
#2
Old 10-22-2015, 12:53 PM
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100ma across the heart is lethal. Doesn't matter how many volts it takes to get there.
#3
Old 10-22-2015, 01:06 PM
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Originally Posted by devineathiest View Post
... He told me the HV capacitor can deliver 1000s of volts and 100s of amps to the body.
Since watts = volts * amps, and since most microwaves use 600 watts of power, his numbers are suspect. While 600 watts is trivial to a 80kg person if spread evenly throughout the body, if may be disastrous if 600 watts are concentrated in one place (like the muscles of your heart).

Last edited by bizerta; 10-22-2015 at 01:07 PM.
#4
Old 10-22-2015, 01:20 PM
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.1 to .2 amps through the heart can put it into ventricular fibrillation, which means you'll die unless someone gets there quick with a defibrillator. How much voltage is needed to get that .1 to .2 depends on a number of factors.
#5
Old 10-22-2015, 01:24 PM
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Electricity is a funny thing. People survive lightning strikes, but can die from relatively minor shocks.

Firstly, it was good advice. If you don't know what you're doing you should never dismantle mains electronics.

If you touched the terminals of a charged HV capacitor, you would certainly feel it, and you wouldn't have time to save yourself from the shock. Assuming the machine was unplugged, though, the shock would be short lived (capacitors hold charge, and shorting the terminals will discharge them through you).
If you touched both terminals with the same hand, you'd get a jolt, but probably nothing more. If you managed to touch the terminals with different hands, the charge would discharge across your body, and your heart. This could potentially be fatal.

Not that HV caps may have insulated terminals that allow you to plug leads into them but that make it difficult to touch the terminal ends otherwise.
#6
Old 10-22-2015, 01:29 PM
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Quote:
Originally Posted by devineathiest
If you were to touch the terminals accidentally would you feel it and would you have enough time to save yourself
Time? No, if if it stops (or screws up) your heart it's game over.
I don't know what the innards of microwaves are, but a high voltage capacitor may not store a lot of charge. If you got a shock off that it might hurt, but since it can only supply a small amount of energy you'll live.
An electricians' "trick" is to keep your left hand in a pocket. That way if you do get a shock it doesn't go across your chest (read heart), but from your right hand to the floor. Odds are better you'll live.
That is just a precaution to take when tinkering with unknown circuitry. It is not safe technique for day-to-day working with live hv electricity.

Quote:
...one of the most deadliest things in the house
I think that'll be the stairs.

Last edited by Small Clanger; 10-22-2015 at 01:30 PM. Reason: because
#7
Old 10-22-2015, 01:39 PM
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The cap in a microwave could bite you. You'd have to be unfortunate to be killed, but yes, the potential is there. Most likely some minor burns where you made contact.

How dry your skin is, and where specifically you make contact matter a lot. Also what part of the cycle your heartbeat is on when it happens.

Most people that have been killed messing around with microwave ovens were doing it "hot" probing around while it was plugged in. In that case it isn't just one quick pop, but a sustained current, and the power supply in a microwave as plenty of umph to kill in that case.
#8
Old 10-22-2015, 01:41 PM
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Quote:
Originally Posted by Teuton View Post
If you touched the terminals of a charged HV capacitor, you would certainly feel it, and you wouldn't have time to save yourself from the shock. Assuming the machine was unplugged, though, the shock would be short lived (capacitors hold charge, and shorting the terminals will discharge them through you).
If you touched both terminals with the same hand, you'd get a jolt, but probably nothing more.
Years ago I was trying to repair my broken film camera. As I was disassembling the case, the flash popped up and the circuit started charging the capacitor, with that classic increasing-pitch whine. I thought nothing of it and continued to dig into the camera, and at some point my finger bridged both terminals of the capacitor. Felt like a fucking bee sting, and when I looked at my finger there were two tiny burn marks where the terminals had made contact with my skin. According to this article, the voltage may have been as high as 380 volts.

