Simple Physics: vacuum vs. air pressure

I was trying to think of a simple, easy, back-yard or classroom experiment that would conclusively demonstrate that “suction” isn’t the correct model, but “air pressure” is.

We naively think we “suck” liquid up through a straw, but really air pressure “pushes” the liquid up through the straw.

But…both models predict the same results.

Then I got really hung up on the Magdeburg Hemispheres. In essence, two hollow hemisperes (cups, so to speak) are put together and the air is pumped out. Now the two hemispheres are pushed together by air pressure. (Or…held together by suction…)

And here’s where I get crazy. What if you take two solid hemispheres, solid iron, and put those together. There’s no air between them, so the air pressure outside them should push them together. But it doesn’t! They fall right apart.

What am I doing wrong?

(I PROMISE this will not turn into a six-hundred-post thread where I refuse to listen to people who really know their physics! I am not a creationist, Einstein denier, quantum revisionist, or Flat Earther.)

(Well, okay, yes, back in 1990, I joined Charles Johnson’s “International Flat Earth Research Society,” but only playing “high weirdness by mail,” and not seriously. Also, it wasn’t any fun. Very boring.)

The solid spheres would work if they were perfectly smooth. I mean perfectly, down to the molecular level. In reality, the two halves are going to be bumpy on very small scales. When you “stick” them together, the rough edges will allow air to seep between them. Imagine if this weren’t the case: you would have trouble lifting anything up off the ground due to the weight of the air on top of it.

The Magdeburg hemispheres work despite this lack of smoothness because of the vacuum you create inside. Of course it’s not a perfect vacuum, some air is still being exchanged between the inside and out, but it’s enough to create a big pressure difference.

The two models do not predict the same results.
Eg your column of mercury in an inverted tube… If vacuum is the cause of any amount of sucking up the mercury, why is the height only due to the outside air pressure ? Wouldn’t it be due to the “outside air pressure + suction due to vacuum”?
But what you are doing wrong ? What looks to you like very very flat steel is actually not… Just put some grease on and then squash that down reallly really firmly… now how stuck is it ?

Hence the saliva (orally produced grease replacement) for use in human applied suction cups… eg windscreen …

Simplest way I think of it is that suction or vacuum cannot ever be thought of as a ‘thing’ because gas molecules cannot ever ‘pull’ other matter along with them via kinetic energy (movement). They can only ‘push’.

Even if you get say two pieces of plate glass they are smooth enough to be very difficult to pull directly apart. You can do it by starting at one edge so that the glass flexes very slightly allowing air to seep in at that edge but it’s not easy.

Neither. They are held together by a pressure differential. If you were to reduce the pressure outside the hemispheres to the same as inside they would fall apart. That’s why the solid hemispheres won’t stick together; there is no pressure differential.

I think actually looking at suction itself might be a more effective demonstration. Consider a straw, a normal straw is easy to suck through, but the longer the straw gets, the more difficult it gets. There’s actually a theoretical maximum at which a straw will work because even with a perfect vacuum, the atmospheric pressure is not high enough to push counteract the increasing weight of the liquid going up the straw.

So, in that regard, if suction really was a thing, then as long as you could suck with enough force to lift the weight of the liquid, then there shouldn’t be a maximum height. But there is, because it’s the atmosphere pushing the liquid up the straw. And this should be fairly easy to demonstrate for a backyard or classroom experiment with some long straws, possibly even hooking up a strong vacuum to one side. You’d have to do calculations to figure out how long the straws and all need to be. You can get by with some relatively inexpensive rubber tubing.

I’ve seen this demonstrated in youtube videos. If I recall correctly, Veritasium (one of my favorite physics channels) did this demonstration and went over the explanation and calculations for it.
And I would recommend this over the plates because, as mentioned in the above posts, even some tiny imperfections can affect the seal. This, straw demonstration still suffers from imperfect vacuum as well, but that would just be that the liquid’s height would come up a little short of the theoretical one. In fact, that could make for an interesting corollary, comparing the theoretical and experimental heights to determine just how close to a true vacuum you were able to achieve.

