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I was reading around about the Radio Frequency and its effect on the GAS/Petrol forecourts, suddenly i came across something posted by someone,

"I personally went on a trip with some friends, one who received a nasty shock every time he tried to enter the car. Since he was the only one receiving these shocks I theorize that the source of his shocks were the shoes he was wearing. I told him to hold his key out toward the door and a spark jumped between the key and the body of the car. This prevented him from receiving the painful shock. But I could see how this could also ignite a gas fire."

I couldn't understand how the static electricity could cause the shock from the car ? and how could be the source of this is Shoes ? and how holding the key could prevent someone from this shock ?

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Related: – Qmechanic Jul 11 '12 at 18:14

3 Answers 3

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I couldn't understand how the static electricity could cause the shock from the car ?

Although the car looks like it should be insulated by the rubber tires they are actually quite good conductors so touching the car is a lot like touching any other grounded metal object.

and how could be the source of this is Shoes ?

Shoes, especially with synthetic soles that are being rubbed along the ground can build up a static charge.

and how holding the key could prevent someone from this shock ?

The key is metal and is held firmly in your hand, so the current flows smoothly from your hand to the key. The spark then jumps from the end of the key to the metal object. It's the very high field where the spark forms that hurts if it comes directly from your finger.

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"rubber tires [..] are actually quite good conductors" - I am not disputing this statement, but is there something that proves it? Because I have seen different opinions on that matter. – Fermi paradox Sep 14 at 11:56
Dry pure rubber in a materials data book is an insulator. Car tires are a mix of rubber, carbon (to make them black) steel wires etc. and the surface of a real tire has a film of water + salts + dirt. The surface area of the tire and the contact area with the ground is relatively large. So real car tires in the real world (especially in winter driving conditions) are quite a good ground path. – Martin Beckett Sep 14 at 15:55

Shoes which isolate something will allow the build-up of electricity.

Rubber tires are usually not conductive although special ones are being manufactured to make them conductive for sensitive situations such as delivering gas. In the USA they are marked 'UL'. Since it was a car it's a near certainty that they were not conductive.

Therefore the material above the shoes and the material above the tires will build up a high potential difference in a relatively short time and at a different rate if the mass is different.

If there is a potential difference between two items and a path to bridge them a current will flow to balance it with a spark if the p.d. is high enough.

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I work in electronics and on petrol forecourts. Tyres are insulators as they are made of rubber. The static charge usually comes from clothing as you slide out of the car, he may have held the charge until he returned to the car. The material you are wearing and the seat material are usually what create the charge, material with nylon in it probably being the worst. The question says “as he gets into the car”. He may have clothing or shoes and socks that causes a charge when he walks and then it discharges to the car when he touches it. Much like rubbing a plastic pen on your sleeve and picking up tiny bits of paper with the charge. Holding the key stops the muscle spasm the shock creates as you will be gripping the key already tensing the muscles.

You may have seen a small anti-static strap or chain hanging from the back of a car touching the ground. People attach these to stop a build up of static.

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protected by Qmechanic Mar 25 at 7:17

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