A microwave oven includes a voltage-doubling circuit that takes a couple thousand volts from the main transformer and doubles it to give the magnetron what it needs for operation. So basically the cap could have dangerously high voltage stored on it. Unclear what the typical capacitance is, and therefore how much current it could deliver (or for how long). But if my little camera's capacitor was able to give me a nice snakebite-burn, it seems like that's the least you should fear from a cap in a circuit that handles 1000-1500 watts. Safe service would require bridging the terminals of the cap with a mega-ohm resistor to bleed off any remaining charge before laying hands on it.
#9
Old 10-22-2015, 01:49 PM
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Originally Posted by Small Clanger View Post
Time? No, if if it stops (or screws up) your heart it's game over.
I don't know what the innards of microwaves are, but a high voltage capacitor may not store a lot of charge. If you got a shock off that it might hurt, but since it can only supply a small amount of energy you'll live.
An electricians' "trick" is to keep your left hand in a pocket. That way if you do get a shock it doesn't go across your chest (read heart), but from your right hand to the floor. Odds are better you'll live.
That is just a precaution to take when tinkering with unknown circuitry. It is not safe technique for day-to-day working with live hv electricity.

I think that'll be the stairs.
What would happen if you touched the capacitor while the microwave was plugged and running?

Last edited by devineathiest; 10-22-2015 at 01:50 PM.
#10
Old 10-22-2015, 02:24 PM
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Originally Posted by Ex_Bubblehead View Post
100ma across the heart is lethal. Doesn't matter how many volts it takes to get there.
is not true, more important of Amp and Voltage is total power.

If you have 0.1 A x 1 Volt = 0.1 Watt is not enough to filling that current

Last edited by KaleMan; 10-22-2015 at 02:24 PM.
#11
Old 10-22-2015, 02:25 PM
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Originally Posted by devineathiest View Post
What would happen if you touched the capacitor while the microwave was plugged and running?
you get chocked but its not kill you
#12
Old 10-22-2015, 02:27 PM
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Quote:
Originally Posted by Small Clanger View Post
An electricians' "trick" is to keep your left hand in a pocket. That way if you do get a shock it doesn't go across your chest (read heart), but from your right hand to the floor. Odds are better you'll live.
That is just a precaution to take when tinkering with unknown circuitry. It is not safe technique for day-to-day working with live hv electricity.
I remember once visiting the test bay of a couple of co-workers who were designing a new power supply for an X-ray tube. This was a serious power supply - 120kV @ 400mA continuous. I asked if this was one of those "keep one hand in your pocket" deals. Their answer: "No, wouldn't make a difference."

(Os course there were tons of safety interlocks and procedures to make sure nobody was near the supply while high tension connections were exposed and the supply was on...)

Basically anything with a vacuum tube in it (CRT, magnetron, old television or stereo) has potentially lethal voltages running around inside it, and probably filter caps with that hold at least painful voltages long after the device has been turned off or unplugged. People who service such devices has the correct equipment to safely discharge the filter caps...
#13
Old 10-22-2015, 02:29 PM
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Originally Posted by bizerta View Post
Since watts = volts * amps, and since most microwaves use 600 watts of power, his numbers are suspect. While 600 watts is trivial to a 80kg person if spread evenly throughout the body, if may be disastrous if 600 watts are concentrated in one place (like the muscles of your heart).
agree .. also need take to account of body resistance
#14
Old 10-22-2015, 02:42 PM
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Originally Posted by KaleMan View Post
is not true, more important of Amp and Voltage is total power.

If you have 0.1 A x 1 Volt = 0.1 Watt is not enough to filling that current
Total power has nothing to do with it, it's the current, and 100-200ma across the heart is lethal no matter the voltage or total power.
#15
Old 10-22-2015, 03:12 PM
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Originally Posted by Ex_Bubblehead View Post
Total power has nothing to do with it, it's the current, and 100-200ma across the heart is lethal no matter the voltage or total power.
We have Ohm's law: Current = Voltage/Resistance
which must be met.

