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[Phys-L] Re: Lightning Blows Hole in Windshield, Rubber Tires Saved Driver



On 05/07/05 18:16, Michael Edmiston wrote:

As far as the rubber tires go, I always tell people... "The lightning
has the ability to go through several hundred feet of air. I don't
think one foot of air from the chassis to the ground is going to
deter the lightning. The tires don't enter into the picture."

Agreed.

As far as the windshield goes, who knows. Electric discharges do not
always go the way you think they are going to go. If lightning
"hits" the roof of the car most people assume it will follow the
metal roof pillars down to the chassis and then jump to ground (or
vice-versa). However, as I just mentioned above, the lightning has
the ability to go through hundreds of feet of air. It doesn't need
to follow the metal. It can jump (either way) between the roof and
the hood. It can go along the windshield or through it. The
windshield can shatter from the heat or from the percussion.

That's true as stated, but should not be interpreted
as saying the lightning can do "anything" without
regard to logic or physics.

Electric discharges do not
always go the way you think they are going to go.

That depends on how hard you think. Lightning is
still governed by the laws of physics.

One piece of physics that non-experts commonly
overlook is the tremendously fast rise-time of the
lightning bolt.

Applying this to the present example: the door post
has an inductance on the order of microhenries, and
the bolt has a rise time on the order of microseconds,
so it is easy to imagine the impedance of the post is
significant, i.e. enough to cause a nontrivial fraction
of the current to find other paths.

It can go along the windshield or through it. The
windshield can shatter from the heat or from the percussion.

My bet is that it went along the windshield
(not through it). There was presumably a nice
conductive layer of water on the outside of the
windshield. A strike through the water will
boil the water, creating tremendous percussion.
In the pictures there is a hole in the windshield,
but I don't see any evidence that the main bolt
went _through_ that hole. Indeed, if it had
done so, the driver wouldn't have needed bystanders
to tell her the car had been hit ... it would have
been obvious.

(There's all kinds of delicate electronics in the
air-bag controller, so the fact that the air-bag
went off doesn't say much about what the main bolt
was doing.)

One must also consider the hypothesis that the
windshield wiper acted as a sort of lightning
rod. On a modern "aerodynamically clean" car,
there won't be a lot of corona points, and under
some conditions a corona point can be effective
at emitting ions in the seconds before the main
strike, creating a conductive channel in the
air, attracting lightning to the area. (Hint:
if you're outside and you feel your hair starting
to stand on end, take cover immediately!) I
have my doubts about this hypothesis because
the car was in motion, and the relative wind
would tend to blow the corona-ions away.

Ditto for radio antennas, which sometimes take
the form of "printed circuits" printed onto the
windshield.

I understand that when a person is killed by lighting, but there are
no burn marks, the cause of death is suspected to be a heart attack
caused by the sudden percussion on the chest.

Maybe percussion ... but one should also consider
the possibility of ventricular fibrillation induced
by electrical shock. Those cardioversion machines
you see on the TV drama shows are roughly as
effective for causing fibrillation as curing it
(so you don't want to apply them to a healthy
patient). The amount of current needed to do
this is only milliamps (depending on the geometry)
... very small compared to the amount needed to
cause a perceptible burn.

It seems more likely
that the windshield took the brunt of this strike and it was the
windshield that saved the driver... not the tires.

For sure not the tires. But I'll bet the lion's share
of the protection came from the Faraday cage effect,
which would have still operated even if the windshield
had been removed (leaving a big open hole).
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