Wikipedia:Reference desk/Archives/Science/2016 April 22

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April 22

Special relativity. Simultaneity 3

Question

Remark

According figures 31-38, 31-39 on pages 492-493 "Physics for the Inquiring Mind" (by Eric M. Rogers), before Lorentz contraction where must be situated clocks in ε' coach for correct observation of desynchronization? Like that : https://archive.org/download/feynmanlectures_631/160422072400.PNG ? If yes, then we have next table: ε' frame ε frame time coordinate clock reading & x-coordinate time coordinate clock reading & x-coordinate $t'=0$ $\theta _{(t'=0),self}^{[}=0$ $_{(x'=-L'/2=-{\tfrac {L/2}{\sqrt {...}}})}$ $\theta _{(t'=0),self}=0$ $_{(x'=0)}$ $\theta _{(t'=0),self}^{]}=0$ $_{(x'=L'/2)}$ $t=0$ $\theta _{(t=0)}^{[}=?$ $_{(x=-L/2)}$ $\theta _{(t=0)}=0$ $_{(x=0)}$ $\theta _{(t=0)}^{]}=?$ $_{(x=L/2)}$ $t'={\tfrac {L'/2}{c}}$ $\theta _{(t'=L'/2c),self}^{[}={\tfrac {L'/2}{c}}$ $_{(x'=-L'/2)}$ $\theta _{(t'=L'/2c),self}={\tfrac {L'/2}{c}}$ $_{(x'=0)}$ $\theta _{(t'=L'/2c),self}^{]}={\tfrac {L'/2}{c}}$ $_{(x'=L'/2)}$ $t=\tau _{1}={\tfrac {L/2}{c+u}}$ $\theta _{(t=\tau _{1})}^{[}=\theta _{(t=0)}^{[}+\theta _{(t=\tau _{1})}$ $_{(x=-L/2+u\tau _{1})}$ $\theta _{(t=\tau _{1})}=\tau _{1}{\sqrt {...}}$ $_{(x=u\tau _{1})}$ $\theta _{(t=\tau _{1})}^{]}=\theta _{(t=0)}^{]}+\theta _{(t=\tau _{1})}$ $_{(x=L/2+u\tau _{1})}$ $t=\tau _{2}={\tfrac {L/2}{c-u}}$ $\theta _{(t=\tau _{2})}^{[}=\theta _{(t=0)}^{[}+\theta _{(t=\tau _{2})}$ $_{(x=-L/2+u\tau _{2})}$ $\theta _{(t=\tau _{2})}=\tau _{2}{\sqrt {...}}$ $_{(x=u\tau _{2})}$ $\theta _{(t=\tau _{2})}^{]}=\theta _{(t=0)}^{]}+\theta _{(t=\tau _{2})}$ $_{(x=L/2+u\tau _{2})}$ ... ... $t={\tfrac {L/2}{u}}$ $\theta _{(t={\tfrac {L/2}{u}})}^{[}=\theta _{(t=0)}^{[}+\theta _{(t={\tfrac {L/2}{u}})}$ $_{(x=0)}$ $\theta _{(t={\tfrac {L/2}{u}})}={\tfrac {L/2}{u}}{\sqrt {...}}$ $_{(x=L/2)}$ $t'=A$ $\theta _{(t'=A),self}^{[}=A$ $_{(x'=-L'/2)}$ $\theta _{(t'=A),self}=A$ $_{(x'=0)}$ $\theta _{(t'=A),self}^{]}=A$ $_{(x'=L'/2)}$ $t=B$ $\theta _{(t=B)}^{[}=\theta _{(t=0)}^{[}+\theta _{(t=B)}$ $_{(x=-L/2+uB)}$ $\theta _{(t=B)}=B{\sqrt {...}}$ $_{(x=uB)}$ $\theta _{(t=B)}^{]}=\theta _{(t=0)}^{]}+\theta _{(t=B)}$ $_{(x=L/2+uB)}$ Note 1. ${\sqrt {...}}$ means ${\sqrt {1-u^{2}/c^{2}}}$. Note 2. $L'={\tfrac {L}{\sqrt {...}}}$. Note 3. $\theta$ means readings of clocks of ε' coach. "[" means left clock according picture (hind), "]" means right end clock (front). $\theta _{self}$ means readings of clocks of ε' coach seen from ε' coach (frame). We neglect time needed light to reach eye (there are no observers). Is table correct? How to calculate $\theta _{(t=0)}^{[}$ and $\theta _{(t=0)}^{]}$ analytically?