So what is the resistance across the heart? You're not going to get 100-200 ma at low voltages.
#16
Old 10-22-2015, 03:32 PM
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At the cellular level, everything typically works with membrane potentials on the order of a few tens of mV. That includes the nerves and muscles cells in your heart. If you had electrodes to the right (or wrong) part of the heart, a mere 100 mV would be bad news. Even 10 mV applied at the exact wrong time would seriously screw up the heart's rhythm. On a larger scale, skin has a pretty high resistance but underneath we're bags of salty conductive fluid arranged in all sorts of complicated channels. I don't have a cite but my understanding was that a 12 V car battery can be plenty dangerous if the voltage is applied, ah, internally.
#17
Old 10-22-2015, 03:49 PM
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Originally Posted by Ex_Bubblehead View Post
Total power has nothing to do with it, it's the current, and 100-200ma across the heart is lethal no matter the voltage or total power.
Okay you mean two AAA batteries from your TV remote control can kill you?????
#18
Old 10-22-2015, 03:55 PM
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Sure, if you stab someone in the heart with electrodes wired with that AAA battery. Pacemakers use lil' bitty 2-3 V batteries, around the same size as a AAA battery, that last for decades.
#19
Old 10-22-2015, 03:59 PM
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Originally Posted by KaleMan View Post
Okay you mean two AAA batteries from your TV remote control can kill you?????
you need sufficient voltage to "push" the lethal amount of current through your heart. that could be as low as a couple of volts (like from your AAA batteries) if done through electrodes embedded in your chest, to hundreds of volts if applied hand-to-foot through dry skin. As with anything, the resistance of the circuit is key. there's a reason it's "Ohm's law" and not "Ohm's general observations."

Last edited by jz78817; 10-22-2015 at 04:00 PM.
#20
Old 10-22-2015, 04:09 PM
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OK, here's a link which gives a decent explanation:

https://physics.ohio-state.edu/~...l_current.html

Interestingly, there is a "sweet spot" between 100-200mA which is enough current to produce ventricular fibrillation and certain death (unless you have a defibrillator, and someone to use it, handy). Over 200mA the heart muscle "clamps" and won't go into fibrillation, but your breathing has likely stopped with predictable results in the absence of CPR.

Regarding the question of the resistance of the heart: I found a paper which has measurements of the resistance of a dogs heart (http://ncbi.nlm.nih.gov/pmc/articles/PMC1802529/) which gives an average of 50 ohms. Assuming a human heart has a resistance on the order of 100 ohms, then a voltage on the order of 10 volts applied directly across the heart will produce a current on the order of 100 mA.

What saves you in real life is your skin resistance, which can range from around 1 kohm for wet skin to hundreds of kohm for dry (see the "Fatal Current" citation above).

Last edited by Marvin the Martian; 10-22-2015 at 04:09 PM.
#21
Old 10-22-2015, 04:26 PM
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Quote:
Originally Posted by Marvin the Martian View Post
OK, here's a link which gives a decent explanation:

https://physics.ohio-state.edu/~...l_current.html

Interestingly, there is a "sweet spot" between 100-200mA which is enough current to produce ventricular fibrillation and certain death (unless you have a defibrillator, and someone to use it, handy). Over 200mA the heart muscle "clamps" and won't go into fibrillation, but your breathing has likely stopped with predictable results in the absence of CPR.

Regarding the question of the resistance of the heart: I found a paper which has measurements of the resistance of a dogs heart (http://ncbi.nlm.nih.gov/pmc/articles/PMC1802529/) which gives an average of 50 ohms. Assuming a human heart has a resistance on the order of 100 ohms, then a voltage on the order of 10 volts applied directly across the heart will produce a current on the order of 100 mA.

What saves you in real life is your skin resistance, which can range from around 1 kohm for wet skin to hundreds of kohm for dry (see the "Fatal Current" citation above).
Now I get it my dry skin safe me

Last edited by KaleMan; 10-22-2015 at 04:27 PM.
#22
Old 10-22-2015, 04:33 PM
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Originally Posted by KaleMan View Post
Now I get it my dry skin safe me
But sweat just a little and you're toast...
#23
Old 10-22-2015, 05:01 PM
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Electricity tends to kill you in two different ways.

The first way is that it screws up your heartbeat. Hit the heart with a shock at just the right time, and it will go into fibrillation, where it just sits there and shakes and isn't pumping blood. Our heart has kind of a funny design in that this fibrillation state is stable, meaning that the heart generally won't go out of fibrillation on its own once you get it there. Since the heart isn't pumping blood, you pass out in a few seconds (maybe 10 to 15 seconds tops) and then die a few minutes later. Better hope there is someone nearby with a portable defibrillator.

It takes a surprisingly small amount of current to throw the heart into fibrillation. Electrical standards in the U.S. are generally based around currents under 5 mA being "safe", though not a lot of human testing has been done to confirm this for ethical reasons that I hope should be obvious. The numbers are based more on animal studies than human guinea pigs.

Even though a 5 mA current can theoretically kill you, it's not very likely to. As the current increases, so does the risk of fibrillation, up until you get around the 50 to 100 mA range mentioned upthread, at which point the risk of fibrillation is much more severe. Fibrillation is kinda hit and miss though. Your heart is much more sensitive to disruptions at certain times during its cycle than others, so there's a lot of luck involved with the timing of exactly when the shock hits your heart.

A funny thing happens as you increase the current though. At higher current levels, the heart doesn't go into fibrillation. Instead, all of the heart muscles just clamp. At that point the heart isn't pumping blood, so again you will pass out and die if the source of the current isn't removed. However, once the current is removed, the heart generally goes back into a normal rhythm.

As you keep increasing the current, you start getting into the second way that electricity kills you. It literally cooks you to death. This is how the electric chair kills you. It's also how lightning usually kills you. There's a wicked amount of energy in a lightning bolt - hundreds of millions of volts and hundreds of thousands of amps. At those extremes, lightning becomes very weird and unpredictable. The electric chair is pretty much always fatal, and that's only a few thousand volts (over a much longer duration). Lightning, even with its bizarrely high voltages and currents, will sometimes fry you to a crisp and will sometimes leave you relatively unscathed.

So overall, what happens is that as you increase the current (generally by increasing the voltage), the fatality rate starts off near zero, the rises, then drops for a bit as you get into the range of the heart clamping, then rises again as you get into heat damage.

Quote:
Originally Posted by Ex_Bubblehead View Post
Total power has nothing to do with it, it's the current, and 100-200ma across the heart is lethal no matter the voltage or total power.
While it is true that it's the current that matters in this case, voltage and current aren't exactly unrelated. All other things being equal, a larger voltage will result in a larger current. All other things are not always equal though.

Quote:
Originally Posted by PastTense View Post
So what is the resistance across the heart? You're not going to get 100-200 ma at low voltages.
The "resistance" of the human body varies. Humans aren't simple ohmic resistors. A commonly used model of the human body is a resistor in series with a resistor and capacitor in parallel, but the values of those components varies depending on the circumstances. At low voltages, the "resistance" will be several k to several megs. At higher voltages, the "resistance" of the body drops to somewhere around 500 to 1000 ohms, which is a pretty significant difference.

I've heard about someone killing themselves with low voltages (like 3 to 6 volts from batteries, somewhere in that range) by using something that stuck needles or probes into their skin. It might just be an electrical urban legend though as I've never been able to confirm it.

Last edited by engineer_comp_geek; 10-22-2015 at 05:03 PM.
#24
Old 10-22-2015, 05:06 PM
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Originally Posted by KaleMan View Post
Okay you mean two AAA batteries from your TV remote control can kill you?????
It could, but you'd probably need to have the electrical contacts pierce your skin to overcome your skin's resistance. Generally speaking, you need to get somewhere in the range of 50 volts or so before the voltage will overcome your skin's resistance and the resulting current starts to get dangerous.
#25
Old 10-22-2015, 05:07 PM
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Skin resistance may save your heart but you'll probably sustain very bad burns. Though I'd take that over cardiac arrest.
#26
Old 10-23-2015, 01:12 AM
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There is an ancient (no cite) story of a US Navy recruit in training to be an Electrician.
One day, they were issued volt/ammeters and had a discussion about safety, including the fact that human skin was actually a pretty decent insulator.
At this point, for those who don't know - the instrument has a small battery used to place voltage across a circuit to measure resistance.

This is the set-up. Pondering the question of skin and resistance, the story goes, he poked small holes in his index fingers, and placed the leads in the holes to measure the resistance of human meat.

The voltage (from a tiny battery, 9v 'transistor' batteries, for instance) passed through his heart.
As mentioned above, it takes virtually no current to stop a heart - esp. when those are pure DC (as from batteries).

