A couple of things: don’t touch the metal ends of the cables or the battery terminals. You could be in for a shock.
12 volts isn’t high enough to produce any sensation under normal circumstances (the only time I’ve noticed so much as a tingle was after diving in the ocean for an hour)
Touching the ends to each other, however, will send sparks flying.
Touching 12 volt battery terminals is indeed very, very unlikely to harm you.
However, cars are equipped with ignition coils to produce a spark for the spark plugs. When a car is running the ignition coil is producing output of 20-30 kV and back EMF in the hundreds of volts. Normally this is filtered out from the 12-volt side in order to protect the electronics and battery, but I would not recommend trusting those filters with your life.
Not just sparks. The electrical system of one or both cars can be severely damaged.
Also, you shouldn’t attach the negative cable to the negative terminal on the dead car, but to exposed metal of the car’s chassis (i.e. structure). This is to avoid a spark igniting any leaking fluid from the bad battery. Flaming battery acid = not fun.
It’s worth noting that, while 12 volts won’t normally penetrate dry skin under most humidity conditions, you really do need to be careful. Pressure increases surface-to-surface contact, which decreases resistance, which lowers the voltage threshold. So can moisture, like even small amounts of sweat. And a car battery does have sufficient current to injure or kill a human being quite easily. (Voltage penetrates insulators, current actually does damage. The zap you get from static electricity is in the range of thousands of volts, but the current is negligible.)
Yeah, car batteries can do about a kiloamp into a dead short so we can treat them as ideal voltage sources for this ‘application’. However, even with wet hands and solid contact, 12 volts is too low to get much current flowing.
Soaking my hands in saturated salt water got my hand to hand resistance down to 10-20kohm (0.5-1ma), which is still at least a factor of 250 above the 40 ohm resistance you’d need to draw 300ma, which is the lower figure wiki gives for DC caused fibrillation. Putting one hand on either terminal didn’t get me so much as a tingle.
I know tingle levels are possible when soaking for longer (hours) and 9v will tingle your tongue (2-4milliamps), but it seems exceedingly hard to get to a dangerous level, considering that most models I’ve seen had the internal resistance of people at hundreds of ohms (350 is the number that sticks in mind). Also nerves are somewhat AC coupled, which brings the fibrillation limit up and makes people push away from the source instead of clinging on.
I guess it might be possible for someone with thin skinned thoroughly soaked hands making good contact and having a poorly shielded sensitive heart, but I’d call it a ‘freak accident’.
It’s worth noting that the reason we use clamps on the ends of the jumper cables is because pressure increases surface area in contact, which decreases resistance for the simple reason of Ohm’s law applied to parallel resistors. (Three 1k Ohm resistors have a parallel resistance of only 333 Ohms. It’s meaningless to give a single figure for copper → wet skin resistance without also giving the surface area for which the figure is valid.)
This means that incidental touching of metal is extremely unlikely to kill anyone, but accidentally clamping your finger, gripping metal tightly, or anything else that applies pressure to your skin will dramatically raise the risk.
And a car battery does have sufficient current to injure or kill a human being quite easily. (Voltage penetrates insulators, current actually does damage.
Fortunately current and voltage are not independent features of a power source—and in the case of current not even something that can be meaningfully measured without specifying the load! A car battery does not “have sufficient current to injure or kill a human quite easily” because of the human part of the (I=V/R) equation.
It does if the skin is wet. Once you’re through the skin, the human body’s resistance is quite low, in the single-digit kiloohm range at most, because the human body is mostly salt water (a fantastically good conductor by non-metallic standards). The biggest barrier to current is the upper layer of dead, dry cells on the epidermis. And lead-acid batteries have a fairly low internal resistance, which allows them to produce high currents if the load is also low resistance (a required feature when cranking the engine).
salt water (a fantastically good conductor by non-metallic standards).
Physics question (for anyone who knows the answer): when lightning strikes somewhere in the ocean, why isn’t every living organism in the entire ocean electrocuted? How far away do you have to be to avoid being fried? How does one calculate this?
