• Question: Why does a kettle get quieter before boiling?

    Asked by jameskeywood123 to Mike, Pip, Tianfu, Tim, Tom on 23 Jun 2012.
    • Photo: Tim Stephens

      Tim Stephens answered on 23 Jun 2012:

      I think that the noise of the kettle is from bubbles forming on the heating element or bursting on the surface as the water turns to steam. As the water boils, the bubbles get bigger, and the frequency of the sound decreases which makes it sound like the noise goes away.

      EDIT:
      Almost right! – there’s a great explanation here: http://ilovebacteria.com/boiling.htm

    • Photo: Tom Lister

      Tom Lister answered on 25 Jun 2012:

      The sound is caused by temperature gradients, just like a jet engine.

      The bottom of the kettle (at the element) is hottest, and the top of the water is coldest. The biggest temperature gradient is after a short time, when the element has got to its maximum temperature but some of the water is still cool. Just before ‘boiling’, all of the water is quite hot, so there isn’t such a difference in temperatures between the water near the element and the water at the top.

      So why do temperature gradients make noises? When something changes temperature, it also wants to change size (hot things get bigger, cold things smaller). When something hot hits something cold, then they both change temperature quickly and so they both expand or contract quickly – that’s the sound!

    • Photo: Philippa Bird

      Philippa Bird answered on 25 Jun 2012:

      To tease you because you so desperately want your cup of tea!

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