240 volt and a pair of scissors(Teaching tip)



A story that can be varied

You might use a variation on this story:

My next door neighbour is a man called Don. Don can get a bit impatient with things at times. A couple of weeks ago he was vacuum cleaning the carpet in the front room. He switched on the vacuum cleaner and it worked OK for a minute or so but then stopped. Don bent down to see what was the matter and the vacuum cleaner just started up again.

Don resumed with his job, but just as quickly the vacuum cleaner stopped again. After this had happened no fewer than four times, stopping and starting, the cleaner seemed to die altogether. Don was getting mightily irritated by all of this and decided that there must be a bad connection in the plug. He spotted a pair of scissors lying on the chair, picked them up and cut off the plug. Unfortunately for Don, this was at the very moment when the vacuum cleaner started up again. There was a huge bang and a flash.

Now Don isn't small by any means. He probably weighs over 15 stone. Even so he was lifted off his feet as the scissors cut through the live cable. He was lucky to be thrown free because he showed me the blades of the scissors afterwards and they were melted into a crazy shape.

The mains supply provides 240 volt, or 240 joule of energy per coulomb of charge. If you have lots of coulombs of charge arriving in a big current with this amount of energy, the effect can be exceptionally dangerous. Just ask Don!



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