My Q: anyone have a cite for this story of volt meter killing an inquisitive Electrical student in the USN?
#27
Old 10-23-2015, 10:19 AM
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Originally Posted by Ex_Bubblehead View Post
100ma across the heart is lethal. Doesn't matter how many volts it takes to get there.
Yep. And it is not possible (at least right now) to calculate how many volts is necessary to achieve 100 mA or whatever.

In order to determine the voltage necessary to achieve a certain current through the body, we would need an accurate electrical model of the human body. Such a model does not exist at this time.

The human body cannot be modeled as a simple resistor. Or a resistor & capacitor. Furthermore, the current in the human body is not due to mobile electrons but mobile ions. Scientists have spent the last hundred years studying the electrical impedance of saltwater in the lab, and they still don't have a good handle on it.

So when it comes to voltage and the human body, about the best you can do is go by some rules-of-thumb. ECG brought up one: voltages less than 50 V are generally considered safe, for most people, in most circumstances, most of the time.

Last edited by Crafter_Man; 10-23-2015 at 10:21 AM.
#28
Old 10-23-2015, 10:48 AM
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Quote:
Originally Posted by Ex_Bubblehead View Post
Total power has nothing to do with it, it's the current, and 100-200ma across the heart is lethal no matter the voltage or total power.
Through, not across!
Voltage is across, current is through.
#29
Old 10-23-2015, 11:04 AM
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Remember that a sharp blow to the chest at just the right spot and the right time can stop the heart, but it's a rare occurrence. I'm sure most electrocutions involve higher voltages and currents, and probably AC. There have been plenty of electricians who tested for hot wires by touching them, and a friend of mine who works for National Grid on the heavy power lines says he doesn't even care about household electricity because even 240 volts isn't going to kill him. I'd be a little more cautious myself, but it's an indication of how rare electrocutions are under normal circumstances.
#30
Old 10-23-2015, 11:29 AM
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The large capacitor has enough energy to electrocute by affecting the heart electrochemistry...

It also has enough energy to cause painful burns to the skin, and cramp up your biceps and other muscles.. This cramping can then have serious results.. eg disabling injury to muscles.. tear something (muscle,ligament, joint tissues, break a bone ), pinch a nerve, wry neck....

Worse, if you are working near exposed mains power, you might then be touching mains power.... repeat for other dangers (even the floor is a danger.. if you fall over and bang your head on it... falling over rotating fast enough to hit head first.. )
#31
Old 10-23-2015, 11:38 AM
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Quote:
Originally Posted by usedtobe View Post
My Q: anyone have a cite for this story of volt meter killing an inquisitive Electrical student in the USN?
This is basically what I was referring to in the last line of my post #23 above.

I've heard several variants on this story. The most common is that it was an electrical technician of some sort, sometimes in the USN, sometimes not. The other variant that I've heard was that it was some sort of device used for sexual pleasure, and not a meter. The key in every version though is that the skin was somehow punctured in order to overcome the skin's resistance.

I've never been able to confirm any version of this story, which is why I'm wondering if this is an electrical urban legend.
#32
Old 10-23-2015, 11:42 AM
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This is the USN version, commonly known because it has been passed around as a Darwin award.

Quote:
(1999) A US Navy safety publication describes injuries incurred while doing don't's. One page described the fate of a sailor playing with a multimeter in an unauthorized manner. He was curious about the resistance level of the human body. He had a Simpson 260 multimeter, a small unit powered by a 9-volt battery. That may not seem powerful enough to be dangerous… but it can be deadly in the wrong hands.

The sailor took a probe in each hand to measure his bodily resistance from thumb to thumb. But the probes had sharp tips, and in his excitement he pressed his thumbs hard enough against the probes to break the skin. Once the salty conducting fluid known as blood was available, the current from the multimeter travelled right across the sailor's heart, disrupting the electrical regulation of his heartbeat. He died before he could record his Ohms.

The lesson? The Navy issues very few objects which are designed to be stuck into the human body.
From here: http://darwinawards.com/darwin/darwin1999-50.html

Note that the Darwin Awards page lists it as "unconfirmed".

