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Almost as soon as I turn my kettle on it starts to make the familiar kettle noise, yet very shortly after turning off the power the boiling noise stops and the kettle is totally silent. The temperature of the water is (almost) the same as when I turned it off. So why is there only noise when energy is being added to the water?

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Perhaps you're asking a different question (does the sound start immediately upon heating the kettle?), but there's some related info over at this question. –  chase Mar 5 at 5:05
    
It is the normal boiling kettle sound. I was wondering what causes the noise as before I really thought about I assumed it was because the water was hot. But when I turn off the power it is still the same temperature but no bubbles or noise. So my next hypnosis is it is something to do with the addition of the energy which maybe can't all be transferred to heat quickly enough so some of the energy is converted to sound. –  RedPython Mar 5 at 17:35
    
This question is not the same as Why is boiling water loud, then quiet?. That question asks why the water becomes quiet just before it boils, while it is still being heated; this one asks why the water becomes quiet after boiling when it is no longer being heated. These are completely different phenomena. –  MJD Apr 2 at 10:25

2 Answers 2

The noise is either from the AC electricity, which would be a 60Hz buzzing, or from small bubbles forming on the heating element itself. When the electricity stops, both the buzzing and the bubble formation will stop as well.

Bubbles create sound due to quickly expanding from a small nucleus.

Here's a book I found with a section on noise from bubble creation

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Your kettle needs incoming energy from the heating element to turn water in to steam. Steam bubbles forming and collapsing make the familiar sound. Early on many of the steam bubbles don't make it to the top because they cool off when they rise away from the heating element. This is why the familiar rumbling sound starts way before the water boils. The water is actually boiling around the surface of the heating element but is cooler away from it.

The reason is stops right away when you turn off the power is because water needs energy to transition from liquid to steam. Even though boiling water and steam are actually both at 100 degrees Cecilius steam has more kinetic energy. This is why steam burns can be worse than the boiling water. For water to turn to steam it must acquire enough energy to jump the energy gap. This is why the kettle needs a constant input of new energy to create new steam even though the steam and water are at the same temperature. Turn it off and the process stops and it stops quickly because the water not balanced precariously on the edge between liquid and steam. There's an energy gap to overcome.

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