• Question: What does the bell jar experiment demonstrate?

    Asked by cbond on 27 Apr 2020.
    • Photo: Kim Liu

      Kim Liu answered on 27 Apr 2020: last edited 27 Apr 2020 9:15 am


      There are many variations of bell jar experiments – most demonstrate the effects of applying a vacuum, or removing all the air and other matter from the space under the jar. A common demonstration is to put a ringing alarm clock or bell in the bell jar, and when the vacuum is created, you can no longer hear the sound of the clock/bell.

      This demonstrates that air is necessary for sound to travel – sound cannot travel in a vacuum. Sound waves form by vibrating air particles on the way to your ear. If there are no particles to vibrate, sound cannot travel. This contrasts light, which does not require particles and can travel through a vacuum.

      Do leave a comment if you were thinking of another experiment! (maybe a demonstration of how the lungs work?) Long story short – increases vacuum, decreases pressure, allows air to suck into the balloon.

    • Photo: Patricia Brown

      Patricia Brown answered on 27 Apr 2020:


      One bell jar experiment that I know of is to place a bell jar over a burning candle. After some time, the flame will eventually die out. This demonstrates that the combustion reaction needs oxygen! There is a limited amount of oxygen inside the bell jar, so when the combustion reaction has used it all up, it can’t continue anymore and the flame dies out.

    • Photo: Nina Rzechorzek

      Nina Rzechorzek answered on 30 Apr 2020:


      My favourite bell jar experiment is the one that helps explain how air is drawn into the lungs when we breathe in. This happens because expansion of the rib cage and contraction of the diaphragm (the muscle that separates the thorax/chest from the abdomen) work together to increase the volume of the thorax, thus creating a negative pressure in the thorax. This produces a pressure gradient that draws air into the lungs and so they inflate. For people who are unable to breathe on their own, ventilators fill the lungs with air. Ventilators also work using a pressure gradient, but this time the air is ‘pushed’ into the lungs via positive pressure created by the ventilator, rather than being ‘drawn in’ by a negative pressure in the thorax. Nice summary with diagrams here:
      https://www.bbc.co.uk/bitesize/guides/zsry39q/revision/4

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