Electricity Question from Bear Grylls "Worst Case Scenario"

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feedme
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Electricity Question from Bear Grylls "Worst Case Scenario"

Postby feedme » Sat May 08, 2010 3:45 pm UTC

Ok, I know it's Bear Grylls, so that's a clue right there that things may not be entirely accurate.

But one of the scenarios is a car hit a powerline and is now electrified. That I'm fine with. Then he says that the ground around the car is electrified and is dangerous up to 30 feet away.

I'm just generally confused. First of all I thought the tires would insulate it from ground so the electricity doesn't have a path. Two, I thought Earth was an "infinite ground" so even if it did have a path to ground, that wouldn't charge the ground around the accident.

Also, he later jumps off the trunk a few feet, and then takes baby steps until he's clear of the dangerous 30 foot radius.


Now....is any of that true?

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Re: Electricity Question from Bear Grylls "Worst Case Scenar

Postby Hawknc » Sat May 08, 2010 4:00 pm UTC

Electricity will take the path of least resistance to ground. If the voltage is high enough, it could arc from the chassis to the ground, but obviously you'd see that. A more likely explanation is that there is some metal from the car touching the ground on account of it having crashed into a power pole. So there is definitely the possibility of electricity leaking to ground. The 30 feet thing I am skeptical about (I agree with your interpretation), but maybe someone more knowledgeable can answer that.
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Re: Electricity Question from Bear Grylls "Worst Case Scenar

Postby Charlie! » Sat May 08, 2010 4:07 pm UTC

And, of course, why would electricity decide to go up from the ground into his body (enough to cause damage, anyhow)? Capacitance of the human body isn't THAT big.
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Re: Electricity Question from Bear Grylls "Worst Case Scenar

Postby Sockmonkey » Sat May 08, 2010 4:30 pm UTC

I guess it usually wouldn't go into you but you're supposed to be careful just in case of the rare event it does go for you.

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Re: Electricity Question from Bear Grylls "Worst Case Scenar

Postby 2.71828183 » Sat May 08, 2010 5:05 pm UTC

Ground (as in dirt or whatever) is resistive, so the electricity is earthed through a resistor. Two different points on that resistor will have different potentials, and if your feet are the two points, your body is a resistor in parallel with the original resistor. Hence current can flow.

Now, the human body (especially with rubber shoes or whatever) is fairly resistive, and the electricity can propagate through the earth in all directions, so it would take a very high voltage to pose an actual danger. I know lightning strikes can harm people in this way, but I'm not so sure about power lines. Maybe the big high-voltage transmission lines, I really don't know. The remedy is supposed to be to stand on one foot (and for lightning, obviously hunker down so you aren't a less resistive path to ground than the air is), so there's no current path. (There would still be a current path between the front and back of your foot, but that would require a far higher voltage still to create a dangerous current where your organs and such are.)

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Re: Electricity Question from Bear Grylls "Worst Case Scenar

Postby GeorgeH » Sat May 08, 2010 6:44 pm UTC

feedme wrote:Two, I thought Earth was an "infinite ground" so even if it did have a path to ground, that wouldn't charge the ground around the accident.

Also, he later jumps off the trunk a few feet, and then takes baby steps until he's clear of the dangerous 30 foot radius.


These are related; imagine current going down a small wire. If you touch the side of the wire, you'll get shocked (obviously assuming you've positioned yourself such that's it's possible for you to be shocked by the circuit in question.) Now gradually increase the size of the wire where you touch it until it has a 30 foot radius - you'll still get shocked.

What will be different is that you won't get shocked as badly (assuming a real wire, not a theoretical 0 resistance wire.) As you get farther away from the center of the wire, the current going down a section of that wire will decrease; you can get a basic sense of "why" by applying V=IR. The voltage drop down the wire is constant, but the path 30 feet out to the edge is longer, so it's more resistive and there's less current going down any given section.

By taking small steps, he was trying to minimize the difference in current traveling through the sections of ground that he was connecting with his feet. V=IR again can explain why this is important; if the resistance of a foot-shaped piece of ground is constant, different amounts of current means different voltages. Connect two different voltages with your feet, and you can get shocked.

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Re: Electricity Question from Bear Grylls "Worst Case Scenar

Postby Cobramaster » Sat May 08, 2010 7:37 pm UTC

He simply mis-informed people in a way that would make them understand the level of danger. What really is going on is that the electricity going into the car can jump from the car into you up to 30 feet away due to the very large charge imbalance present in the area now.
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Re: Electricity Question from Bear Grylls "Worst Case Scenar

Postby wst » Sat May 08, 2010 9:40 pm UTC

2.71828183 wrote:Ground (as in dirt or whatever) is resistive, so the electricity is earthed through a resistor. Two different points on that resistor will have different potentials, and if your feet are the two points...
So... what if you hopped?
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Re: Electricity Question from Bear Grylls "Worst Case Scenar

Postby pizzazz » Sun May 09, 2010 2:35 am UTC

Electricity is dangerous stuff. The 30 feet warning is probably excessive, like many safety tips, because the point is to reduce your chances of being hurt as much as possible, rather than to say, 25% chance of being shocked because that's still very high. As someone else pointed out, it is definitely a good idea to take baby steps away from the car.

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Re: Electricity Question from Bear Grylls "Worst Case Scenar

Postby drLagrangian » Sun May 09, 2010 4:02 pm UTC

Cobramaster wrote:He simply mis-informed people in a way that would make them understand the level of danger. What really is going on is that the electricity going into the car can jump from the car into you up to 30 feet away due to the very large charge imbalance present in the area now.


no he didn't. I took training as a firefighter some years ago, and we had a section on dealing with downed power lines (hint, don't try to put out any fires with a water hose).

And his safety measures are accurate. While it may be possible for electricity to arc through the air that way, the main reason is because it doesn't dissipate through the earth easily.

The standard power line is so strong that the voltage needs space to dissipate through the earth (the standard safety level is 30 feet away, where the energy has dissipated enough).

electricity always takes the path of least resistance. Well asphalt or earth is very resistive, so if it encounters anything less resistive (like a mostly water living thing) while its trying to dissipate through the ground, It will jump through that living thing from one leg to the other, just to skip that 8 inches of resistive earth ... and then continue on its way from there as normal.

Basically, the system is more complicated than a grounded wire. The current flows from the wire, and forms a complex circuit with resisters and links between everything nearby (trees, cars, you, the air). Thats why its so dangerous to get near one. people think that if you don't touch the arc of electrons you won't get hurt, but even if you are feet away, you are still part of the circuit, and the only thing keeping you safe is that you are too far away for it to overcome the air's resistance and jump to you.


In other news: seeing a downed power line is awesome (from far away). the arcing is brighter than magnesium white from a half block away, and will light up the neighborhood like a spotlight. It also jumps around like crazy, so don't get near it.

In more news: firefighters hate downed power lines. We have to sit there for like 5 hours while PSEG comes to fix it. We can't go near it to put it out, but we can't leave cause its dangerous. Its boring.

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Re: Electricity Question from Bear Grylls "Worst Case Scenar

Postby wst » Mon May 10, 2010 11:34 am UTC

drLagrangian wrote:In other news: seeing a downed power line is awesome (from far away). the arcing is brighter than magnesium white from a half block away, and will light up the neighborhood like a spotlight. It also jumps around like crazy, so don't get near it.

In more news: firefighters hate downed power lines. We have to sit there for like 5 hours while PSEG comes to fix it. We can't go near it to put it out, but we can't leave cause its dangerous. Its boring.
Link related.

Always awesome to watch that video.
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