I’m not a physics major, but this is how I would reason: a regular human usually survives a lightning strike, IIRC. Why would fish be any different? It might hurt them but they have simpler nervous systems to boot. So my initial guess is that no fish at all is hurt, no more than they are fried by the sun unleashing gigawatts onto the ocean.
But that’s a cheap answer, perhaps. Let’s try another route. A human isn’t that big compared to a tuna fish, but is pretty big compared to things like trout or salmon. Let’s say we weigh 100x as much as those small fish.
Lightning is a one-shot packet of energy—like quickly blinking a flashlight. As the light spreads away from the flashlight, it begins to fade out. (Why isn’t the entire earth illuminated?) Well, there’s a fixed number of photons released, and the sphere/area they are spread over keeps getting bigger as they go—it increases as the square of how far away they are. It’s like gravity: you get an inverse square law. Squares increase pretty fast − 2^2 = 4, 3^2 = 9, 4^2 = 16 etc.
So if we humans are 2 feet around from the ‘epicenter’, how many units of 2 feet do we have to go to cut the strength by 1⁄100 and give the little fish a little fish-sized dose? Well, the square root of 100 is 10, so 10 2-feets is 20 feet.
In other words, by this reasoning, I’d expect even little fish to survive a lightning strike around 20 feet away. 20 feet is much smaller than an ocean.
This is all high-school physics at best; all you really have to do is think about why gravity follows an inverse square law, analogize a space-filling gravity to light, and guess some numbers in the best spirit of Fermi calculations.
Is this a correct model, though? My understanding is that the relevant figure is the potential difference between various points the creature’s body touches during the lightning strike. (Just like if a high voltage power transmission cable snaps and falls to the ground close to where you stand, you’re in much worse danger of electrocution if you stand so that one of your feet is closer than the other to a point where the cable touches the ground.)
Considering that the fish are usually elongated and the potential is (presumably?) distributed in a radially symmetrical way, they will be struck much worse if they happen to be swimming directly towards or away from the lightning strike point. [Edit: This is incorrect—please see the further discussion with Constant below.] Ignoring this and assuming spherical fish, the danger seems to be proportional to D*dV(r)/dr, where V(r) is the potential as a function of distance on the radial between the strike point and the current position of the fish, and D is the diameter of the fish. Now does anyone know what V(r) is supposed to look like?
Vladimir, I’m not sure about the orientation bit. Imagine constructing a sphere of fish around the lightning strike, so that the fish tile the sphere and are flat against the sphere (actually, hemisphere). Necessarily, all the electricity flows through the fish, because they completely tile the hemisphere. Now re-orient the fish without otherwise changing their location. Now, because the fish are thin, they no longer cover the sphere, and between them is a lot of seawater. So only a small fraction, now, of the electricity flows through the fish, and the rest passes by them in the seawater.
Meanwhile, of course, that small fraction of electricity is staying in the fish for much longer, because each unit of power is flowing the entire length of the fish, from head to tail, whereas when the fish are placed sideways relative to the flow of electricity, each unit of power is only flowing from one side to the other.
At first glance, it seems to cancel out.
Imagine the following: each fish is made out ten unit cubes placed next to each other. They can either be placed perpendicular to the flow of energy, so that each cube gets the energy that flows through one unit square. Or, they can be placed parallel to the flow of energy, so that they all share the energy from a single unit square, which flows through all of them one after the other.
It seems to come to the same thing.
But here’s one further complication: if the fish is a better conductor than the seawater, then the energy will tend to re-direct to seek out the fish (more electricity will flow through the better conductor, cet.par.), so that placing the fish parallel to the flow of energy rather than perpendicular to it will not entirely protect it from the neighboring energy. In short, if the fish is a better conductor than the seawater, then it is better for the fish to be oriented perpendicular to the flow of energy.
But, without going into details, I hastily extrapolate that if the fish is a poorer conductor than the seawater, then it is better for the fish to be oriented parallel to the flow of energy (i.e. facing the lighting strike point, or facing away).
But, without going into details, I hastily extrapolate that if the fish is a poorer conductor than the seawater, then it is better for the fish to be oriented parallel to the flow of energy (i.e. facing the lighting strike point, or facing away).
