Amps kill you, Volts don't?

I often hear people say (regarding electrocutions), it’s not the volts that kill you, it’s the amps. This doesn’t really make sense to me. If current and voltage are related by Ohms law ( V = IR ), then shouldn’t you automatically end up with more current flowing through you when you’re exposed to a larger voltage? (assuming your body provides a constant resistance.) Why would being exposed to high voltage be any less dangerous than being exposed to a large amount of current?

If I may venture a WAG:

It’s the actual current that flow through you that kills you, not the potential difference. Amps measure the current, volts the potential difference, so that’s what gives rise to the saying.

For example, you could move between a pair of capacitors with a large potential difference (measured in Volts) but if you were moving through a non-conducting medium you’d be OK. If the capacitors were dropped in a conducting medium (like a swimming pool) a current would flow (measured in amps) and you would be cooked.

hopefully this will do till someone who really knows what they’re talking about comes along.

Both amps and volts will kill you if they are large enough.

Considering that the body has a fixed resistance, 40,000 volts at 0.001 amps will not kill you whereas 200 volts at 1 amp will.
This is why, IMHO, we consider amps to be more important than current.
If someone tells you that he was exposed to a live wire and amazingly wasn’t harmed you should ask how many amps the wire was carrying and not how many volts.

In utterly non-technical terms, the voltage is the amount of ooph available. The amount of current that flows depends on the resistance between the points you apply the voltage to. The resistance is simply how hard it is for electricity to get from one point to another.

What kills you (in the usual senario) is the amount of current flowing across your heart, which interferes with the heart’s own electrical system which works with relatively tiny currents. If you get a shock between two fingers (don’t try this) you’ll probably be OK as the current misses your heart. You’ll still get burnt fingers. If you get a shock between your hands or between your left hand and feet you are in trouble.

Other things to consider:

If you get a shock from a build up of static electricity the voltage will be pretty high (in the region of 10,000 volts) but it won’t kill you***** because there isn’t enough charge there to generate a decent current.

The source resistance. You could have a whacking great voltage between terminals but it might not be able to supply more than a certain amount of current. The mains supply is not like that. If you jam a crowbar across a mains socket it will try to keep the voltage between the terminals at 230/120 volts (it’s AC but that doesn’t matter here). Depending on the resistance of your crowbar - which will be very low - the current it will try to supply will be enormous, hundreds of Amps. What happens of course is that a fuse blows (or breaker throws). If you stick your spare crowbar across the fuse then most likely your wiring will catch fire (that’s why the fuse is there). In principle the mains has negligible source resistance, that’s why it is so dangerous. In practice it’s usually limited to 13 Amps, but that is way more than is needed to kill you.

Back to the OP. It is the current that kills you but you need to apply enough Volts to the right place. A car battery can supply hundreds of Amps but at only 12 Volts it can’t overcome your (dry) skin’s resistance sufficiently to kill you. If you have wet or sweaty hands this may not be the case. I don’t recommend experimenting.

*****Unless it’s lightning. Don’t play with lightning.

When I was a pupil, we played around with a Van der Graaf generator (holding hands to form a chain to the metal doorknob, so the next person entering would get a shock - and so would we all).
I understand this is a lot of (static electricity) volts, but it just made our hair stand on end.

Is that correct?

glee, have a look here Van der Graaf generators Goto the safety page for reassurance (and warnings).

Apparently a human chain is not such a good idea, with enough people charged to a high voltage you can store a heck of a lot of energy. And it all gets discharged though the last sap in the chain. From one hand to the other (v.bad see previous post or read the link). You really don’t want your heart in the circuit.

When it comes right down to it, there needs to be a certain amount of current that flows through you in order for you to be electrocuted. In other words, the relationship between “getting zapped” and “electricity” is primarily defined in terms of current, not voltage. Anything over 0.01 A is “risky territory.”

So let’s say you’re grounded and you touch a live wire. Or you touch both terminals of a battery. Or whatever. So how much current is flowing through you?

  1. If the power source approximates an ideal voltage source, then the current following through your body is I[sub]B[/sub] = V[sub]S[/sub]/ R[sub]B[/sub], where V[sub]S[/sub] is the voltage of the ideal voltage source, and R[sub]B[/sub] is your body resistance.

