Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: chris on 28/02/2008 04:02:20
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When you boil a kettle ("put t'kettle on" for the benefit of visitors from Yorkshire) the note changes just before the kettle starts to boil. Why is this?
Chris
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Is it because the water has heated just to the point of boiling so it has expanded and pushed the air up eliminating air space and changing the water level of the water making it closer to the top and changing the sound and pressure.
Kind of like when you put a blind fold on and try to pour a cup of tea, by listening to the change of water level by the sound of the water coming closer to the surface. The pitch and sound changes as the water approaches the top of the cup or container.
With the hot steaming water, I would think the pressure would build like a pressure cooker, and change the pitch! Just a guess though!
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Ah 'ood answa t' question, bur as ah li' i' Yorksha ah'm offended by t' question. ah'm off naw ta race uz pigeons afowa doin eur shift daahn t' pit.
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Two possible reasons spring to mind. The pressure goes up as the boiling starts - that can change the note. Also, as the air gets expelled from the3 kettle and is displaced by the steam, the density changes. Both these factors can change the resonant frequency inside the whistle.
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People from Yorkshire never pronounce the "t" as in put "t" kettle on.
Its a silent "t" and only gets pronounced by comedians to add t`effect. [;D]
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Ah 'ood answa t' question, bur as ah li' i' Yorksha ah'm offended by t' question. ah'm off naw ta race uz pigeons afowa doin eur shift daahn t' pit.
No whippets?
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Ah 'ood answa t' question, bur as ah li' i' Yorksha ah'm offended by t' question. ah'm off naw ta race uz pigeons afowa doin eur shift daahn t' pit.
No whippets?
It et uz fla' cap sa it 'ed ta be pur daahn.
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T' question does not seh wha' type o' kettle is bein used, ah 'ood think 'a' t' answa aboon by wee Andrew relates ta eur new type electric kettle. if t' question is relatin teur t' owd type o' kettle wi' eur cap/lid o'a t' spart then t' change i' pitch is caused by eur reed int' cap. as t' steam bulids up it forced o'a t' reed 'n dis gives theur t' whistlin kettle soun'.
For those who are not from Yorkshire
The question does not say what type of kettle is being used, I would think that the answer above by Andrew relates to a new type electric kettle. If the question is relating to the old type of kettle with a cap/lid over the spout then the change in pitch is caused by a reed in the cap. As the steam bulids up it forced over the reed and this gives you the whistling kettle sound.
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T' question does not seh wha' type o' kettle is bein used, ah 'ood think 'a' t' answa aboon by wee Andrew relates ta eur new type electric kettle. if t' question is relatin teur t' owd type o' kettle wi' eur cap/lid o'a t' spart then t' change i' pitch is caused by eur reed int' cap. as t' steam bulids up it forced o'a t' reed 'n dis gives theur t' whistlin kettle soun'.
For those who are not from Yorkshire
The question does not say what type of kettle is being used, I would think that the answer above by Andrew relates to a new type electric kettle. If the question is relating to the old type of kettle with a cap/lid over the spout then the change in pitch is caused by a reed in the cap. As the steam bulids up it forced over the reed and this gives you the whistling kettle sound.
Merci Beaucoup Paul.. The Translation is appreciated!
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When you boil a kettle ("put t'kettle on" for the benefit of visitors from Yorkshire) the note changes just before the kettle starts to boil. Why is this?
Chris
If you mean a common electric kettle (not any kind of whistling kettle), and you are talking about the throaty-hissing sound which goes quiet just before it boils...
...then I suspect the answer may be to do with dissolved gas (oxygen) coming out of the water. The oxygen starts coming out long before the water gets to boiling, and is (I think) responsible for the early hissing. I was going to say that just before it boils that all the oxygen has come out so it goes quiet - but I know this can't be right because you have to boil water for quite a few minutes to really get the oxygen out.
It could also be to do with the dynamics of the water locally boiling around the heating element. When the kettle is in the earlier stages of heating then there will be a steep thermal gradient in the final couple of mm of water around the element, which probably means that any bubbles of steam will have to be rather small. However as the water approaches boiling the temperature will be uniformly 98-100C, and the bubbles of steam get a lot bigger, which I guess could cause a different sound.
Any experts on the subject?
Now if I had one of those professional audio recorders (or a walkabout laptop computer) I'd go and make a recording of my kettle and do an audio spectrum-analysis on it and see if that shed any light.
Could be quite interesting.
If anyone else wants to try it, note that steam and microphones are a bad combination, so keep the microphone somewhere dry.
You can download a realtime Fourier spectrum-analysis program from my web page: http://www.techmind.org/audio/
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An interesting topic because recently a "quiet" kettle has been marketed and I have got one and it works.
As I understand it the "singing" of a kettle is caused by tiny bubbles of steam forming close to the hot element (or kettle base for on heated from below) and then collapsing violently as they try to expand into the cold water further away from the hot element or base. As the water heats up the bubbles of steam can expand more and collapse slower so the note gets progressively deeper and quieter.
The quiet kettle has a "shielded" element. This allows the liquid near the element to rapidly approach boiling point and therefore enter the quieter phase of operation but also allows the almost boiling water to escape by convection slowly enough to ensure that cold water does not come into contact with the hot element. A brilliant and effective idea.
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That sounds a fair explanation. You can suppress the 'singing' by rapid stirring in a saucepan; that would support your ideas.
Perhaps a 'silent, propellor assisted' kettle would sell well.
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It might be sommat to do with latent heat at near boiling point.
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It might be sommat to do with latent heat at near boiling point.
Wouldn't latent heat have to be at boiling point, i.e. change of state?
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I don't know when latent heat starts to kick in. It maybe that the energy is being absorbed in the change of state process and less is generating turbulance and sound.
I know that vapour cooling phase which uses latent heat is very similar to a kettle continously boiling. If you use water cooling you need to circulate a hell of a lot more water. With vapour phase you are continously replacing the boiled off water with cold water gravity fed from a tank. No pumps. The water is not very turbulant in the boiler. It does remind me of a kettle just about to boil.
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Maybe not latent heat. The temperature v time curve is weird. I have just tried and then temp climbs quite slowly and seems to be linear with time and then it accelerates like mad after about 82 degs getting to 100 very quickly. Bit like half a bath tub curve. The note doesn't really change a lot with this kettle. Shape and material might come into it.
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[url]http://www.engineeringtoolbox.com/water-thermal-properties-d_162.html[/url
The thermal properties of Water have been amongst the most studied bits of Science ]for centuries. The steam engine and its commercial applications ensured this.
The above link shows you the actual variation of SHC with temperature and it's not a lot for a kettle of water.
I think that the natural convection in the vicinity of the heating element must account for a lot of the differences; you can't be sure of the temperature distribution. If you stir the water in your experiment you will probably find you get the same sort of result from your heating curve experiment as the conventional one.
Whether the kettle is a standard or a whistling kettle I still think the conditions above the surface will affect the sound that the kettle produces. As more and more water evaporates and drives Oxygen and Nitrogen out of the kettle, the density of the 'atmosphere' in the container reduces (H2O molecules are lighter than O2 and N2 molecules). This affects the resonance in the container and, just like in a loudspeaker, the coupling of the water vibrations to 'sound' vibrations in the air outside is affected by all sorts of factors. Certainly, the sound from an open saucepan is very different from the sound from a kettle.
Then there is the effect on the situation inside the container due to condensation around the inside. This will have an effect on the resonance of the metal skin by loading it with extra mass; kettles are thinner than saucepans which could also account for some differences. This condensation will go when the container gets hot enough and alter things.
A real can of worms, actually.