33 feet or so. Might need to move to something more substantial than a drinking straw. If the OP can get hold of a bunch of mercury, though, all he needs is a 30inch or so tube.

Which is too bad, because that’s probably the simplest and most obvious way to demonstrate the difference.

The ability to stick 2 smooth flat surfaces together naturally DOES happen in the “wringing” of gauge blocks, very precise measurement standards used in industry, but the surface adherence is actually caused be a thin oil coating.

I really like the 30" of mercury as a demonstration of maxmimun “suck”, maybe you could locate an old chem/physics lab barometer.

You need to use washers.

And chicken.

I think I grok… This is the deal I wasn’t getting.

True that! I’ve had that experience with a stack of plates of window glass. You can’t just “pick one up” off the top of the stack. You have to slide it free.

Thank you all! The little refrigerator light is now illuminating the cold caverns of my mind.

(See? I promised you I wouldn’t fight against this for 600 posts!)

Fight, damn you! Fight!

Um… “But suction!” Slurps a chocolate milkshake through a straw. “See, it goes right through.” Now slurps a cherry milkshake through a straw, which gets clogged with cherry bits. “But now it doesn’t! So why doesn’t the air pressure cause the milkshake to rise up in the cup outside the straw, when it can’t rise in the straw? The air pressure should make the shake spill over the lip of the cup.”

Also… “I put a lid on the cup, but the straw still works.”


There is no pressure differential exerted on the milkshake outside the straw, so it won’t go anywhere.

Not a tight enough seal, air leaks in. Pressure differential is maintained between the air outside the straw and the space in your mouth.

Well, there was me and that near-infinite thread about the Stairmaster. What you need is for all the same people who worked on me there to come and work on you here. It took a while because the ignorance was strong in that one and it fought back, but the job finally got done.

So the milkshake is on top of a spinning mirror…

I got nothin’.

The real problem with the “suction” model is that most folks can’t clearly articulate what they think “suction” even is. So it doesn’t matter what behavior they see, their soft and flexible concept of “suction” instantly reshapes itself to conform to the results of the experiment.

If you could use, say, the Socratic method to force them to come up with a firm definition of “suction” then that definition would produce actual testable hypotheses. Which could/would be disproven by well-chosen experiments.

The problem is that by the time someone has the physics *nous *to rigorously define suction, they’ll understand why that model doesn’t work.

Most people are totally stuck in an intuitive experiential universe where air and gravity and friction are universals. Not in the real world we inhabit where the vast majority of the Universe is airless, gravityless, and frictionless. So all their “common sense” notions of physics include all these extra variables and behaviors that are really just manifestations of being at the bottom of an atmosphere and a gravity well.

If the Earth was smaller and/or spun faster, wherein Coriolis effects were obvious everyday experiences, folks’ intuitive physics would have features like “objects in motion tend to turn right but by a varying and unpredictable amount, occasionally zero. But never left.”

Unless they lived in the Southern hemisphere in which case the rule would be “objects in motion tend to turn *left *but by a varying and unpredictable amount, occasionally zero. But never right.”

It’s very very hard to fight ingrained “intuitive physics” with anything but raw mathematical power. But that’ll only work on folks able to visualize the consequences of the equations. Otherwise they just get literally dizzy: that sensation of stuff both moving and being simultaneously stationary. Both conceptions can’t be true, but your eyes see that they are. HALP!!!

There is the demonstration where one places a cheap yardstick or equivalent on a table with its end overhanging about a foot. Whack the end with a hammer, and the stick goes flying. Lay a single sheet of newspaper (a full double-leaf four page sheet) over the stick and smooth it down. Whack the stick again, and it breaks at the table edge. Air pressure holding the paper down is then said to have immobilized the stick.

No, the milkshake is on a treadmill.

which is on a plane full of m*****f****n snakes!! :slight_smile: