There's No Such Thing as "Suction"

There's no such thing as "suction." Now that might seem an odd thing to say as we all use the word and the concept almost daily. Indeed if you are alive you are using is right now, by breathing (inhaling).

So what do I mean?

Well, by suction we imply that a gas can draw other gases or fluids towards itself, as though the body of the fluid were attached to a piece of string and being pulled along. The apparent phenomenon of "suction" is not, however like magnetism or gravity, that attract bodies to each other. "Suction" is not attraction, though, by using the term, we imply that it is.

When you inhale air, or by the same process, drink through a straw, you increase the internal volume of your lungs by muscular action and, as Boyle's Law tells us, because the volume of a given mass of air is inversely proportional to the pressure, we have created a low pressure zone in the lungs. By increasing the internal volume of the lungs, we have lowered the pressure inside the lungs.

Now the relatively higher pressure of the air outside the mouth is able to push air into the lung (or push the drink up the straw). The air, and the drink, are both pushed into the mouth by positive action by the relatively higher pressure air outside the lung exhibiting the tendency of fluids to equalise pressure in a volume. The direction of equalisation of pressure must be from the higher to the lower and that is the direction of flow.

A mercury barometer uses this phenomenon exactly, when the column of mercury is said to be supported by the outside air pressure and not held in the tube by the vacuum above the mercury. That would imply that something with less strength was doing more work, which is a natural and engineering impossibility. Fluids can only be moved by positive, pushing, action. Suction does not hold the mercury in the tube; outside air pressure does. And it is outside air pressure that pushes air into the lungs or drink into the mouth.