You are correct! I hastily analogized from the human step potential, ignoring the fact that fish, unlike humans, may well be much poorer conductors than the surrounding (or, in the human case, underlying) medium. Sadly, it seems the electrical engineering courses I took long ago haven’t left many surviving correct intuitions.
After a bit of googling about this question, I’m intrigued to find out that the problem of electrocuting fish has attracted considerable research attention. A prominent reference appears to be a paper titled Electrical stunning of fish: the relationship between the electrical field strength and water conductivity by two gentlemen named J. Lines and S. Kestin (available ungated here, and with a gruesome experimental section). Alas, the paper says, “No publications appear to be available which identify conductivity measurements of fish tissue at the frequencies being used.” It does however say that we might expect something in the hundreds or low thousands of uS/cm, whereas Wikipedia informs us that the conductivity of seawater is around 4.8 S/m, i.e. as much as 48,000 uS/cm.
So, yes, this was definitely a blunder on my part.
I’m sure it isn’t! But that’s the fun of Fermi problems: reaching not-wildly-incorrect solutions by way of absurdly simplified & wrong models.
For example, I feel sure that if my 20 foot answer is too little, the lethal radius would still be less than 1 larger order of magnitude (200 feet), and if it’s too much, that the lethal radius is still bigger than 1 smaller order (2 feet).
Oh, I like your Fermi model! (And also my above comment was horribly incorrect—see the subsequent discussion with Constant.)
What I was wondering however is whether it might be off too much even by Fermi problem standards, i.e. by multiple orders of magnitude. The trouble is that if the target creatures are vastly better or poorer conductors than the surrounding medium, this greatly influences how the flow of energy through and around them is distributed, possibly making the model based on uniform energy flow across all angles too inaccurate even for a Fermi calculation. (To give an extreme example, a metal wire connecting the poles of a battery draws nearly all energy flow in the circuit through itself, despite being a negligible part of the spatial cross-section.)
Or to put it more precisely, the way a human distorts the flow of electrical energy when surrounded by ground and air may well be extremely different, and possibly go in a totally different direction, from the way a fish distorts it when surrounded by seawater, so your generalization from humans to fish might be problematic.
My initial idea was to attempt another Fermi approach based on guesstimating V(r) and its derivative, but the poor conductivity of fish relative to seawater seems to complicate that one too.
For what it’s worth, it is quite possible to at least stun fish through electricity. I know people who have done this for scientific studies of fish. It’s called Electrofishing and is restricted in most places because it makes over fishing easy and requires some significant caution on the part of the fisher (you’re wearing a backpack designed to produce high voltage while standing in water).
And lead-acid batteries have a fairly low internal resistance, which allows them to produce high currents if the load is also low resistance (a required feature when cranking the engine).
The internal resistance of the body (and wet skin) is sufficient that it and the voltage are the relevant factors. Jimmy even went so far assume an ideal power source—as much current as 12v can get you. The resistance even without the benefit of dry skin is sufficient to keep the current that passes near the heart below the level that will result in fibrillation in healthy humans.
I would have to concur with Jimmy that death by car battery electrocution would qualify as ‘freak accident’. If you want to kill yourself with a battery you could perhaps try balancing it on top of a door and closing it while your head is
According to Wikipedia, the threshold for fibrillation is 60 mA for AC, 300-500 mA for DC. On reflection, it seems I’d previously cached the AC value as the value for all currents, so that was skewing my argument.
Given these figures, a 1k Ohm total resistance (internal plus skin plus body) would lead to a 12 mA current (painful but not fibrillation-inducing), whereas 200 Ohms / 40 Ohms total resistance would be required for 12 VAC / VDC to be potentially lethal. So, yeah, now that I think about it, a car battery probably couldn’t be lethal unless conductors were actually puncturing the skin and touching the bloodstream directly (or covering a HUGE amount of surface area). I retract my claim.
