Why don't electric kettles turn off if the lid is left off?

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Nic Wood

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Nic Wood  asked the Naked Scientists:
   Electric Kettle part 2       
Thanks for the recent explanation on why electric kettles are so noisy, I wonder if you could find out why some electric kettle's don't automatically switch off if the lid is left off.
Also why does hot water sound different to cold water when being poured ?
Finally why can other people hear noise in the water pipes when a cold tap is being run? If the inside bore of the copper pipe is smooth is the noise generated by joints or connections, and can underground water main pipes generate noise that can be heard as the water 'sloshes' past?
Great show.
Kind Regards,

What do you think?



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Why don't electric kettles turn off if the lid is left off?
« Reply #1 on: 04/06/2009 11:55:49 »
An automatic kettle turns off when a pressure of steam builds up, forcing steam down through a pipe onto a sensor. With the lid on, there will not be enough pressure to push the steam where it's needed.

The noises from pipes come from joints and taps. The pressure of the mains is usually at least 1bar but, for hot water systems, it may be less than half that. So the cold water is flowing much faster, producing turbulence.. Notice the difference in noise and fill time between a toilet cistern filling from the mains and from a 'cold tank' in the loft.
Noise can travel through the copper pipes and the water over distances of may metres.