  2. If the power source approximates an ideal current source, then the current following through your body is I[sub]B[/sub] = I[sub]S[/sub], where I[sub]S[/sub] is the ideal current source.

  3. If the power source is modeled as an ideal voltage source in series with a resistor (“source resistance”), then the current following through your body is I[sub]B[/sub] = V[sub]S[/sub]/( R[sub]S[/sub] + R[sub]B[/sub]), where V[sub]S[/sub] is the ideal voltage source, R[sub]S[/sub] is the source resistance, and R[sub]B[/sub] is your body resistance.

  4. If the power source is modeled as an ideal current source in parallel with a resistor (“source resistance”), then the current following through your body is I[sub]B[/sub] = (I[sub]S[/sub] R[sub]S[/sub])/( R[sub]S[/sub] + R[sub]B[/sub]), where I[sub]S[/sub] is the ideal current source, R[sub]S[/sub] is the source resistance, and R[sub]B[/sub] is your body resistance.

Yes. But you should also realize that the voltage across your body depends on more than just the voltage of the voltage source. It also depends on the source resistance and your body resistance. The source resistance and your body resistance act as a “voltage divider.” At any rate, the “severity” of the electrocution is primarily defined in terms of current, not voltage. See my previous comments on how to calculate the current.

As long as the power source can source at least 0.1 A, the maximum current capability does not matter. As an example, a car battery is capable of sourcing 500 A. But this does not matter, since it can only source this much current into a very low resistance load.

This is incorrect. Again, as long as the power source can source at least 0.1 A, the maximum current capability does not matter. A 240 V / 200 A circuit is no more lethal than a 240 V / 1 A circuit. A 120 VAC outlet is no more lethal that the “hot” service wire going into your house from the pole. A car battery that can produce 500 A is no more lethal than two 6V lantern batteries in series.

If someone tells you that he was exposed to a live wire and amazingly wasn’t harmed, you should ask him the following: A) Was he grounded? B) What was the voltage?

My recollection (from my double E for dummies course (okay, Electrical Engineering for non Electrical Engineers)) is that “lethal” amperage is about .1 amps. Less is annoying to damaging and more may have nasty to very nasty effects, but may not kill you.

The human body has a variable, not a fixed resistance. Dry you probably have a resistance of 5000 ohms. Wet, about 1000 ohms. So:

   water + you + 110 volt source = death  (110V/1000 ohms = .11 amps)

Without water, the amperage is just .022 amps, so pain, damage, but not necessarily fatal. Crank the voltage to 500 and you are back to .1 amps. Wear some mack-diggety insulation (rubber sneakers need not apply) and you can probably shift the voltage required by quite a lot.

My one EE prof used to say “its not volts or amps that kills you - its resistance”.

Ta for that.
My school science teachers were not very hot on safety…

It’s not just science teachers. This was a very common demonstration in science museums for many decades. At the Boston Museum of Scicnce, they used to do this every hour or two with groups ranging from a dozen to almost a hundred. AFAIK, there was never a serious injury. The practice didn’t even end when they moved their biggest Van De Graf generator (20-30ft tall) into a specially constructed “Theater of Electricity” to allow higher voltage exhibitions (with Faraday cages protecting the operator and audience). They still periodically set up smaller Van de Graffs (typically 5-6 ft tall) as standing displays and did this demonstration. As far as I know they still haul out the “mini” (3 ft tall, under 100 kV) for school field trips and the school vacation crowds. I don’t know if the still shock schoolkids en masse today, but they were still doing it 10-15 years ago.

(But of course, it takes a lot more to shock kids these days)

I agree with the “it’s the resistance that kills you” to a point. Unfortunately, our bodies use changing electrostatic potentials across cell membranes as a means of sending signals and regulating critical biomechanical processes. This is no more true than in the heart, the proper electrophysiologial function of which is critical to life. So it’s not necessary to fry the individual to kill them; in fact you don’t even have to burn them, just have sufficient current flow through the heart, and you be dead. That critical level isn’t all that high, so if you’re really unlucky, even modest current applied to the right place can cause fibrulation and, subsequently, death.

It’s also possible to burn body parts clean off and not even lose consciousness, depending on where and how the current is applied.

My brother, who is an electrician, witnessed this latter unfortunate scenerio first-hand (not him, a co-worker). I won’t go into the gory details, but lets just say I don’t fool around with much wiring these days after hearing that story.

