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In the Jurassic Park movie, the boy (Tim) is on an electric fence when the power gets turned on. However, birds can sit on power lines. I think this is because they do not touch the ground, only the line. Since Tim is not touching the ground, shouldn't he be safe until he reaches the bottom?

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    power lines, are typically safe to touch as they are encased in a protective covering, an electric fence, is specifically made to shock you. – Himarm Jun 26 '15 at 13:31
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    @Himarm: Power lines are not encased in a protective covering. If you touch one, it will kill you. – Chenmunka Jun 26 '15 at 13:39
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    @Chenmunka false physics.stackexchange.com/questions/28350/… – Himarm Jun 26 '15 at 13:41
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    Birds can sit on ONE power line. If they try to touch two at once (alternate phases) they would be fried. Of course, it would require a very big bird to make that happen but it does happen from time to time when a big eagle or buzzard span their wings in the rain. – slebetman Jun 26 '15 at 17:29
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    @Himarm and Chenmunka — guys guys, we prefer assertions supported by evidence here. If you could both pop out and touch a power line to test your theories, that’d be much appreciated. – Paul D. Waite Mar 23 '17 at 8:48
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Note that he wasn't just holding onto a single line but was in fact standing on one and holding another. It's certainly possible for him to conduct sufficient electricity (between the lines) to have electrocuted himself.

enter image description here

An electric fence designed to stop things jumping into and climbing it would have alternate rails electrified with opposite polarity, or AC current with the opposing phase, resulting in exactly this effect. So, while people often object that it doesn't act like a cattle fence, this is only because cows can't jump or climb, and cow-fences are typically a single rail.

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    Technically, if each rail had opposite polarity/AC phase, he wouldn't have been shocked because there is a rail in between his hands and feet. – Chase Sandmann Jun 26 '15 at 16:45
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    I'm more bothered by the fact that a 10 year old boy was only stunned by a shock designed for a 50 foot T Rex. – Lindsey D Jun 26 '15 at 18:18
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    @LindseyD - The T-Rex is worth billions. Why would you design a fence that would kill it? – Valorum Jun 26 '15 at 19:00
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    @Richard - Quite the contrary, you don't want to kill the T-Rex. But I'd imagine that a shock designed to stun a 50 ft. leather-skinned behemoth would have the effect of killing a puny, fleshy, 10 year old boy. But perhaps that's a question best posed on Electrical Engineering SE. ;) – Lindsey D Jun 26 '15 at 19:27
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    As an electrical engineer, let me chime in that a common way to transmit very high voltage power is three phase power. This uses three or four wires, each transmitting AC voltage, but with their sine waves offset from each other by 120 degrees. This means that there is a HUGE difference in voltage between any of the two lines many times every second (normally 60 times for USA power). If they were using three phase power, and it were a 20,000 volt line, you could experience nearly 40,000 volts across your body. – Dan Jun 27 '15 at 0:37
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You are correct that birds aren't shocked because they aren't grounded.

However, on a typical electric fence, the electric conductor is separated from the fence itself and insulated from it. Not all the fence carries a voltage.

If someone were to touch both the conductor and the metal of the fence then they would be grounded. The current would be able to flow through them to the fence.

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No he wouldn't have gotten shocked because he's not grounded and there's no way there's three phases running across that fence because all the lines are connected by a mesh. The whole fence would be shorted out if that was the case. enter image description here

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    Clearly he was shocked. Presumably that mesh isn't conductive. – Valorum Aug 16 '17 at 8:12
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    He'd also not been able to let go unless his hands vaporized. – Mazura Mar 28 '18 at 2:57
  • but cloning dinosaurs makes total sense ;D – NKCampbell May 3 at 16:11
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Part of the drama of the situation was that the fence was initially off. We can assume this means the fence was at ground state, i.e. zero volts relative to the ground and relative to little Timmy. The tension of the scene comes from Dr. Grant and the kids racing against the restart sequence conducted by the other intrepid heroes, who are about to unwittingly electrocute homo sapiens instead of tyrannosaurus rex.

If the fence lines Tim were touching were opposite polarity, that would have been 50,000 volts from his hands to his feet, with a pass through his torso. Now, the fatality of electrocution is measured in current, not voltage. Apparently as little as 42 volts is enough to push a fatal charge. However, as you point out, it all hinges on whether or not the current would actually transmit to him, while he's on the fence.

