Lots of time I’ve heard about electricity things like: the one that kills you is the current, don’t worry about large voltages (tasers for example have thousand of voltage). But using Ohm’s Law, if the voltage is big, the current is big. Then why it’s said that about current?
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You can not have one without the other. In the case of a taser or other similar devices they can supply a lot of voltage but only in open circuit. As soon as you close the circuit, for example by touching both ends to your body, the power supply in them is not able to sustain the high voltage and the voltage drops to almost nothing. What these devices are rated on is how much energy they can deliver in each pulse. If they keep the energy low enough then it will not cause much harm.
“It’s not voltage that kills, it’s the current” is like saying “It’s the bullet that kills, not the gun”—technically correct, but completely misses the bigger picture. Current and voltage cannot be considered independently, since current only flows as a result of voltage. Electronics like tasers are specifically designed to limit the current flow to non-lethal levels; industrial high-voltage applications will cook your goose without a second thought.
The adage is often “it’s not voltage that kills you, it’s current”, but that’s a drastic over-simplification to the point that it’s pretty much incorrect.
As you noted, ohm’s law helps us understand the relationship between voltage and current. If voltage goes up, current goes up (assume for this discussion that resistance never changes), right?
Well, *kind of*. What about things like Van deGraaf generators, that can generate hundreds of thousands of volts, and give you a shock from several feet away? Or, as you noted, tasers?
Voltage alone is just sort of analogous to pressure- that’s how much the electricity “wants” to flow. Current is how much *actually* flows. You can’t measure current unless it’s flowing.
So consider a voltage source that provides HUGE pressure, but almost no current- like a Van deGraaf generator that makes massive static electricity discharges- when current begins flowing, the voltage “pressure” is massive, but it almost instantly drops to nothing because there’s not enough energy behind it to sustain that pressure. So you get a spark and a shock that stings a bit but does no damage, because the voltage dropped so fast the current was negligible.
But now consider a voltage source that’s very low voltage-like a car battery at about 12 volts. It can generate HUNDREDS of amperes. However, because the voltage “pressure” isn’t very high, it’s much harder to get a shock from it, because your body’s resistance is so high that almost no current can flow.
So, the real answer is that if the resistance of the circuit path is low enough and the voltage source can provide enough current, you can get hurt very badly. But you could get killed by any voltage source that can sustain enough current- which if it goes from hand to hand through your chest and across your heart, is not very large.
Voltage = Current * Resistance (V=IR) So current and voltage are inversely related.
That being said, current (I) is the actual movement of charge (electricity through the medium, but it the wire or your body) so more current is bad.
When you get shocked, you are reducing the resistance of the electric circuit and often shorting electricity to ground. Your body offers very little resistance in comparison to many electrical devices so all the electricty goes through your body. If you make R really small in the equation above, and voltage is constant, current has to get bigger to compensate to make this law true.
The voltage in question really doesn’t matter when R is small, you are going to get hit with enough current to give you a serious zap.
Well, in most cases it’s the current simply because it’s the physical actor on your body. But as other have pointed out, the voltage is basically the force that drives the current so the voltage is definitely part of the equation and I’ll argue that the resistance of your body is too.
Ohm’s Law: V=IR
Super simple. However, take a 5V battery. 5V doesn’t sound like a lot, right? I’d agree, “relatively” it’s pretty small. However, the resistance of the human skin is somewhere in the range of 100,000 ohms give or take. So, solve for I, R = 5/100,000 = 5uA. Super small current. Now take the same human skin and make it *wet*. The resistance falls DRASTICALLY to on the order of 100 ohms. I = 5/100 = 50mA. This is 4 orders of magnitude higher and nearly in the generally considered lethal range of 100 to 200 mA!
You can achieve a higher current in this by also increasing the voltage, i.e. pushing the current faster. A 20V battery would put you in that lethal range if your skin was wet.
But, is that the whole story? Of course not. Given enough voltage, even if your skin is dry and has 100,000 ohms of resistance, what does it take to kill you? Assuming a lethal current is 100mA, V = (100mA)(100,000) = 10,000V. That’s a fairly normal high voltage you’d find in power stations and in power supplies. He’ll, you can easily find higher than that. Easily lethal. But that current *really isn’t that high* when you consider that 10A power supplies are common.
The short of it is that all 3 factors here matter, mathematically speaking. People say it’s the current that kills because it’s the actual physical flow of electrons that stops your heart, but the reality is that the resistance of your skin and the applied voltage dictate that current so all 3 things matter.
It gets even more complicated when you figure that Ohm’s law is a vast simplification of a more complex problem. The human skin is effectively a capacitor and has a breakdown voltage of ~600V. At 600V, the resistance of your skin drops *rapidly* because the voltage is capable of ripping electrons off of the atoms in your body and your body effectively begins to act like a conductor. At that point, of course, your body will start to sustain 100s of times larger currents.
TL;DR – the voltage, the current, and the resistance are all important.
Edit: and this completely disregards AC electricity. This is generally considered more dangerous simply because the alternating current usually stimulates your muscles on and off so you typically end up accidentally gripping the electronic that’s shocking you and your muscles don’t give out. DC wears out your muscles super quickly and you’ll typically go limp and break the connection.
It’s both. A current as low as 10 milliamps can stop your heart. A car battery can provide over 600 amps to start a car. But with the resistance of dry dead skin 12 volts isn’t enough to push any substantial amount of current.
What ultimately kills you is a large current passing through your heart.
However, for that large current to exist, you need enough voltage to counter-balance the resistance of your body. If you’ve got a 12V source, it’s basically impossible to die because your skin resistance is 1k+. So the maximum current that could be passing through your body (given Ohm’s Law) would be 12 / 1k = 12mA. Since it takes about 50mA before you’re in truly dangerous territory, you’re mostly safe (if you reduce skin resistance such as by wetting the skin, this can still be a problem).
On the other hand, even with extraordinary high voltages you could also be safe if the power source has insufficient power. When we refer to the ‘current’ of a power source we’re really talking about the maximum current we can draw from the power source instead of what is actually flowing out of the power source (this misuse of ‘current’ is why so many people get confused).
If you had a power source with 1kV but only able to provide 1W of power, then the maximum current – regardless of resistance in the circuit – that could occur would be 1 / 1k = 1mA.
>But using Ohm’s Law, if the voltage is big, the current is big.
That’s not always true. Remember that Ohm’s Law is voltage = current X resistance. You could have a high voltage , and a very very high resistance and the result would be a low current.
The reason people focus on current is because current is literally the thing that does the damage. Current is electric charge moving through something and when it moves through your flesh that electric charge is literally ripping into your cells generating tons of heat and cooking you from the inside.
Also keep in mind that current is *the result* of the other two parts of Ohm’s Law. When you apply a specific voltage to something with a specific resistance, the *result* is a certain amount of current flow,
So this is why voltage is still important to consider.
Though anyone who is serious about electrical safety knows that there are actually three very important things that determine how lethal electrical exposure is:
* Amount of exposure (current, which is determined by voltage and resistance)
* Time of exposure
* Pathway of exposure
Tasers have both high voltage and high current. but for very very short periods of time and the pathway tends to be across the skin.
The electric chair on the other hand has both high voltage and high current, super long exposure time, and a deliberate pathway through all the most important stuff in your body (brain, heart, lungs).
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