Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: thedoc on 19/07/2016 19:50:02
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Ari asked the Naked Scientists:
During lightning the best place to be is in your car ?
If that is correct why do i get a static shock sometimes when i touch my car ?
its not grounded due to the tyres ( note i disconnected the battery earth wire and it still happens
thanks
What do you think?
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During lightning the best place to be is in your car ? ... its not grounded due to the tyres
If lightning can travel ~100m from the cloud to the car, it will have no problem travelling the 10cm air-gap from the car body to the ground.
https://youtu.be/ve6XGKZxYxA?t=1m40s (https://youtu.be/ve6XGKZxYxA?t=1m40s)
... i get a static shock sometimes when i touch my car ? ... ( note i disconnected the battery earth wire and it still happens
The static shock from the car isn't caused by the battery. The movement of the car causes it to accumulate charge. The solution is an earthing-strap (https://www.amazon.com/s/ref=nb_sb_noss_2?url=search-alias%3Dautomotive&field-keywords=Earth+strap).
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Most of the shocks you experience in a car are due to the generation static electricity built up by your clothing rubbing on the seat it has no relevance to external lightning
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If you are standing on the ground in the open, the lightning travels through your body to the ground, causing electrocution.
In a lightning storm, the car's body acts as a Faraday Cage (assuming it's not a convertible...). The electricity of the lightning tends to go around the metal car body to the ground, rather than through the occupants sitting in the middle of the car.
If you or your car builds up a static charge while driving, sitting in the car is a perfectly safe place to be. It is only when you touch the ground that the charge travels through you to the car.
Also, having the windows closed partially shields you from the deafening bang of a nearby lightning strike.
See: https://en.wikipedia.org/wiki/Faraday_cage
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Most of the shocks you experience in a car are due to the generation static electricity built up by your clothing rubbing on the seat ...
It's the car moving along the road & through the air which causes it to accumulate charge.
Aircraft also ... https://en.wikipedia.org/wiki/Triboelectric_effect#In_aircraft_and_spacecraft
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Ari asked the Naked Scientists:
During lightning the best place to be is in your car ?
If that is correct why do i get a static shock sometimes when i touch my car ?
its not grounded due to the tyres ( note i disconnected the battery earth wire and it still happens
The tyres are rich in graphite and make good conductive ground connection for lightning - which will travel over the surface of the car to ground and won't trouble you inside.
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You are generating the static charge, either by walking towards the car with insulating soles or by moving around inside the car, wearing insulating clothing. When you touch the metal of the vehicle, your static charge flows to earth and you feel the shock of the current flow at your fingertips.
A lightning discharge will flow to earth through the metal and the (conducting) tyres.
Wikipedia has plenty to say about "Faraday cages", including
In 1836, Michael Faraday observed that the excess charge on a charged conductor resided only on its exterior and had no influence on anything enclosed within it. To demonstrate this fact, he built a room coated with metal foil and allowed high-voltage discharges from an electrostatic generator to strike the outside of the room. He used an electroscope to show that there was no electric charge present on the inside of the room's walls.
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Even Ghostbusters (2016) used a Faraday cage...
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its not grounded due to the tyres ( note i disconnected the battery earth wire and it still happens
Actually it is grounded. The potential of the lightning is in the billion volt range. With a voltage that high current can flow through them. Keep in mind that rubber is not a perfect conductor. Almost all materials have at least a small conductivity to them.
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Vehicle tyre rubber is heavily loaded with carbon to make it conductive, and half the mass is steel wire.
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Cars are pretty good, but are not a perfect Faraday shield. The windows have, on rare occasions, allowed lightning to hit someone inside the car, but the lightning is strongly attracted to the metal work, so this is rare.
All this assumes you have a metal car of course, if it's made of composites, all bets are off, also soft-top cars!
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Pure composites may be glass fibre (useless - it's an insulator) or carbon fibre (explodes). Lightning strikes are common on aircraft so they use metal equipotental bonding wires and tapes to protect composite sections (including wood) but strikes on cars are I think sufficiently rare that they probably aren't bonded.