Grades K–2 · Everyday Life
How can a pressure difference raise a drink in a straw?
A drink can climb a straw even though gravity pulls it down. The hidden helper is the air around the cup. Air presses on the drink's surface all the time, even when we cannot feel that push.
If the air pressure above the liquid inside the straw matches the pressure outside, the drink stays at about the same height inside and outside the tube. When someone draws air from the top of a straw into their mouth, the pressure there falls. The outside air is still pressing on the open cup.
That difference in pressure can push liquid from the cup into the straw and upward. The straw gives the liquid a narrow path to follow. It does not pull with a special grabbing force, and it does not make more drink. Liquid that rises inside has come from the cup.
Compare the liquid heights in the two cups. The downward arrows act on the open liquid surface; the upward arrow marks liquid movement in the straw.
The raised liquid has weight, so gravity still matters. A bigger pressure difference can support a taller column, up to a limit. What looks like a drink being pulled up is really a story of unequal pushes and the weight of the liquid between them.
About this explanation
These pictures show a wide enough straw that tiny surface effects can be left out. Gravity limits how high a pressure difference can raise the liquid.
Starting knowledge: Understand a liquid surface and compare a higher and lower level in a drawing.
Sources
- OpenStax, Rice University — 14.2 Measuring Pressure