Magdeburg Hemispheres

 

Pressure is defined as force per area. If you push on an object, there is a certain pressure 
associated with that force. Similarly, when atoms collide with a surface, they exert a 
pressure on that surface. In a balloon, for example, the atoms on the inside of the balloon 
exert an outward pressure on the rubber that keeps the balloon inflated.

Our Magdeburg hemispheres demonstrate the idea of pressure. If you connect the sphere 
halves together but the vacuum pump is not turned on, then you will find that you have 
no difficulty pulling the spheres apart. This is because the pressure inside and outside the 
sphere is the same. In other words, the number of gas atoms colliding with the inner 
surface of the sphere is approximately the same as the number of gas atoms colliding 
with the outer surface of the sphere.

If you now connect the two halves and turn on the vacuum pump, you will discover that it 
quickly becomes quite difficult to separate the pieces. As the vacuum pump removes 
atoms from the inside of the sphere, the pressure inside lowers. If the force per area 
outside the sphere is greater than the force per area inside the sphere, then there will be a 
net inward force. This force brings the sphere halves together and makes them difficult to 
separate.