Edit: OH! Except that Wikipedia says the threshold for fibrillation is a mere 10 µA if the current is from electrodes that establish a circuit through the heart. THAT’s the figure I’d seen before and cached in my head. Still, that’s not a likely situation to arise when using jumper cables, so my claim remains retracted.
12 volts isn’t high enough to produce any sensation under normal circumstances (the only time I’ve noticed so much as a tingle was after diving in the ocean for an hour)
Touching the ends to each other, however, will send sparks flying.
Touching 12 volt battery terminals is indeed very, very unlikely to harm you.
However, cars are equipped with ignition coils to produce a spark for the spark plugs. When a car is running the ignition coil is producing output of 20-30 kV and back EMF in the hundreds of volts. Normally this is filtered out from the 12-volt side in order to protect the electronics and battery, but I would not recommend trusting those filters with your life.
Indeed, a good reason to do this procedure a safe distance from gas pumps.
Not just sparks. The electrical system of one or both cars can be severely damaged.
Also, you shouldn’t attach the negative cable to the negative terminal on the dead car, but to exposed metal of the car’s chassis (i.e. structure). This is to avoid a spark igniting any leaking fluid from the bad battery. Flaming battery acid = not fun.
It’s worth noting that, while 12 volts won’t normally penetrate dry skin under most humidity conditions, you really do need to be careful. Pressure increases surface-to-surface contact, which decreases resistance, which lowers the voltage threshold. So can moisture, like even small amounts of sweat. And a car battery does have sufficient current to injure or kill a human being quite easily. (Voltage penetrates insulators, current actually does damage. The zap you get from static electricity is in the range of thousands of volts, but the current is negligible.)
Yeah, car batteries can do about a kiloamp into a dead short so we can treat them as ideal voltage sources for this ‘application’. However, even with wet hands and solid contact, 12 volts is too low to get much current flowing.
Soaking my hands in saturated salt water got my hand to hand resistance down to 10-20kohm (0.5-1ma), which is still at least a factor of 250 above the 40 ohm resistance you’d need to draw 300ma, which is the lower figure wiki gives for DC caused fibrillation. Putting one hand on either terminal didn’t get me so much as a tingle.
I know tingle levels are possible when soaking for longer (hours) and 9v will tingle your tongue (2-4milliamps), but it seems exceedingly hard to get to a dangerous level, considering that most models I’ve seen had the internal resistance of people at hundreds of ohms (350 is the number that sticks in mind). Also nerves are somewhat AC coupled, which brings the fibrillation limit up and makes people push away from the source instead of clinging on.
I guess it might be possible for someone with thin skinned thoroughly soaked hands making good contact and having a poorly shielded sensitive heart, but I’d call it a ‘freak accident’.
It’s worth noting that the reason we use clamps on the ends of the jumper cables is because pressure increases surface area in contact, which decreases resistance for the simple reason of Ohm’s law applied to parallel resistors. (Three 1k Ohm resistors have a parallel resistance of only 333 Ohms. It’s meaningless to give a single figure for copper → wet skin resistance without also giving the surface area for which the figure is valid.)
This means that incidental touching of metal is extremely unlikely to kill anyone, but accidentally clamping your finger, gripping metal tightly, or anything else that applies pressure to your skin will dramatically raise the risk.
Upvoted for experimenting!
Fortunately current and voltage are not independent features of a power source—and in the case of current not even something that can be meaningfully measured without specifying the load! A car battery does not “have sufficient current to injure or kill a human quite easily” because of the human part of the (I=V/R) equation.
It does if the skin is wet. Once you’re through the skin, the human body’s resistance is quite low, in the single-digit kiloohm range at most, because the human body is mostly salt water (a fantastically good conductor by non-metallic standards). The biggest barrier to current is the upper layer of dead, dry cells on the epidermis. And lead-acid batteries have a fairly low internal resistance, which allows them to produce high currents if the load is also low resistance (a required feature when cranking the engine).
Physics question (for anyone who knows the answer): when lightning strikes somewhere in the ocean, why isn’t every living organism in the entire ocean electrocuted? How far away do you have to be to avoid being fried? How does one calculate this?