https://archive.org/download/PhysicsForTheEnquiringMind/Rogers-PhysicsForTheEnquiringMind.djvu , page 492 Wikipedia:Reference_desk/Archives/Science/2016 March 18#Light path analysis and consequences Wikipedia:Reference_desk/Archives/Science/2016 March 20#Null result of Michelson.E2.80.93Morley experiment extrapolation Wikipedia:Reference desk/Archives/Science/2016 March 28#Special relativity. Derivation of formula Wikipedia:Reference desk/Archives/Science/2016 March 29#Special relativity. Simultaneity Wikipedia:Reference desk/Archives/Science/2016 April 4#Special relativity. Simultaneity 2 Wikipedia:Reference desk/Archives/Science/2016 April 10#Special relativity. Simultaneity 2.

37.53.235.112 (talk) 05:33, 22 April 2016 (UTC)

I think the image [1] does show the thought-experiment you're interested in. But it's not more correct than any other thought-experiment. You could observe the desynchronization (or rather different synchronizations) with the clocks in other locations.

I think the table is okay, but I only spot-checked it. It's similar to the last table and you filled that one out correctly.

One way to calculate $\theta _{(t=0)}^{[}$ is to first write down

$x'(\theta ^{[})=-L'/2$

$t'(\theta ^{[})=\theta ^{[}$

and then use the Lorentz transform to get $t(\theta ^{[})$, and then set $t=0$ and solve for $\theta ^{[}$. -- BenRG (talk) 06:50, 22 April 2016 (UTC)

Thanks. Using your suggestion I derive

$\theta _{(t=0)}^{[}={\tfrac {L'u}{2c^{2}}}$;

$\theta _{(t=0)}^{]}=-{\tfrac {L'u}{2c^{2}}}$.

It works for $\theta _{(t=\tau _{1})}^{[}=\theta _{(t=\tau _{2})}^{]}$, which was derived independently from statement that all reference frames are applied to single world.

But when left clock from ε' arrives to point x=0 , it must show same reading as clock from ε, which is also situaded in x=0. So we must have:

$\theta _{(t={\tfrac {L/2}{u}})}^{[}={\tfrac {L/2}{u}}$ .

Let's check:

$\theta _{(t={\tfrac {L/2}{u}})}^{[}=\theta _{(t=0)}^{[}+\theta _{(t={\tfrac {L/2}{u}})}=$

$={\tfrac {L'u}{2c^{2}}}+{\tfrac {L/2}{u}}{\sqrt {...}}={\tfrac {Lu}{2c^{2}{\sqrt {...}}}}+{\tfrac {L{\sqrt {...}}}{2u}}={\tfrac {Lu}{2c^{2}{\sqrt {...}}}}\cdot {\tfrac {u}{u}}+{\tfrac {L{\sqrt {...}}}{2u}}\cdot {\tfrac {c^{2}{\sqrt {...}}}{c^{2}{\sqrt {...}}}}$

$={\tfrac {Lu^{2}+Lc^{2}(1-u^{2}/c^{2})}{2uc^{2}{\sqrt {...}}}}={\tfrac {Lu^{2}+L(c^{2}-u^{2})}{2uc^{2}{\sqrt {...}}}}={\tfrac {Lc^{2}}{2uc^{2}{\sqrt {...}}}}={\tfrac {L}{2u{\sqrt {...}}}}\neq {\tfrac {L/2}{u}}$.

