We've all seen the add........gas leak & matey just about to turn on teh light!!!!! NO!!

Why does the switch cause a spark???

Something to do with the permittivity of air me thinks....?

Any electrical engineers out there??

Cheers

Matt.

Ooops Switch, not witch!

[Edited by matt.bowey - 6/30/2002 5:00:01 PM]

Because to turn the switch on you're actually moving two metal contacts towards each other, and to turn it off you're moving them apart. There comes a time when the two contacts are close enough together for the resistance to be low enough (resistance is a function of length) for the electricity to jump the gap. Air isn't a particularly good insulator because of all the water vapour in it. For a 133kV power line (as opposed to your 240V mains) it's possible to get the spark to jump several feet.

sure, but are you sure it not something to do with the permittivity of air rather than resistance???

The reason you get a spark is because of the limited dielectric strength of air. Think of this as its ability to not conduct electricity even when the voltage across it is very high.

The resistance of air is normally extremely high. However, when the contacts in the switch are close enough together, the voltage (per unit length) across the gap is so great that the air molecules are literally pulled apart. Electrons are pulled from their atoms and towards the positive terminal. As soon as this happens, the air loses its insulating property and becomes rather a good conductor, so a huge current can flow. This current heats the air, causing it to expand and making the familiar sound of an electric spark.

This hot spark is, of course, what ignites a gas leak, just like it does if you have electric igniter on your gas cooker or boiler. The intense heat can also blow tiny pits or holes in the switch contacts themselves if they're not up to the job. That's why a switch has a maximum voltage and current rating.

Switches designed to switch very large voltages on or off (eg. in power stations) are immersed in oil, which cannot support a spark in the same way.

Andy.