I’m not a physics major, but this is how I would reason: a regular human usually survives a lightning strike, IIRC. Why would fish be any different? It might hurt them but they have simpler nervous systems to boot. So my initial guess is that no fish at all is hurt, no more than they are fried by the sun unleashing gigawatts onto the ocean.
But that’s a cheap answer, perhaps. Let’s try another route. A human isn’t that big compared to a tuna fish, but is pretty big compared to things like trout or salmon. Let’s say we weigh 100x as much as those small fish.
Lightning is a one-shot packet of energy—like quickly blinking a flashlight. As the light spreads away from the flashlight, it begins to fade out. (Why isn’t the entire earth illuminated?) Well, there’s a fixed number of photons released, and the sphere/area they are spread over keeps getting bigger as they go—it increases as the square of how far away they are. It’s like gravity: you get an inverse square law. Squares increase pretty fast − 2^2 = 4, 3^2 = 9, 4^2 = 16 etc.
So if we humans are 2 feet around from the ‘epicenter’, how many units of 2 feet do we have to go to cut the strength by 1⁄100 and give the little fish a little fish-sized dose? Well, the square root of 100 is 10, so 10 2-feets is 20 feet.
In other words, by this reasoning, I’d expect even little fish to survive a lightning strike around 20 feet away. 20 feet is much smaller than an ocean.
This is all high-school physics at best; all you really have to do is think about why gravity follows an inverse square law, analogize a space-filling gravity to light, and guess some numbers in the best spirit of Fermi calculations.
Is this a correct model, though? My understanding is that the relevant figure is the potential difference between various points the creature’s body touches during the lightning strike. (Just like if a high voltage power transmission cable snaps and falls to the ground close to where you stand, you’re in much worse danger of electrocution if you stand so that one of your feet is closer than the other to a point where the cable touches the ground.)
Considering that the fish are usually elongated and the potential is (presumably?) distributed in a radially symmetrical way, they will be struck much worse if they happen to be swimming directly towards or away from the lightning strike point. [Edit: This is incorrect—please see the further discussion with Constant below.] Ignoring this and assuming spherical fish, the danger seems to be proportional to D*dV(r)/dr, where V(r) is the potential as a function of distance on the radial between the strike point and the current position of the fish, and D is the diameter of the fish. Now does anyone know what V(r) is supposed to look like?
Vladimir, I’m not sure about the orientation bit. Imagine constructing a sphere of fish around the lightning strike, so that the fish tile the sphere and are flat against the sphere (actually, hemisphere). Necessarily, all the electricity flows through the fish, because they completely tile the hemisphere. Now re-orient the fish without otherwise changing their location. Now, because the fish are thin, they no longer cover the sphere, and between them is a lot of seawater. So only a small fraction, now, of the electricity flows through the fish, and the rest passes by them in the seawater.
Meanwhile, of course, that small fraction of electricity is staying in the fish for much longer, because each unit of power is flowing the entire length of the fish, from head to tail, whereas when the fish are placed sideways relative to the flow of electricity, each unit of power is only flowing from one side to the other.
At first glance, it seems to cancel out.
Imagine the following: each fish is made out ten unit cubes placed next to each other. They can either be placed perpendicular to the flow of energy, so that each cube gets the energy that flows through one unit square. Or, they can be placed parallel to the flow of energy, so that they all share the energy from a single unit square, which flows through all of them one after the other.
It seems to come to the same thing.
But here’s one further complication: if the fish is a better conductor than the seawater, then the energy will tend to re-direct to seek out the fish (more electricity will flow through the better conductor, cet.par.), so that placing the fish parallel to the flow of energy rather than perpendicular to it will not entirely protect it from the neighboring energy. In short, if the fish is a better conductor than the seawater, then it is better for the fish to be oriented perpendicular to the flow of energy.
But, without going into details, I hastily extrapolate that if the fish is a poorer conductor than the seawater, then it is better for the fish to be oriented parallel to the flow of energy (i.e. facing the lighting strike point, or facing away).
Constant:
You are correct! I hastily analogized from the human step potential, ignoring the fact that fish, unlike humans, may well be much poorer conductors than the surrounding (or, in the human case, underlying) medium. Sadly, it seems the electrical engineering courses I took long ago haven’t left many surviving correct intuitions.
