MadSci Network: Physics

Re: Why did the lid on the milk jug pop up and down?

Date: Fri May 25 05:29:51 2001
Posted By: Sean Hunt, Secondary School Teacher, Science, ISM
Area of science: Physics
ID: 990369231.Ph

thanks for your interesting question.

The answer is fairly straight forward.
When milk is taken out of the fridge (icebox) it is cold. When it is left 
to stand in a warm kitchen the air inside the milk container will warm up 
and expand. When this expanding gas gets enough pressure it will be able 
to just lift the lid. Some of the air escapes  lowering the pressure in 
the container. The pressure is now too low to lift the lid. After a short 
time the pressure builds up again, and so on until the temperature of the 
container is the same as your kitchen. This process is unlikely to happen 
with full containers of milk. You have to have air present.
You can compare this to a tyre on an automobile. It expands when warm and 
contracts when cold. This happens to a very small extent with solids and 
liquids too. A thermometer is an example of liquid expansion. An oven 
shelf is deliberately made small to allow for expansion in a hot oven.

I hope this makes sense.
All the best

Sean Hunt (Science Teacher)

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