If the fence lines were the same polarity, there's still the issue of capacitance. Capacitance is what causes static electricity: a charge builds up on your body while you're ungrounded and discharges when you touch an unsuspecting friend or doorknob. But in this case, it's the fence that's getting charged up and sending a shock into the ungrounded boy. Again, while 50,000 volts seems like a lot, a typical, minor shock that get from touching metal after walking on carpet can be as high as 25,000 volts. Part of what causes the voltage from a static shock to be so high is the collection of a surface charge on the pointy ends of our fingers we tend to touch things with. A charged fence with a large enough current supply pushing electrons into an ungrounded body may behave differently. I'm not qualified enough to tackle that one, so I'd suggest asking again on Physics.SE if you're interested in more.

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Tim would not have been shocked while hanging on the fence. As stated by a previous post, the fence is all at the same potential. Under normal conditions, there is voltage present in the wire, but no current flow. Current will only flow through the fence wiring when a path to ground is presented. A practical example is an electrical receptacle. Voltage is always present at the receptacle, but Current will flow when you plug in a hair dryer or when you stick a fork in the hot side. Tim is not touching the ground; he is touching four different points of the same potential. He should have been fine.

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What you are failing to notice is that all the the larger conductors are connected together via the smaller uninsulated wires. Therefore all of the cables are at the same potential. The boy should not have received a shock due to current flow through his body as all of the cabling is at the potential. However in EHV overhead transmission, you will not see birds perched on the cables due to the strong electric field caused by the high potential (between phases and also to ground). The high strength electric field caused by 50kV would be enough to blow him off the fence, especially when the power was turned on as there would have been a surge of current.

  • If the smaller wires are conductive, the fence wouldn't fill any purpose - this fence is supposedly there to keep dinosaurs in. It does look like regular stainless steel but who can tell. Just because a wire is "naked" and lacks plastic insulation etc, it doesn't mean it is conductive. It could be enameled or similar. – Amarth May 3 at 15:55
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While it's true he isn't grounded and no CURRENT would run through him, it's definitely the reason he's not instantly fried. However, static discharges is something everyone has tried when touching a charged surface, like your car or say escalator-handrails. I doubt this has ever killed anyone, even if (as someone mentioned previously, and this is true) the voltage experienced can be over 25000.

The reason Tim is "electrocuted", though, and rendered unconscious, is the electric POTENTIAL difference (voltage) between him and a suddenly active 50000 volt fence. Tim then becomes a capacitor (a device which stores electric charge) whose capacitance is much greater than that of a bird. The voltage doesn't kill you - the current (if grounded) will.

If you've ever seen live powerline work being performed from a helicopter, you'll notice they use a conductive wand on approach to equalize potential difference between wire and helicopter before "touching" the wires. This neatly visualizes what happens.

For more on that, refer to this question on Physics.SE.

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    Hi, welcome to SF&F. This seems a bit unclear what you are trying to say. That the equalization current is what gave him a shock? You should lead with that and then explain the details. Also, paragraphs really help. Please read How to Answer for more pointers. – DavidW May 3 at 15:08
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However, birds can sit on power lines. I think this is because they do not touch the ground, only the line.

It is enough to touch something with different potential, not necessarily ground/earth. Birds only touch one single conductor so no current passes through them. They have the same potential but far higher resistance than the conductor they are sitting on.

Since Tim is not touching the ground, shouldn't he be safe until he reaches the bottom?

Supposedly the fence would be built to give the dinosaurs a non-lethal shock if they go near it - not to kill them. The specified voltage is completely irrelevant, what matters is how much current the fence can deliver. It would be sensible to have a current limit on it.

In addition such fences will give out short pulses rather than continuous current, to make them less harmful. To get "fried" by the fence isn't going to happen, because why would they want to fry their expensive dinosaurs.

Also regarding "frying", the size of the animal doesn't really matter all that much. Just how conductive their body/skin is. I would imagine that a wet dinosaur standing out in the rain would get a larger trauma than a dry human boy. There is no reason to believe that dinosaurs are thousand times less conductive than humans or cattle, so it's quite silly to use 50000V for dinosaurs but lets say 12V for humans - that has no science behind it whatsoever. Again, only the current matters.

The danger with electricity through a body, human or dinosaur, is mainly if it passes the heart. This might cause the sinoatrial node to go crazy, leading to heart failure. Similarly, electricity through the heart can cause the sinoatrial node to "reboot", which is the purpose of a defibrillator.

When touching two fires like the boy do, you have a high chance of this happening. Wheras a dinosaur brushing against the fence would only have electricity pass through its nose, tail or side of the body.

protected by TheLethalCarrot Jul 7 at 19:53

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