Jeff Aziz asked:
Why are electric kettles so noisy?
Chris Smith - Ah, yes my kettle is no exception and the reason that kettles are noisy, they make that sort of thumping and bashing noise as they boil and then the noise intensifies as they warm up and then it goes silent as they boil. It is because of the way that the heat is being transferred into the water. So you have an electric element inside the kettle, a high current is passing through that element which makes it get hot. The heat from the element is therefore transferred by convection and conduction locally onto the water molecules around the element; they then get excited and get hot. So you have a bubble of hot water around the element which tries to expand and it also floats upwards away from the element because, of course, warm things rise but as it rises of course it loses its heat again to all of the surrounding water. So this bubble of water and water vapour collapses in on itself very quickly and that’s cavitation and you get a shocking, sort of knocking noise. So those thumps that you hear and the sort of ‘shhh’ hissing that you hear, as the water vapour bubbles collapsing on themselves and emitting some sound waves, that’s what the sound is.
Dave - Sounds of the bubbles, as the water is boiling, forming little bubbles of steam and as they rise up in the colder water they shrink and collapse and then smash into one another and make lots of noise. So it gets a lot quieter once bubbles get all the way up to the surface and they stop cavitating.
Helen Scales - It makes more noise I think when I have less water because I try and have as little as I need for a cup of tea, obviously covering the element but not lots more to save on energy and I think that’s noisier.
Chris Smith - It could be there’s a bigger rezonant cavity inside the kettle because the bubble makes a sort of note inside the cattle and if you got lots of free space then the air will help to move around and bounce and sound around inside the kettle like an echo chamber. So that might be why you are hearing more sounds, probably what’s going on.
jeffaziz asked the Naked Scientists:
When you heat water on the stove, the layer at the bottom is the first to boil, meaning it turns into a gas. The water vapor collects into bubbles, which rise toward the surface, passing through cooler water en route. The lower temperature causes the vapor to recondense into liquid and the bubbles collapse, making a noise. This gets gradually louder, as bubble production increases, until the water is so uniformly hot that the bubbles make it to the top without popping. At this point the noise diminishes. But it takes a moment before the vapor pressure builds up sufficiently in the top of the kettle to make it start whistling...