After a bit of googling about this question, I’m intrigued to find out that the problem of electrocuting fish has attracted considerable research attention. A prominent reference appears to be a paper titled Electrical stunning of fish: the relationship between the electrical field strength and water conductivity by two gentlemen named J. Lines and S. Kestin (available ungated here, and with a gruesome experimental section). Alas, the paper says, “No publications appear to be available which identify conductivity measurements of fish tissue at the frequencies being used.” It does however say that we might expect something in the hundreds or low thousands of uS/cm, whereas Wikipedia informs us that the conductivity of seawater is around 4.8 S/m, i.e. as much as 48,000 uS/cm.
So, yes, this was definitely a blunder on my part.
I’m sure it isn’t! But that’s the fun of Fermi problems: reaching not-wildly-incorrect solutions by way of absurdly simplified & wrong models.
For example, I feel sure that if my 20 foot answer is too little, the lethal radius would still be less than 1 larger order of magnitude (200 feet), and if it’s too much, that the lethal radius is still bigger than 1 smaller order (2 feet).
Oh, I like your Fermi model! (And also my above comment was horribly incorrect—see the subsequent discussion with Constant.)
What I was wondering however is whether it might be off too much even by Fermi problem standards, i.e. by multiple orders of magnitude. The trouble is that if the target creatures are vastly better or poorer conductors than the surrounding medium, this greatly influences how the flow of energy through and around them is distributed, possibly making the model based on uniform energy flow across all angles too inaccurate even for a Fermi calculation. (To give an extreme example, a metal wire connecting the poles of a battery draws nearly all energy flow in the circuit through itself, despite being a negligible part of the spatial cross-section.)
Or to put it more precisely, the way a human distorts the flow of electrical energy when surrounded by ground and air may well be extremely different, and possibly go in a totally different direction, from the way a fish distorts it when surrounded by seawater, so your generalization from humans to fish might be problematic.
My initial idea was to attempt another Fermi approach based on guesstimating V(r) and its derivative, but the poor conductivity of fish relative to seawater seems to complicate that one too.
For what it’s worth, it is quite possible to at least stun fish through electricity. I know people who have done this for scientific studies of fish. It’s called Electrofishing and is restricted in most places because it makes over fishing easy and requires some significant caution on the part of the fisher (you’re wearing a backpack designed to produce high voltage while standing in water).
Google: lightning sea
I tried it and got good hits, which is why I suggest it. Variants will probably work too.
The internal resistance of the body (and wet skin) is sufficient that it and the voltage are the relevant factors. Jimmy even went so far assume an ideal power source—as much current as 12v can get you. The resistance even without the benefit of dry skin is sufficient to keep the current that passes near the heart below the level that will result in fibrillation in healthy humans.
I would have to concur with Jimmy that death by car battery electrocution would qualify as ‘freak accident’. If you want to kill yourself with a battery you could perhaps try balancing it on top of a door and closing it while your head is
According to Wikipedia, the threshold for fibrillation is 60 mA for AC, 300-500 mA for DC. On reflection, it seems I’d previously cached the AC value as the value for all currents, so that was skewing my argument.
Given these figures, a 1k Ohm total resistance (internal plus skin plus body) would lead to a 12 mA current (painful but not fibrillation-inducing), whereas 200 Ohms / 40 Ohms total resistance would be required for 12 VAC / VDC to be potentially lethal. So, yeah, now that I think about it, a car battery probably couldn’t be lethal unless conductors were actually puncturing the skin and touching the bloodstream directly (or covering a HUGE amount of surface area). I retract my claim.
Edit: OH! Except that Wikipedia says the threshold for fibrillation is a mere 10 µA if the current is from electrodes that establish a circuit through the heart. THAT’s the figure I’d seen before and cached in my head. Still, that’s not a likely situation to arise when using jumper cables, so my claim remains retracted.
I think we’re on agreement with all those figures. Embedding electrodes into your chest or using AC are definitely asking for trouble!