Why? 37.53.235.112 (talk) 12:02, 22 April 2016 (UTC)

I think you calculated both clock readings correctly, and they are indeed different. Here's a "Euclidean spacetime diagram" of the situation (later times on top):

   |    E    |
   |   /|   /|
   |  / |  / |  /
   | /  | /  | /
   |C   |/   |/
   A    O    B
  /|   /|   D|
 / |  / |  / |
   | /  | /  |

The unprimed clocks at A, O, B show 0, and the primed clocks at C, O, D show 0. If the clocks at E had the same reading, that would mean CE=OE, which clearly isn't the case. In Euclidean geometry, $CE/OE=\cos \theta$ where $\theta$ is the angle between the lines. In relativistic/Minkowskian geometry, $CE/OE=\cosh \alpha =1/{\sqrt {\cdots }}$ where $\alpha$ is the rapidity. -- BenRG (talk) 19:27, 22 April 2016 (UTC)

I don't understand such diagrams. Are lines t-axes or trajectories (world lines)? What do you mean about 'unprimed' and 'primed'. Why point C is above horizontal? I imagine this diagrame next way : https://archive.org/download/feynmanlectures_631/160423120000.PNG . So point C (where hind (left) moving clock shows 0) is below x-axis.

37.53.235.112 (talk) 10:16, 23 April 2016 (UTC)

Well, some calculations show that

$\theta _{(t=\tau ) \atop (x=0)}^{[}=\tau$

only when $u\to 0$. But it's very strange, as all moving clocks from ε' coach we believed were synchronized manually with middle clock of ε coach. So in few moments after synchronization of middle clocks none of clocks are synchronized.

37.53.235.112 (talk) 11:58, 23 April 2016 (UTC)

The lines are world lines. "Unprimed" means the ε coordinate system (or the clocks at rest wrt ε) and "primed" means ε'. "Prime" refers to prime (symbol). This is standard terminology.

My "Euclidean spacetime diagram" is not at all standard. The reason C is above A is that it's the Euclidean version of the problem, where the "moving" set of clocks is just rotated. I thought it would be easier to see in the Euclidean diagram that there's no symmetry that would make the clocks agree at E. OCE is a right triangle, not an isosceles triangle. -- BenRG (talk) 07:51, 24 April 2016 (UTC)

Flu shot...

I am having a hard time finding a reliable source for this one.. "How many people die from the flu shot every year?" Such a simple inquiry... unfortunately the prevalence of anti-vaxers on the internet is equivalent to the quantity of porn on the internet... I'm just curious as to how many actually DIE from it.. (If they died from the flu shot there is no way they would survive an influenza infection!) 199.19.248.20 (talk) 05:50, 22 April 2016 (UTC)

Death would most likely be from a severe allergic reaction I'd think, so they might have survived the flu itself. I'll see if I can find some numbers for you. EvergreenFir (talk) Please {{re}} 05:54, 22 April 2016 (UTC)

Closest thing I can easily find is VAERS. It's a reporting database of adverse events associated with vaccines. It does not indicate that death occurred because of the vaccine however. It allows you to specify the types of events, type of vaccine, date range, etc. EvergreenFir (talk) Please {{re}} 06:07, 22 April 2016 (UTC)

Assuming there have been people whose death has been attributed to the flu vaccination, the answer to the OP's question is that the number who die per year is a random variable. If the number who die every year is a random variable then there is no answer to the question "How many die every year". It might make some sense to ask "How many died in 2015" but asking how many die every year makes no sense. Dolphin (t) 11:39, 22 April 2016 (UTC)

Wouldn't an average be able to be given though? "An average of X number of people have died per year from Y over time span Z". Dismas|^(talk) 13:02, 22 April 2016 (UTC)

[2] is a study of the deaths reported to VAERS over a period of about 15 years. It doesn't only include deaths reported after having an influenza vaccine but does seem to provide some related statistics. They didn't detect any concerning pattern suggested that very very few, if any, of the deaths were caused by the vaccine but that obviously isn't a number. I didn't look in to the actual paper so I'm not sure if they attempted to estimate number of deaths actually due to the vaccine or anything of that sort. And they could obviously only look at deaths where there was an available report.

