In my backyard I have about 8 pieces of 10 foot long rebar stuck in the ground. The rebar goes down about 4 feet and is used as my tomato plant supports. I’ve always wondered what would happen if lightning struck the rebar. How would the electricity be directed during the strike? I met a guy who installed lightning rods(not exactly an authority) who said it would “protect the plants”(not my concern) and lighting wouldn’t hit it unless it was going to hit the area anyway. So I’m just wondering, a lighting strike in a 10 foot piece of rebar driven 4+ feet into the ground, what’s going to happen?
A typical lightning bolt has 1E9 J of energy, which is more than enough to vaporize a piece of rebar. The problem is figuring out how much of that energy gets dumped into the rebar, vs. heating the air, making noise, heating the ground, inducing currents in nearby objects, etc.
Vaporize? Cite, please. Based upon the lightning protection systems which I’ve seen in place which would hopefully conform to NFPA 780, the attractive electrodes and connective/grounding conductors were equal to or smaller than 5/8" rebar. How is it that they are still in place, absent physical damage?
Because, as stated, not all the energy was dumped into the conductors. Most of the energy is lost elsewhere. But, if all the energy of a typical lightning bolt DID dissipate entirely with a piece of, say, 5/8" rebar it WOULD vaporize it. A billion Joules is a lot of energy.
1E9 to 10E9 J
Standard ASTM Reinforcing Bar sizes
10 feet of #4 rebar is approx. 3 kg
Boiling point of Iron is 2750°C
Calculation of specific heat capacity with iron
3704250 J to heat 3 kg of iron by 2750°C
So if I have a 10 foot piece of rebar stuck 4 feet into the ground, what’s usually going to happen if it takes a hit?
Well, for one thing I think the point of the rebar will increase the electric field intensity in its vicinity and discharge the cloud, or whatever at a lower energy level than the typical lightning strike that takes place in the absence of a pointy object.
So your rebar will most likely not be damaged any more than any other lightning rod.
The energy converted to heat in the rebar will only be that produced as a function of resistance. Since rebar is a pretty good conductor, it’ll certainly get hot, but not enough to vaporize the rebar. Most of the energy, in the form of electricity, will be conducted to ground, leaving an intact hunk of rebar. Otherwise, every grounding bar/lightning rod would need to be fully replaced after every strike, and would not be able to protect the structure on which it is placed to protect in the first place…
“Martha, the steeple lightning rod has vaporized, I’m heading up to replace it before the next strike…” :rolleyes:
Now, I’m not going to even attempt to refute the evidence above, that if ALL the energy was discharged, and converted to heat, ONLY in the rebar, without a discharge to ground, that it would vaporize. I’m sure it would. But in real life, it’d heat up some, and shunt nearly all that energy to ground.
I’d also not put good odds on a bountiful harvest of tomatoes following a strike.
-Butler
If all the energy that went into a structure were to be turned into heat, they’d always be repl;acing the TV antennae and the repeaters atop the Empire State Building and other takk city buildings. In most cases the current is routed harmlessly to ground (although I’ve seen some spectacular shots of shingles blown off the sides of houses by lightning bolts that passed through nearby lightning rod-to-ground cables)
I’m not sure that there’s much point to a four foot high lightning rod meant to protect your garden. Unless your garden is out in the middle of a big flat area, it doesn’t seem very likely it’d be hit.
And even if lightning strijkes a rebar rod near your tomatoes, I doubt if it’d do much harm.
Dover’s got a couple of good books out on lightning. They’re worth a look.
There is no point to having lighting rods protecting my garden and due to this I don’t have any. I use the rebar to support my plants (as stated in the OP). As stated, I don’t care about protecting my plants, a person who installed lighting rods said that is what would happen. Also, the rod is 6 feet out of the ground since it is 10 feet total with 4 feet in the ground.
Thanks for the answers but…
Everyone is misreading the question. I asked, "How would the electricity be directed during the strike?” not “what’s going to happen to the rebar?”. I can buy rebar for $5 a section so replacing it doesn’t matter. Where’s all that juice going if it hits the bar? If I’m 20 feet from it when it gets hit am I going to also be fried or will the 4-foot part in the ground be enough to protect anyone in the area? Assume it’s been raining heavily before the strike. I’m pretty sure they have taken hits already and they are still standing. New Orleans gets nasty electrical storms on hot summer afternoons.
I think the answer to “what would happen” ranges fromnot much to “fresh tomato sauce”.
First of all, there are many, many chain link fences whose metal posts are essentially the same as your embedded rebar. They don’t seem to attract excessive amounts of lightning. So unless your rebar is the tallest object for 50m around, you’re not increasing the probability of a hit.
However, in the event of a direct strike, I doubt the tomato plants at ground zero would be protected. The ground is not a very good thermal conductor and all the energy in the bolt has to go somewhere. So I imagine that a small area around the rebar is going to get very hot, probably killing the roots of the tomato. Maybe even a small steam explosion might occur.
If a person were standing nearby, they’d probably be in the same danger as if lightning had struck the ground nearby. The ground is also not a very good electrical conductor so voltage falls off rapidly with distance from the strike leading to steep voltage differentials. If you happen to be near your tomatos when lightning strikes, bring your feet close together and get on the balls of your feet to lessen the current across your body.
As I understand it the rod, being a point, raises the field intensity in its neighborhood and increases the chances that the first voltage breakdown of the air will be there. However I think people in the vicinity of the rod will be no more protected than people in the vicinity of a tree or other object that sticks up above the surroundings.
The excess electrons on the ground, brought there by the falling rain, return to the cloud through the ionized air and I don’t believe that ionization occurs only at a point.
Fried green tomatoes, no doubt.
IANAGFE (I am not a ground fault expert) but my BIL is. Since he’s written papers on the effects of lightning, I asked him the question and his reply follows.
Wow. A real answer. Thanks!
No problem! 
BIL and I don’t agree politically, but since this is *General Questions * and not *GD * then I’ll quote him as an expert. 
Thought about replacing them with wooden stakes?
Tomato supports? I’ll see your 10’ iron rebar and raise you 12’ copper pipes. As I understand it, unless I were to install something made of silver, nothing’s a better conductor than copper. (And as much as I enjoy my tomatoes, spending money on silver has to involve something a bit higher on the Random Heirarchy of Needs.)
The copper’s been there 4 years. I’m in the Midwest. We get the occasional electrical storm. Nothing vaporized or killed yet. (Mind you, the idea of a lightning bolt frying one of those GD squirrels that are eating my Brandywines is pretty damn appealing.)