Keri Teavae, of Dunedin, asks :-

Why do I get an electrical shock when I touch the car door to get out?

John Campbell, a physicist at the University of Canterbury, responded.

Because of frictional charging.

When two non-conductors of electricity are rubbed together, some electrical charge transfers from one object to the other causing both to become electrically charged, one negative and one positive. You can show this by rubbing a nylon comb on wool then placing it a few millimetres from some very small pieces of paper (confetti). It will attract them. Or hold the comb near your hair whilst you watch in a mirror and you will see strands of hair stand up towards it.

Synthetic materials are very good non-conductors of electricity. So if you wear clothes of such materials you charge them up everytime you brush against another non-conductor. So sitting (and sliding) on a car seat made from synthetic materials is a sure way to charge you up. As you touch the car door there is a discharge between you and the door which you feel as a shock. When wearing a nylon shirt or slip undress in the dark. You will often see and hear a spark. As you walk across a carpetted floor in a library then reach out to a metal shelf you will also get a little shock.

These effects are most pronounced when the air is very dry and hence doesn't conduct electricity very well itself. Cold, frosty mornings, or here in Canterbury warm Nor-West winds, give the best conditions for a shock. A bolt of lightening is the result of large scale charge separation in the drops of water in clouds.

The biggest shock I have ever received is when holding a metre long sheet of perspex in one hand while I ripped off the protective paper with the other. Frictional charging can lead to disasters, such as when pouring dry grain into silos. When the difference in charge relieves itself through a spark, the air/grain dust mixture can explode, destroying the whole silo. Next time you see a petrol tanker refill a storage tank at a garage notice that the first thing the driver does is to earth the tanker before having the petrol flow. Ditto when an aircraft is being refilled.