Note as the EvergreenFir said, although on average you should reduce your chances of dying even if you're a healthy and fit young adult by receiving the vaccine, if anyone does die due to the vaccine it's possible they will not have died if they didn't receive it. (Whether because of different circumstances at the time they received the vaccine, the cause of death which as mentioned would probably be an allergic reaction is something which may not have resulted even if they were infected with influenza or simply because they wouldn't have been infected anyway.)

Nil Einne (talk) 13:29, 22 April 2016 (UTC)

Your paper says that in 2009-2010 there were 107 deaths that occurred shortly after receiving an influenza vaccine (among 15 million doses delivered). This was apparently an unusually high number. Dragons flight (talk) 13:51, 22 April 2016 (UTC)

I also found [3] which says "The CDC is aware of four deaths linked to influenza vaccination from 1990 to 2005". Unfortunately I haven't been able find any sign of this statistic elsewhere so don't know where it came from. The author still appears active on Quora so you could ask them. While I know usage of the flu vaccine has increase, it seems unlikely this is simply the number of reports to VAERS unless the criteria for reporting has change very significantly.

BTW according to [4] (far from a RS but it links to an RS which unfortunately isn't working for me) there were 3 deaths compensated due to the flu vaccine in the period covered under the so called Vaccine court system in the US. According to our article, for favourable adjudication the claimant "must show a causal connection; if medical records show a child has one of several listed adverse effects soon after vaccination, the assumption is that it was caused by the vaccine". This ia a data point, even if considering the nature of such court proceedings and the laws governing them, it's difficult to say for sure how many deaths were really caused by the vaccine; and it's likewise difficult to know how many deaths cause by the vaccine either never went to the court or didn't receive favourable adjudication. Also it only applies to children I believe.

Nil Einne (talk) 14:29, 22 April 2016 (UTC)

Well, that's the rub, isn't it? Post hoc ergo propter hoc means that simply showing that a child died after receiving a flu shot doesn't mean they died because they received the flu shot. There is a convoluted set of questions that this comes down to:

• People who died after they received a flu shot
• People who died after they received a flu shot for whom it was reported, reported, or claimed that happened (that is, someone needs to make a record of the temporal relationship between the flu shot and the death, and not miss it or ignore it)
• People who received compensation after dying after getting a flu shot (either voluntarily, or court ordered).

Not that none of this is a record of people for whom it is known that the flu shot caused the death. For that to be shown to be true, we need more than a temporal relationship (that is, death coming after flu shot). We also need a mechanism; that is the means by which the flu shot caused the death, such that we could show that if they had not received the flu shot, they would not have died. That data is often quite lacking, because largely that group 2 listed above is a MUCH smaller subset of group 1, and even THAT group may not have had a causal relationship. --Jayron32 15:41, 22 April 2016 (UTC)

This is why they ask you if you're allergic to chicken eggs, as the strains are often grown in petri dishes containing egg derivatives. ←Baseball Bugs ^{What's up, Doc?} carrots→ 13:30, 22 April 2016 (UTC)

Usually not in petri dishes, but in actual hen's eggs [5] neat, isn't it? SemanticMantis (talk) 14:29, 22 April 2016 (UTC)

Ha, I must have heard wrong. So the yolk's on me! ←Baseball Bugs ^{What's up, Doc?} carrots→ 14:31, 22 April 2016 (UTC)

That joke leaves me petrified. StuRat (talk) 16:20, 25 April 2016 (UTC)

Can thinking about sex reduce pain?

Can pain or discomfort be reduced simply by thinking about erotica? I don’t know how somebody could find this topic on Wikipedia—if it’s already here. --Romanophile (talk) 12:44, 22 April 2016 (UTC)

Thinking about erotica is essentially to increase dopamine levels in the blood. Dopamine plays an important role in pain regulation. So yes, it's possible.196.213.35.146 (talk) 12:52, 22 April 2016 (UTC)

Original Research: the answer to any question involving humans and sex is "Yes, probably, for some people." I approached the OP's question from the pain side. Our article on Pain management has a reasonably well-sourced section on psychological approaches - nothing specific to sex. "Got Pain? Think Sex" from WedMD is journalistic, but cites university research. "Does using fantasy to outwit pain and fear sound far-fetched? It's not, claims a study presented at the annual meeting of the American Pain Society in October 1999. A research team from Johns Hopkins and other universities found that people who fantasized about a highly pleasurable sexual scenario experienced the least pain." And here's a New Scientist article on sexual arousal and brain changes, including pain perception. These offer a start, at least. Carbon Caryatid (talk) 14:08, 22 April 2016 (UTC)

What if you're a masochist? ←Baseball Bugs ^{What's up, Doc?} carrots→ 14:32, 22 April 2016 (UTC)

Thinking about sex won't reduce pain if one suffer's from a penis injury. StuRat (talk) 23:51, 22 April 2016 (UTC)

I often thought that some sort of diversionary stimulation would be good whilst being drilled and probed in the dentist's chair. But with the cost of basic treatment these days, I doubt if I could afford the additional cost of that luxury. :( --178.108.238.49 (talk) 21:57, 24 April 2016 (UTC)

First device to produce non-static electricity

Historically, what was the actual first device to generate non-static electricity (direct current)? As I understand it, Francis Hauksbee's generator from c. 1705 still generated only static electricity known to the ancients. Was it Faraday's dynamo or there were previous rough, but working devices? Thanks. 93.174.25.12 (talk) 19:47, 22 April 2016 (UTC)

Was not it a chemical battery? Ruslik_Zero 20:37, 22 April 2016 (UTC)

Indeed, before the experiments of Luigi Galvani and Alessandro Volta, the electricity was either produced by friction between a suitable pair of materials, or (far less controllably) made briefly available by the use of a lightning rod. Lightning rod pre-dates voltaic pile by several decades; however, lower atmospheric electricity is also static in its origin. On a closely related subject, storage of (static) electricity in what we now call a capacitor also pre-dates voltaic pile by a few decades, see e.g. Leyden jar. --Dr Dima (talk) 21:02, 22 April 2016 (UTC)

And please see the Georg Wilhelm Richmann article for a good reason to use a Voltaic pile rather than a lightning rod o_O ... --Dr Dima (talk) 21:11, 22 April 2016 (UTC)

(EC) A static electric generator basically produced very high voltage electricity(kilovolts) at very low current (microamperes typically).The Leyden Jar or capacitor, invented 1745, could accumulate the electricity produced by a static electric generator. Once the Leyden jar was charged, it could be discharged through a conductor in a diminishing surge of high current until it was discharged. In the right circuit the discharge might oscillate for a bit back and forth. In the 1770's Joseph Priestly used a set of Leyden Jars as a source of high current to make pieces of wire glow and melt ,a graphic demonstration that it was a high current of electricity.Edison (talk) 22:02, 22 April 2016 (UTC)

PS: Lest you should think that there is a gap in Wikipedia's comprehensiveness, the link is Leyden jar. Alansplodge (talk) 09:10, 23 April 2016 (UTC)

The artifacts known as the Bahgdad batteries have been considered to be a very early form of galvanic cell, but this is no longer the academic consensus. (The poster formerly known as 87.81.230.195} 90.199.208.67 (talk) 21:57, 23 April 2016 (UTC)

Contractile vacuole in Paramecium.

1. Why is the contractile vacuole in Paramecium star shaped as depicted in this image:

2. Do Paramecium have eyes? How do they know what they are doing? --Augustous (talk) 22:00, 22 April 2016 (UTC)

Paramecium doesn't have eyes - it's a single-cell organism so it cannot have organs, only organelles - but it does have a photoreceptor and it does exhibit phototaxis. --Dr Dima (talk) 22:49, 22 April 2016 (UTC)

Erythropsidinium: Just because it's a single cell doesn't mean it can't have an eye with a lens and retina! More "eye candy" at ocelloid.

The first one is an interesting question - certainly I had not looked up anything about contractile vacuoles in ages, and our own article certainly is not leaving me with a settled feeling. Apparently the structure is quite a nuisance to try to isolate. In part this is because it is a dynamic structure, not a physical one - the CV is surrounded by acidocalcisomes, and it does not have a continuous channel to the outside, at least in certain random organisms, not paramecia, I was just reading about. See [6] for a CV, surrounded by acidocalcisomes, caught in the act of kissing one of them. The paper also describes the structure as a "transport hub" for membrane proteins to the surrounding acidocalcisomes.

Looking at the Paramecium article itself, they have a hint of starriness but certainly don't look like in the illustration. My guess from what I just read over is that the distribution of acidocalcisomes around the CV, and periodic connections made with them, might give it a very roughly center-and-petals type arrangement when viewed overall over time. But I don't really know this at this point.

On the second one, dinoflagellates famously have eyes - even with lenses! - but I don't think any such thing has been observed for paramecia. Wnt (talk) 22:51, 22 April 2016 (UTC)

Each Contractile vacuole has a high surface area to volume ratio for efficiency when it periodically gathers water from the cell and then expels water by contraction. Paramecia are occupied 24/7 with maintaining their osmotic balance with their environment, swimming using their cilia in search of edible microorganisms like bacteria, algae, and yeasts, and occasionally backing up when they feel a collision with an obstacle. Paramecia have so little concern with Circadian rhythm, identifying prey without tasting it or optical rangefinding that to grow a multicellular photorecepter such as an eye never occurs to them. See Eye#Relationship to life requirements. AllBestFaith (talk) 22:59, 22 April 2016 (UTC)

This video and some others by the same author are really pretty nice - at exactly 0:45 the paramecium rolls and if you hit the space bar you can see the CV surrounded by six other little structures, which I assume are the acidocalcisomes? It's not really a regular star but it looks pretty damn cute anyway, much better than most of the shots I was looking at. Wnt (talk) 23:11, 22 April 2016 (UTC)

Great video Wnt! Regarding the Erythropsidinium or ocelloid-bearing dinoflagellates in general - yes, their ocelloid (and surrounding structure) is currently hypothesized to work like an eye for all practical purposes, and certainly bears similarity to an eye; but it is not an eye. By the strict definition eyes are organs - multicellular structures - which ocelloids are not. The boundary may be rather tenuous, though, because plastids, classified as organelles, very likely evolved from endosymbionts. --Dr Dima (talk) 23:33, 22 April 2016 (UTC)

That is not uncommon as a definition, but it doesn't seem very useful to me. Eyes can be immense, as in humans, or be made up of just a few cells as in ocelli/stemmata, various lancelet visual organs, etc. If the term can encompass that level of diversity of size and function, I don't see a use for an arbitrary boundary at the one-cell level. To me the concept of an "image-forming eye", i.e. something with a lens, seems a more fundamental distinction. Similarly, it would seem more useful to me simply to admit that paramecia have mouths and anuses, rather than resorting to awkward circumlocutions. Wnt (talk) 10:44, 23 April 2016 (UTC)

I seem to remember during my biology degree that contractile vacuoles were always depicted as stars. Perhaps it is a convention historically adopted by biology illustrators. DrChrissy ^(talk) 00:01, 23 April 2016 (UTC)

OK, I just found a video that knocks the socks off the last one I posted: [7] In this one you can really see the "star" type arrangement, and in particular, the filling of the surrounding chambers that then fuse into the CV. Wnt (talk) 02:38, 23 April 2016 (UTC)

It's not the amps that kills you, it's the power

So from my understanding, there's a misconception that electrocution is caused by amps rather than volts. But instead the truth is that the power of the current is actually what kills you, not merely the amps. Let's take a taser that has a voltage of 50,000 and a current of .0021. So this should produce a power of 105 watts. If we were to reverse this and have a taser with a voltage of .0021 volts and a current of 50,000 amps this should still be "relatively" less than lethal right? If so, how would this affect a person if he was shot by reversed taser like this? Would he still feel pain? ScienceApe (talk) 23:02, 22 April 2016 (UTC)

I think you need to review Ohm's Law. Voltage and current are not independent variables. --Trovatore (talk) 23:06, 22 April 2016 (UTC)

As Trovatore mentions above, you need to understand Ohm's law. Even though it's the current that can be lethal, you can't get a current of 50000 amps through your body with a voltage of only 0.0021V, simply because the current directly depends on the Voltage applied AND the resistance of your body. The resistance of skin has a 'range' of resistance of about 50Kohm to 10Mohm, depending on many factors like stress, how wet the skin is, how much pressure is applied on the contact points, etc... Even though a car battery can deliver 100s of Amps, it can't kill you because applied to your body, 12V will only produced a few micro-amps through dry skin. Also, if a taser can generate 50000V, but is limited to 0.0021 Amps, then as the current flows through a body, the voltage decreases (down to about 4000V). We can estimate that with a body resistance of 2Mohm for instance, the power going through it would be only about 8.8W (not 105W). Dhrm77 (talk) 01:03, 23 April 2016 (UTC)

Normally, people say: "It's not the volts that kill you, it's the amps" --Llaanngg (talk) 23:37, 22 April 2016 (UTC)

I've heard "it's the volts that jolts; it's the mills that kills", as in "milliampere". But when you're talking about electrocution there are other things to take into account. Organisms aren't ideal spherical masses of uniform density. The path the current takes through the body is one important factor. Even a fairly small current can kill you if it travels through your heart and induces cardiac arrest. Conversely a very large current can just injure you if it travels through your extremities. This is why some people survive lightning strikes and others don't. --71.110.8.102 (talk) 00:14, 23 April 2016 (UTC)

Another point: Electricity can hurt you in more than one way. You can have your heart rhythm disrupted, which is the case that people are often concerned about because it takes a relatively small current. (And I've read, but I don't know where, that it even has a random element, depending on when in the heart's cycle the shock comes.) But you can also simply be burned to death, and that is a matter of power. --69.159.61.172 (talk) 02:58, 23 April 2016 (UTC)

I think what a lot of people don't understand when they talk about the effects of "voltage" versus "current" is twofold. First of all, for direct current anyway, the two things are proportional to one another. You can't change one without changing the other in lockstep.

So what does the voltage-v-current question even mean? Well, the thing is, very rarely do you have either a pure voltage source or a pure current source, something that will deliver the same voltage or the same current regardless of the resistance.

Instead, you have electricity sources with internal resistance. If you put an infinite resistance across its poles, you get the nominal voltage, but the current is zero. If you short-circuit it (give it zero resistance), you get the maximum current it can deliver, but now the whole voltage drop is internal; there is no potential difference between the poles at all.

Here's an extreme example: In ordinary circumstances, the electric potential of the air at the height of your head is maybe a couple hundred volts different from ground, depending on your height. (It could be millions of volts in an electrical storm, before you actually get a lightning bolt.)

Now, if you put a voltage source with 200V DC to your bare scalp, and your feet are in wet dirt, maybe it won't kill you (but I take no responsibility if it does), but in any case I'm pretty sure you're gonna feel it.

So what gives? Why aren't you getting shocked all the time?

Well, the thing is, air has a high resistance (much higher than your skin). So a few inches from your head, you've got charged air at a potential of 200V relative to ground. But right at your head, as soon as a tiny trickle current starts to flow, it uses up all the charge carriers from the air touching your head, and more cannot easily flow in to replace them. So there's still a circuit with a 200V potential difference that has your body as part of it, but almost none of that potential difference is actually across your body. The tiny current that does flow through your body corresponds to the tiny potential difference between the skin of your head and the skin of your feet; the rest of the potential difference is explained by that tiny current trying to make its way through the unyielding air.

Does that help? --Trovatore (talk) 03:44, 23 April 2016 (UTC)

Hang on, if I hold one probe of a multimeter at head height and one at the ground, it reads less than 0.01 mV . Not 200V, so what are you talking about? Greglocock (talk) 08:01, 23 April 2016 (UTC)

The impedance of your multimeter is far too low to make that measurement; in effect you are shorting out the voltage. If you had more suitable test equipment, you would measure somewhere in the genertal neighborhood of 100 Volts per meter. See our article on Atmospheric electricity and our Wikiversity page on the Natural electric field of the Earth. --Guy Macon (talk) 08:18, 23 April 2016 (UTC)

More than pure statics, you need to look at dielectric breakdown. Tasers work on a principle similar to how lightning and arcing work. See Van de Graaff generator for an example. Voltage can build up to very large amounts and then collapse when a conduction channel is formed in the dielectric. Once formed, the low resistance channel conducts. A taser starts at a high voltage, senses the channel and shuts down both the voltage and current. It cannot sustain the same voltage after dielectric breakdown reduces the impedance. The pointy tips of lightning rods increase the Volts/meter field difference to be maximimum near the tip so dielectric breakdown happens at the lightning rod before surfaces that aren't as pointy. That breakdown starts the chain reaction and ultimate discharge. --DHeyward (talk) 09:05, 23 April 2016 (UTC)

The article on electric shock goes through some of the basics. The power causes electrical burns, using the body as an ordinary heating element. (However, fibrillation is more dangerous, which is a different phenomenon) As others say, voltage and amperage are not independent variables -- if we denote the resistance of the body as R, the voltage logically as V, the current or amperage not so logically as I, then V = IR and the power is V² R or I²/R. But R is also not dependent of voltage - as explained at the article, the skin degrades and the resistance gets less and less with higher voltage. So there is some function R(V) you could write. Because resistance goes down with high voltage, there should still be a one-to-one relationship between voltage and amperage. So given a value for the voltage or the current, and the position of the electrodes on the body implying some resistance, you should be able to work through the math and figure out how much heat is being liberated. The catch is that voltage is a quantity known in advance - you can put a HIGH VOLTAGE sign on a piece of equipment. The amperage is an empirical measurement of exactly what the resistance really is, which makes it more accurate at predicting how badly a person will be hurt .... if you happen to have a multimeter rigged to him at the moment of his injury. However, that is not particularly uncommon, since there are fuses in many pieces of equipment one might unwisely poke into. Since the fuse is very low resistance at one end of the circuit, it measures power and amperage interchangeably based on its internal resistance. Since the article says that 30 milliamps of alternating current or 500 milliamps of direct current to the heart frequently cause death by fibrillation, it should be enlightening to compare the rating of the fuse to the rating of the heart!

Bottom line - it's true that "it's the amperage the kills" in the sense that the voltage is the same whether you are wearing rubber gloves and rubber shoes or have a death-house electrode smeared with conductive jelly strapped to your shaved head. But ordinarily it is not practical to measure the amperage, and if you knew all the circumstances it would simply be a function of voltage. Wnt (talk) 10:31, 23 April 2016 (UTC)

And resistance Shirley.!--178.108.238.49 (talk) 20:28, 24 April 2016 (UTC)

The problem with Wnt's formulation is that it doesn't emphasize that voltage, in any sense that matters for the purposes under discussion, is a difference between two points in the circuit, and it's terribly important which two points you pick.

In the case with the rubber gloves and shoes, the voltage drop between the poles of the voltage source may be the same, but the voltage drop across your heart is much less. Let's consider the DC case for simplicity; electrocution for capital punishment actually generally uses AC, but that's a distraction here.

In the rubber-glove case, there's a big voltage drop across the gloves and the shoes, and a much smaller drop across your actual body. Then there's also a drop across your dry skin before the electricity gets into the more conductive wet interior; some of that drop would be removed by the conductive gel in the electrocution case.

So if you look at the part of the circuit where your heart actually is, it makes no difference whether you consider voltage or current. The two things are directly proportional.

But it's hard to measure that, whereas it's somewhat easier to measure the voltage difference between the poles of the source. That's presumably the reason people say that it's the current that kills. It's easier to know what current is flowing through the circuit, than it is to know how much of the voltage drop is taken up by resistive elements along the way (like the gloves and shoes). --Trovatore (talk) 21:42, 24 April 2016 (UTC)