3
$\begingroup$

One question which has recently been interesting me is: Why exactly do people get thrown back when they are shocked by electricity.

I have done some research to find the answer to this, and i am yet too find an answer. My personal and initial speculations were that the phenomenon was related to muscle spasms, however i highly doubt that clenched muscles would produce such a result.

Some of the material i have come across, have mentioned that the phenomenon is restricted to only AC power as opposed to DC. Whilst interesting, exactly how electric power is converted to kinetic energy when an individual touches a live wire or socket still remains unknown to me. To summarise:

- Is this entirely fictitious and not actual science, merely a by-product of Hollywood?

- Why do people go flying off when they touch a live wire?

- Is this restricted to AC currents?

- How is the electrical energy converted to kinetic energy which results in the victim flying?

I should also mention that my experience with physics is really limited to watching Brian Cox documentaries and basic knowledge/interest, this being said, please don't feel like you have to simplify anything; I am very interested.

Thanks in advance.

$\endgroup$
  • 2
    $\begingroup$ I would also keep an eye out for the possibility that this phenomenon is either greatly exaggerated, or completely invented, by the movies that display it. $\endgroup$ – Emilio Pisanty Sep 3 '15 at 10:34
  • $\begingroup$ I was taught when working in the lab with electricity to approach any possible live wire with the back of the hand. That is because the body goes into spasm with electricity and if the finger are around the wire the spasm closes the hand and one cannot let go. Most of the effect is spasms from the body, not the energy of electricity , imo, though I have no links $\endgroup$ – anna v Sep 3 '15 at 10:40
  • $\begingroup$ That is fair enough @Emilio_Pisanty, all manner of media loves to dramatise it, however i have heard multiple reports go this actually happening. With this in mind, i will alter the question $\endgroup$ – user4493605 Sep 3 '15 at 10:42
  • 1
    $\begingroup$ There's a huge danger with regard to high voltage electrical lines. The most likely outcome is that on grabbing that line, your muscles will uncontrollably contract. You are latched tightly to the line and you die. This is why electric fences are typically pulsed. $\endgroup$ – David Hammen Sep 3 '15 at 11:31
4
$\begingroup$

Essentially right. Luigi Galvani first noticed that, if a frog's legs were given an electric shock, they would twitch, even if the frog was dead. Same is true in live humans - bigger muscles, bigger twitch. Whether the shock throws you back or makes to grasp the conductor even tighter is probably a matter of luck - certainly any time I've had a shock from 250V mains AC, I've recoiled quite sharply. I don't imagine that DC would be much different. I've also used the 'back of the hand' technique but don't recommend it - a voltmeter is much safer.

Wiki 'Galvani' and 'electrophysiology' for much more info.

$\endgroup$
  • $\begingroup$ ah ok, so the electrical-kinetic energy transformation occurs within the muscles, and hollywood largely dramatises the effect. $\endgroup$ – user4493605 Sep 3 '15 at 11:14
  • $\begingroup$ AC can couple better through the insulating skin through capacitive coupling, but I was told (30 years ago) DC tends to be more dangerous once a circuit through the body is established: muscle spasms do not relax whereas there are zero crossings in AC which can allow you to relax your grip and can thus be life saving. When I studied engineering three decades ago, I was tutored by a masters student whose thesis was "breakdown of the resistance of human tissue during electric shock phenomena". A creepy topic and a very creepy guy who loved sidetracking tutorials to describe macabre details. $\endgroup$ – WetSavannaAnimal Sep 3 '15 at 11:59
1
$\begingroup$

Here's a different way of looking at it. A current of around 30mA at 230V is likely to be fatal if applied for any length of time. However, that's only a power of 0.03 x 230 = 6.9W.

A power of 6.9W isn't going to be hurling anybody across the room, so there must be something else. That something else is the muscles of the person being electrocuted.

$\endgroup$
1
$\begingroup$

An associated phenomena is arc flash, which has generally only been recognized as a separate type of even in the last ~20 years, but is now definitely part of the US electrical code. Some number of earlier 'shocks' were, in fact, arc flashes, just not recognized as such.

On example can be found here: Arc Flash video - note this may be disturbing to the viewer since the event was fatal.

In an arc flash, something triggers the formation of a plasma between electrical contacts. This can result in extremely large currents flowing, and hence very high power release (consider a 13kV panel with 100's of amps, which is a megaWatt output). In addition, melting bus bars will cause an expanding metal vapor cloud. Blunt force trauma is the leading form of injury (from the force of the blast), followed by burns, as we;; as inhalation of metal vapor. The event most certainly can, and will, throw the person backward away from the arc flash.

One should never approach a potentially live wire greater than 50V with your hand or any other part of your body. This is not the way to do zero energy verification!

$\endgroup$

protected by Qmechanic Nov 17 '15 at 22:34

Thank you for your interest in this question. Because it has attracted low-quality or spam answers that had to be removed, posting an answer now requires 10 reputation on this site (the association bonus does not count).

Would you like to answer one of these unanswered questions instead?

Not the answer you're looking for? Browse other questions tagged or ask your own question.