Saturday, September 30, 2017

How does a straw really work?

Most people don't think about how straws work or believe that they are somehow pulling the liquid up in some way.    But the truth is that atmospheric pressure is doing the "heavy lifting".   And did you ever wonder how tall could a straw be?

These serious issue were ones I talked about in Atmospheric Sciences 101, which I am teaching now at the UW.

One starts with a straw in a liquid.  The liquid doesn't move by itself, obviously.  There is pressure on the liquid...atmospheric pressure, which is typically around 14.7 pounds per square inch (psi) near sea level.  This pressure is communicated into the liquid.  Since the straw is open to the atmosphere, the same pressure is pushing down into the straw.

So, there is no reason for the liquid should rise in the straw since the pressure of the liquid is the same as the air pressure in the straw.  

But now the fun starts.  A human puts her mouth on the straw and starts to suck air by expanding her diaphragm;  sucking the air causes the pressure in the top part of the tube to drop.   What is the limit for humans in reducing pressure in a straw by sucking?  Checking around on the web, the general finding was that a human can drop the pressure about around 7 psi.

So we have full pressure (14.7 psi) in the liquid, but substantially less than  that in the straw.  Thus, there is a difference in pressure and fluids (and air) are moved by differences in pressure, moving towards lower pressure.  Thus, the liquid thus rises in the straw (see below).

If there was no atmospheric pressure, we would not be able to use straws, although that is kind of academic since without pressure our demonstrater would not be alive.

The insightful among you would note that the fluid in the straw has weight, which contributes to a downward pressure.  It turns out that a vertical column of 30 feet (about 10 meters) would produce a downward force equal to typical sea level pressure (again, about 14.7 psi).  So, even if one could create a total vacuum in the upper portions of the straw, one could never suck a fluid higher than 30 feet--not that anyone is interested in such long straws!

But this issue does effect the depth that suction pumps can draw up water.... again, no more than 30 feet (and actually that is not possible since pumps are not perfect).  This Honda suction pump can only do 23 ft.
You will never think of a straw the same way.


Unknown said...

The first time she uses her diaphragm to generate the suction, she'll inhale water, cough miserably for several minutes, and learn to generate suction by expanding her cheeks, instead... #pedant

David Cuthbert said...

... not that anyone is interested in such long straws!

Ah, but there are crazy folk who are! Derek Muller, the host of the You Tube channel Veritasium, explored this. Though, of course, he does it in SI rather than our strange units. :-)

He follows up with the question: How can a tree be taller than 10.3m? I don't want to spoil the answer here, but it's surprisingly complex (and I have to admit that I don't have a decent grasp on the mechanics).

typingtalker said...

Cliff wrote, "The insightful among you would note that the fluid in the straw has weight ... "

The fluid in the straw has mass. Weight is a measure of the force of mutual attraction (gravity) between the earth and the fluid. This may sound pedantic but it is important that university students know and understand the difference. They should have learned this in high school.

Eric Blair said...

The science is settled - humans could draw more liquid into straws if AGW hadn't occurred. If we don't act now to halt it no one will be able to draw liquids into a straw in the first place.

Eric said...

Next question: How do trees suck water up much higher than 30 feet? It was only recently discovered!

Somebody said...

Trees use osmosis and water's affinity for sugar to lift water much higher. The forces involved with that are surprisingly powerful. Yes powerful enough to lift water to the top of Redwood trees.

Somebody said...

I will never forget my college chemistry professor's saying. "Nothing can't do nothing"... Meaning the vacuum doesn't do the lifting, it is atmospheric pressure that does it.

Mark MacIntyre said...

Cliff: I assume you’re aware that your app has quit working on Apple devices since IOS 11 was launched.