It’s actually worse than that, since instantaneous currents are lethal. In your example, the body would see instantaneous current magnitudes of 0.156 amps.

My understanding is as follows:

The combination of the voltage and the resistance determine whether or not you get a shock. At a high voltage (>500 Volts) you, as a person, will probably get a shock under normal cercumstances.

Once you get a shock, it’s the current that matters. A small current is no problem (like a static spark) and a big current is a real problem (like an electric chair).

So basically, the voltage is either a yes or a no. Do you get shocked? Yes or no. Then, you look at the severity of the shock.

Low voltage will stop you from getting shocked in the first place, low current will minimize the damage if you do get shocked.

High voltage will cause you to take a shock, high current will kill you.

If we want to be even more precise, it’s really the lack of resistance that kills you, right? If your body had very high resistance, than having a certain potential difference placed across you would only generate a little current. But if your resistance is lower, the current produced by that potential difference is much greater.

The opposite of resistance is conductance, is that right? (I used to know these things, but it’s been a while.) So I guess it’s really the conductance that kills you?

We say the inverse, not the opposite. But yes, it’s conductabnce. The unit of conductance was once the mho (ohm spelled backwards–EEs have a sense of humor), but these days, you’re more likely to hear seimens. Both represent the same quantity.

This statement is nonsensical. In order to get 40,000 volts at 0.001 amps the body would have to have a resistance of 40 megaohms, but to get 200 volts at 1 amp, the body would need to have a resistance of only 200 ohms. Forty million is not equal to 200, so this is inconsistent with “Considering that the body has a fixed resistance”.

And the skin’s resistance can vary, but current through or voltage across the skin is not going to kill you (unless you manage to burn the skin entirely off). What matters is generally the heart, which does have a fairly constant resistance (it’s always wet). So an increase in current through the heart is directly related to an increase in voltage across the heart. You cannot have a high voltage across the heart but a low current through it, nor vice versa, so it is meaningless to say that it’s the current or the voltage that kills you. To say that it’s not the volts that kill you, it’s the amps is analogous to saying that it’s not the milage that puts wear on your car, it’s how far you drive.

The other main way to get killed by electricity, other than by throwing off the heart’s pacemaker, is for the electricity to cook you. Here, it would be accurate to say that it’s the watts (power) which kill you. Power can be expressed by the equations P = VI, P = V[sup]2[/sup]/R, or P = I[sup]2[/sup]R. So, if exposed to a constant voltage source, a low resistance element will cook faster than a high resistance element, but if exposed to a constant current source, a high resistance element will cook faster than a low resistance element. Practically, constant voltage sources are more common than constant current sources, so a lower resistance element would be at greater risk. So it would probably be more accurate to say that conductance kills, rather than resistance kills.

Well I am pretty confused now. Lets put it as simply as possible.

I have an electric chair, and I flip the switch but OOPS, the guy is still alive (I am NOT trying to start a debate about capital punishment. Go to IMHO for that). What do I need to do?

1 - increase the voltage
2 - increase the current
3 - doesn’t matter (but they are different things)
4 - doesn’t matter (they are dependant, increasing one will increase the other)

To my understanding, voltage is the propensity for electricity to want to travel and current is the quantity of the flow.

If you wanted to kill someone using “the chair,” voltage would not be the key factor. High Voltage requirements are circumvented by using a wet sponge, specifically a sponge moistened with undestilled water, applied to the poor bastard’s head in between hte skin and the elctrode. As many of the geniuses explained in massive detail, this circumvents the protective and resistance of human skin. Basically, the water creates a “path through iron gates,” reducing the importance of having an energy delivery system that is high in voltage.

The current aspect of the delivery system is more relevant however in such a situation. The higher the current, the more energy going into the "system, or in this case, the axe murderer. You want as much energy flowing depending on the task. As people stated, if you want to distrupt his heart, the amount of amerpage isn’t that high since it does not have to make its way through the skin. Then again, if you want to cook him ala ‘green mile’ use all the current you can pump out.

Keep in mind I am an EE student and I used to have a nasty habit of falling asleep in my Circuits class :wink: so I might be totally RIGHT, or not…:smiley:

Or you could just through a bucket of water at the guy…make sure it isn;t a metal pail and that you are wearing gloves though, Mr. executioner :wink: