Atmospheric Pressure

We are living at the bottom of a 'sea' of air called the atmosphere, which exerts a pressure on us (just as the sea squeezes a diver).

Experiment 1:
Remove the air from the inside of a can by connecting it to a vacuum pump. (Alternatively, the air can be removed by boiling some water in the can so that the steam drives out the air before a bung is used to seal the can.)

What would happen? Before the pump is switched on, molecules are hitting the outside and the inside of the can with equal pressure. After the pump is switched on, there are almost no molecules inside the can and the pressure of the molecules outside the can crushes it!

Experiment 2:
Put together two Magdeburg hemisheres and use a pump to take all the air out and so form a vacuum.

Questions from experiment 2:
Hold the handles and pull. Can you pull them apart? Why not? What happens if you let the air back in?

Answer to question from experiment 2: The pressure exerted on us by the atmosphere is about 100000 newtons per square metre (100kN/m²)! On a dining table of area 1 square metre, the air pushes down with a force of 100000 newtons ( the weight of more than 100 people). Fortunately, the air under the table exerts an equal force upwards.