Last edited by engineer_comp_geek; 10-23-2015 at 11:42 AM. Reason: fixed quote tags
#33
Old 10-23-2015, 12:48 PM
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Darwin Awards, aka "funny stories people made up".
#34
Old 10-26-2015, 02:34 PM
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Originally Posted by engineer_comp_geek View Post
It could, but you'd probably need to have the electrical contacts pierce your skin to overcome your skin's resistance. Generally speaking, you need to get somewhere in the range of 50 volts or so before the voltage will overcome your skin's resistance and the resulting current starts to get dangerous.
how people can survive after million volts go through the body

https://en.wikipedia.org/wiki/Roy_Sullivan

Last edited by KaleMan; 10-26-2015 at 02:35 PM.
#35
Old 10-26-2015, 02:44 PM
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Originally Posted by KaleMan View Post
how people can survive after million volts go through the body

https://en.wikipedia.org/wiki/Roy_Sullivan
People survive getting shot, too.
#36
Old 10-26-2015, 02:56 PM
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Originally Posted by Crafter_Man View Post
Yep. And it is not possible (at least right now) to calculate how many volts is necessary to achieve 100 mA or whatever.

In order to determine the voltage necessary to achieve a certain current through the body, we would need an accurate electrical model of the human body. Such a model does not exist at this time.

The human body cannot be modeled as a simple resistor. Or a resistor & capacitor. Furthermore, the current in the human body is not due to mobile electrons but mobile ions. Scientists have spent the last hundred years studying the electrical impedance of saltwater in the lab, and they still don't have a good handle on it.

So when it comes to voltage and the human body, about the best you can do is go by some rules-of-thumb. ECG brought up one: voltages less than 50 V are generally considered safe, for most people, in most circumstances, most of the time.
Were you part of this thread? Your input would be perfectly on point (and an excuse to zombifie it is always welcome):
How many people connected in series to power a lightbulb?
This sounds like the beginning of a joke, but, given GQ standards, it's deadly serious.

There's a sole 60-watt light bulb that must be turned on for a minute or everyone will die of fear of darkness. You have insertable single head-plugs, luckily made of lithium, and you have a ton of connection wire. The head of each plug is 2 centimeters in radius.

You decide everyone must chip in and give their electroconductive best, and be plugged in in series to generate their share of amps. How many people would be needed to save the day?
http://boards.academicpursuits.us/sdmb/...d.php?t=553857
#37
Old 10-26-2015, 03:04 PM
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Originally Posted by jz78817 View Post
you need sufficient voltage to "push" the lethal amount of current through your heart. that could be as low as a couple of volts (like from your AAA batteries) if done through electrodes embedded in your chest, to hundreds of volts if applied hand-to-foot through dry skin. As with anything, the resistance of the circuit is key. there's a reason it's "Ohm's law" and not "Ohm's general observations."
It should really be Ohm's general observation. Most complex things like say biological tissue do not obey Ohm's law very closely. It is a lot like Hook's law springs only Obey Hook's law over a smallish range.
#38
Old 10-26-2015, 03:06 PM
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Quote:
How many volts and amps does it take to kill a human?
Rare, medium-rare, medium, or well done?

* * * * *

My father had an old anecdote about a college prof who used to charge up a Leyden jar (those were the days) and demonstrate resistance in series by having the students stand in a circle holding hands and he and the other end of the circle would touch the opposite contacts of the jar, and everyone would feel a slight tingle. Then to demonstrate the RC constant effect, he would drain the remaining charge by holding the one contact with his finger and touching the other contact close to his nose for a small visible spark.

One clever student heard about this demonstration beforehand and persuaded the fellow next to him not to hold hands... Then, after the RC demonstration, when the prof regained consciousness...

Last edited by md2000; 10-26-2015 at 03:07 PM.
#39
Old 10-29-2015, 09:55 AM
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People survive getting shot, too.
seven times in head?
#40
Old 10-29-2015, 12:44 PM
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Originally Posted by beowulff View Post
People survive getting shot, too.
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Originally Posted by KaleMan View Post
seven times in head?
...pausing only once to reload...
#41
Old 10-29-2015, 12:53 PM
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Lightning is so high in voltage that many of the normal rules of electricity don't apply any more and it just gets weird and extremely unpredictable. Things that you normally consider to be insulators (like several miles of open air) suddenly become conductors. Lightning can also take very weird paths. A friend of mine was struck while opening his oven. Lightning hit his garage, traveled up the underground electrical wire from the garage to the house, went through the electrical ground to his oven, then arced over and zapped him, knocking him onto his backside. You'd have expected the bolt to just go into the earth and dissipate instead of doing that, but as I said, lightning is weird.

A lot depends on the path that the lightning takes through your body. Some people end up with small burns on their head and feet and little damage elsewhere. Some people end up with severe burn damage and spend months in the hospital recovering. Some people get killed instantly.

Poor Roy started thinking that the lightning was out to get him and started getting more than a little paranoid about it.
#42
Old 10-29-2015, 02:53 PM
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Quote:
Originally Posted by gazpacho View Post
It should really be Ohm's general observation. Most complex things like say biological tissue do not obey Ohm's law very closely. It is a lot like Hook's law springs only Obey Hook's law over a smallish range.
No, it pretty much is a law. Ohm's Law works exactly for metallic conductors and many other things. According to Wikipedia, Ohm's law works for silicon wires as small as four atoms wide and one atom high. It works exactly to within the limits of measurement for resistors made of metal film. It does not work exactly for old carbon film resistors and the like, but that's the resistors' fault, not Ohm's.

Nobody expects biological tissues to obey Ohm's Law.

As to the OP, the microwave is almost certainly the most dangerous electrical item in the house. The voltages used are very likely to be lethal.
I'm an electrical engineer and I wouldn't dream of trying to fix one.

Last edited by Bert Nobbins; 10-29-2015 at 02:58 PM.
#43
Old 10-29-2015, 07:09 PM
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Quote:
Originally Posted by Bert Nobbins View Post
No, it pretty much is a law. Ohm's Law works exactly for metallic conductors and many other things. According to Wikipedia, Ohm's law works for silicon wires as small as four atoms wide and one atom high. It works exactly to within the limits of measurement for resistors made of metal film. It does not work exactly for old carbon film resistors and the like, but that's the resistors' fault, not Ohm's.
We're getting a bit O.T. here, but it's apparent many people have misconceptions about Ohm's Law, including many EEs:

1. V = IR is only absolutely true for an ideal resistor. All real resistors approximate Ohm's Law. Some resistors do a very good job of coming close to obeying Ohm's Law over a given range, but no real resistor has ever been made that perfectly obeys Ohm's Law, even over small ranges.

2. The primary purpose or "utility" of Ohm's law is to allow you to predict the current through a resistor if you know the voltage across it, or predict the voltage across a resistor if you know the current through it.

3. If you have an unknown 2-terminal device, it is considered bad form if you simply measure the voltage across it (at a known current) and proclaim the resistance to be V/I. While you can certainly make that calculation, it may be meaningless if the component does not approximate Ohm's Law. A good example of this is a diode. As an example, if you source 100 mA through a diode, and the voltage across it is 0.614 V, you might be tempted to say, "The resistance of the diode is 6.14 ohms," but it wouldn't have any real meaning or utility. Even worse, you'll simply look dumb in front of well-seasoned EEs. (A slight improvement might be, "The resistance of the diode is 6.14 ohms @ 100 mA," but even that has little to no utility.)
#44
Old 10-30-2015, 10:10 AM
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The frequency of alternating current is a factor in how dangerous it is. For humans, 60 hz is one of the more lethal frequencies.
#45
Old 10-30-2015, 12:23 PM
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Quote:
Originally Posted by Crafter_Man View Post

...but no real resistor has ever been made that perfectly obeys Ohm's Law, even over small ranges.
I don't think you are correct. To the best of my knowledge, Ohm's Law works exactly for metallic conductors, with constant temperature assumed, etc. Cite?

Quote:
(A slight improvement might be, "The resistance of the diode is 6.14 ohms @ 100 mA," but even that has little to no utility.)
On the contrary, that could be a very useful spec for the dynamic resistance of something like a Zener diode.
#46
Old 10-30-2015, 07:41 PM
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Quote:
Originally Posted by Bert Nobbins View Post
I don't think you are correct. To the best of my knowledge, Ohm's Law works exactly for metallic conductors, with constant temperature assumed, etc. Cite?
Ohm's Law was derived based on the Drude Model of Conduction in 1900. The pertinent word there is model. As with any model, there are flaws with it.

