How many volts and amps does it take to kill a human?

[engineer_comp_geek]

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.

[Bert_Nobbins]

@gazpacho

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.

[Crafter_Man]

@Bert_Nobbins

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.)

[Mr.Duality]

The frequency of alternating current is a factor in how dangerous it is. For humans, 60 hz is one of the more lethal frequencies.

[Bert_Nobbins]

@Crafter_Man

…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?

(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.

[Crafter_Man]

@Bert_Nobbins

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.

@Bert_Nobbins

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.

[jz78817]

@Crafter_Man

Please feel free to PM me for more info.

why? so you can be an information hoarder?

[Teuton]

@Bert_Nobbins

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:

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.

[Crafter_Man]

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 = I[sup]2[/sup]R = V[sup]2[/sup]/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 V[sup]2[/sup]/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.

[casdave]

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.

[Crafter_Man]

@casdave

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.

Nitpick, but I think you mean 1 joule = 1 watt·second, not 1 joule = 1 watt/second.