I encountered a puzzle for which I'm sure of the answer. It's simple:

Which cow will survive? Why? Assume that single strike of lightning is 10^20 electrons.



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    \$\begingroup\$ The one on the left already looks like it's not gonna make it. \$\endgroup\$ – Dmitry Grigoryev Apr 7 '16 at 14:23
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    \$\begingroup\$ Trick question -- there is only 1 cow. The one on the left is a bull. \$\endgroup\$ – Reinstate Monica Apr 7 '16 at 14:25
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    \$\begingroup\$ This isn't an "Electrical Engineering Puzzle" at all. \$\endgroup\$ – DerStrom8 Apr 7 '16 at 14:36
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    \$\begingroup\$ The taller cow (one standing up) will get struck directly and die. The lightning will not split the distance between the cows. \$\endgroup\$ – MikeP Apr 7 '16 at 14:45
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    \$\begingroup\$ Both will survive with that little current and the huge distance. \$\endgroup\$ – PlasmaHH Apr 7 '16 at 14:59

This site called "facts and figures about lightning" states that the current in a lightning strike typically ranges from 5,000 to 50,000 amperes depending on the strength of storm.

10 raised to the power 20 electrons is a charge of 160 coulombs i.e.

\$10^{20} \times 1.6 \times 10^{-19}\$ = 160 coulombs.

The site mentioned above states that a lightning strike can last 10 to 50 microseconds so for a big strike if we assume 50 us the current from 160 coulombs is 3.2 million amps. If we assumed 10 us then the amperage is 5 times higher.

NASA has recorded strikes of 100,000 amperes and there are other reports of strikes over 200,000 amperes.

Based on the extraordinarily high level of amperage in the question I think it's fair to assume that both cows will be toasted.

  • \$\begingroup\$ Doesn't this assume that the lightning strikes one or both of the cows? The illustration shows that the bolt already struck the ground in between. \$\endgroup\$ – pipe Apr 7 '16 at 17:46
  • \$\begingroup\$ @pipe it assumes that a lightning bolt of approximately 16 to 80 times the current of any previously recorded is going to do some serious damage up to at least 20 m away. \$\endgroup\$ – Andy aka Apr 7 '16 at 20:57

The left cow will survive because the lighting doesn't cross her: she has only one hoof on the ground so the current can't flow through her.

The right cow will die because she have 2 hoofs on the ground and the current will flow from the first (from left) to the right. That will happen if the impedance of the cow is lower than the impedance of the ground between the 2 hoofs.

  • 1
    \$\begingroup\$ You're assuming that the current from the strike will flow in the direction of the cows. But if that was the case, the lightning would strike one the cows, not the ground. \$\endgroup\$ – Dmitry Grigoryev Apr 7 '16 at 14:36
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    \$\begingroup\$ This is the answer expected by whoever wrote the question, although the reasoning behind it could be explained and bit more clearly. \$\endgroup\$ – The Photon Apr 7 '16 at 14:50
  • \$\begingroup\$ ... Except that cow hooves are pretty good insulators. Even if the right cow's nose is touching the ground, it isn't in any significant danger. \$\endgroup\$ – Dave Tweed Apr 7 '16 at 14:55
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    \$\begingroup\$ @DmitryGrigoryev - Sorry, but no. Think of the lightning strike as a massive current pulse. The electrons will spread out in all directions, creating a nominally symmetric transient electric field. The 1 meter connections of the cow on the right will (relatively speaking) short the current flow through the cow. \$\endgroup\$ – WhatRoughBeast Apr 7 '16 at 15:30
  • \$\begingroup\$ @WhatRoughBeast I've been near a cable with 3000A current pulses and survived somehow. Where did you get the idea that massive current creates symmetric electric field? You seem to assume that the ground below the cows has uniform resistance, but in that case the strike would be on one of the cows, traveling the shortest path possible. \$\endgroup\$ – Dmitry Grigoryev Apr 8 '16 at 6:01

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