Quote:
Originally Posted by Bert Nobbins View Post
On the contrary, that could be a very useful spec for the dynamic resistance of something like a Zener diode.
By itself, the absolute current and voltage at one point on the IV curve for a diode says nothing about the dynamic resistance. Dynamic resistance is the slope of the IV curve at a given point (or over a very small range). To determine the dynamic resistance of a diode at a given point on the IV curve, measure the voltage and current at two points on the curve that are very close together, and then calculate ΔV/ΔI.

At any rate, we are way OT here. Please feel free to PM me for more info.
#47
Old 10-30-2015, 10:16 PM
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Quote:
Originally Posted by Crafter_Man View Post
Please feel free to PM me for more info.
why? so you can be an information hoarder?
#48
Old 10-31-2015, 05:31 AM
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Quote:
Originally Posted by Bert Nobbins View Post
I don't think you are correct. To the best of my knowledge, Ohm's Law works exactly for metallic conductors, with constant temperature assumed, etc. Cite?
From here:

Quote:
The law is strictly true only for resistors whose resistance does not depend on the applied voltage, which are called ohmic or ideal resistors or ohmic devices. Ohm's law is never completely accurate, if R is assumed to be constant, for "real world" devices, because no real device is an ohmic device for every voltage and current - at some level, the device will open or short, for example, by burning up or arcing.

Crafter_Man above says that Ohm's Law was derived in 1900, but it was described in 1827, per my link.
#49
Old 10-31-2015, 09:39 AM
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The resistance of a real resistor is a function of, well, just about everything: temperature, humidity, mechanical stress & strain, age, phase of the moon, etc. And that's for DC operation. There are a lot more variables when operating the resistor at AC.

I will address two of these:

1. The resistance of a resistor is a function of its temperature. This is called the resistor's Temperature Coefficient (TC). The temperature of a resistor is determined by the temperature of the environment and the self-heating of the resistor. The influence of the latter is a function of the power dissipation of the resistor (P = I2R = V2/R), its surface area, geometry, emissivity, etc. Even when the environment is kept at a constant temperature, the temperature of the resistor will always be a function of the current through it (or equivalently the voltage across it), and thus its resistance will always be a function of the current through it. (Though I suppose you could try and actively adjust the temperature of the environment to compensate for the self-heating of the resistor and thus maintain a constant temperature inside the resistor. But the response time would be very slow and imperfect.)

2. Let's pretend we have a resistor that is made of a high-tech alloy that has a TC = 0. Do we have a perfect resistor, then? No. You still have to deal with the fact that the resistance of a real resistor is a function of the voltage across it, and this has nothing to do with temperature, heat dissipation, or V2/R. It is called the Voltage Coefficient of Resistance (VCR). Most of the time we only pay attention to it with high voltage resistors in the gigaohm range, though it is always a factor in all resistors.
#50
Old 11-02-2015, 01:42 PM
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Instead of talking about current, volts, and power we are looking at this slightly wrongly.

The absolute killer is the amount of energy that is delivered within a period of time.

You can have a large amount of energy in total, but delivered over such a long period of time that it had no effect, you can have a fairly small amount or energy delivered in a microsecond, and its fatal.

We can calculate rates of current flow using ohms law, or we can take into account frequencies, but these are largely simplifications in this particular case.

This is why capacitors can be so dangerous, they can deliver their energy in a very short timeframe.

Instead of using Watts, or Amps or Volts, we need to use Watts per second - or a derivative. That's when we tend to use other units to calculate the rate of energy delivery, we use Joules. Sometimes we will use the term 1 Watt/Second to describe one Joule.

You may have noted on some of those ER shows that they use defibrillators that the senior nurse, EMT etc will call out the settings on the machine, and this is given in Joules - we are still talking about electrical energy here, but now we are setting that delivery timeframe.

An analogy might be to imagine a vehicular impact into a human, if the vehicle strikes a person for a week at a delivery rate of .1 metres per second, it will just push the person along and the human frame can cope with it, and the total energy delivery could still be large, however if the same vehicle delivers the same energy at 1000 times the speed, that's quite different.

We tend to get a little mixed up in our terms when discussing electric shocks because we tend to examine it from the engineers perspective, rather than from the physicists perspective.
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