A helium balloon floats...it is lighter then air and a vacuum is lighter then He

[Gyro]

I mean, even large cathode-ray tubes don't float, so it would have to be *really* big.

I remember reading that the overall pressure on a 12" CRT is something like 13 Tons.

[Rick Law]

A very interesting thing to try :

A free floating helium balloon in the middle of the car with all windows closed, and the brake is applied to stop the car quickly.  Where would that balloon go?

(best if someone else is driving and the observer is alert and ready to prevent the balloon from obstructing the driver)

[0culus]

I mean, even large cathode-ray tubes don't float, so it would have to be *really* big.

I remember reading that the overall pressure on a 12" CRT is something like 13 Tons.

Seems about right. CRT implosions are violent and even though it's not technically an explosion at all, it will still get glass absolutely everywhere.

[Stray Electron]

  FWIW  My pop used to work in a large TV shop in the early 1960s and they replaced a lot of TV CRTs. They took them to a pit that they had out back and would throw a large piece of steel at them.  When they broke the CRT they would explode and throw heavy thick pieces of glass 15 feet or more in every direction. I worked in a TV shop about 15 years later and we destroyed CRTs by breaking the small glass tit on the very back of the neck.  We used any kind of metal like a large screw driver and just "chopped" the tit off and air would rush into the CRT for several seconds and then stop and none of the rest of the CRT would break or explode.  Yes, I know it's not actually an explosion since the CRTS are filled with a vacuum and not pressure but it certainly looked like one.

[apis]

I mean, even large cathode-ray tubes don't float, so it would have to be *really* big.

I remember reading that the overall pressure on a 12" CRT is something like 13 Tons.

At sea level with the equivalent of 10 ton per m² it would correspond to a surface area of the CRT of 13/10 = 1.3 m², sounds about right.

[amyk]

Exercise for the reader: What compressive strength and other properties would a material have in order for it to be able to create a buoyant vacuum balloon?

Yes, I know it's not actually an explosion since the CRTS are filled with a vacuum and not pressure but it certainly looked like one.

There's a term for that, the "opposite of an explosion" is an implosion.

[T3sl4co1l]

A better question why do all three of the videos have the exact same number of views and likes and dislikes?

They don't.  YT glitch for you?

Tim

[T3sl4co1l]

A very interesting thing to try :

A free floating helium balloon in the middle of the car with all windows closed, and the brake is applied to stop the car quickly.  Where would that balloon go?

(best if someone else is driving and the observer is alert and ready to prevent the balloon from obstructing the driver)

I already know the answer to this as my car's cabin air is highly stratified in wintertime.  That is to say, the hot air rises to the roof and the cold air stays at my feet.

When taking off, the air suddenly gets pleasantly warm in the front seats.  Why is that?

Tim

[raptor1956]

A very interesting thing to try :

A free floating helium balloon in the middle of the car with all windows closed, and the brake is applied to stop the car quickly.  Where would that balloon go?

(best if someone else is driving and the observer is alert and ready to prevent the balloon from obstructing the driver)

The air density will be higher towards the front of the car because the acceleration will do that.  The He balloon will then move backwards in the opposite direction.  Gravity is indistinguishable from acceleration and, according to general relativity, gravity and acceleration are the same thing.  So, if an He balloon moves up against the accelerating force of gravity then it will move backwards against the accelerating force of braking!




Brian

[james_s]

Seems about right. CRT implosions are violent and even though it's not technically an explosion at all, it will still get glass absolutely everywhere.

Well it's easy to calculate, ~14.7 pounds on every square inch, so add up the number of square inches of the entire surfae area of the CRT, it's substantial.

It starts out as an implosion, but the bits of glass don't stop when they get to the middle, they pass by one another and for all practical purposes it becomes an explosion. About 20 years ago I was disposing of a badly worn out 19" CRT from an arcade monitor and I forgot to crack the pip off the neck before I threw it away. I heaved it toward the dumpster but saw it was coming down a little short and spun around just as I saw the bell of the tube contacting the hinge on the lid. There was a deafening BOOM as glass sprayed around about a 15' radius from where I was standing. I stood there in shock for a moment as I checked to see if I was injured and somehow I escaped unscathed. My hands were shaking for a good 20 minutes after that though and my left ear was ringing for quite some time.

I remember hearing a similar story from an old TV tech who *twice* tried flipping the mounting ears around by removing the implosion protection band holding them from around the circumference of a tube and the tube imploded in the process. He also walked away unscathed both times, not knowing how. CRTs are pretty amazing devices, an incredible amount of engineering went into getting them to where they were at their peak. Sadly they are vanishing very quickly and much of that knowledge will be lost.

[Gyro]

I forgot to crack the pip off the neck before I threw it away.

That was a lucky escape.

My favourite (and possibly safest?) method was to run a fine pcb drill through the middle of the metal EHT contact on the bell of the tube. It really would hiss for quite a long time, especially if you left the drill in.

I always though it was a waste of useful reservoir of hard vacuum. If you kept the system volume very small and were quick with a pipe and bung arrangement, you could probably use it for some physics experiments, a low pressure discharge tube for instance. Certainly sticking a damp finger over the contact gave a good impression of what it would feel like to be being exposed the the vacuum of space.

[james_s]

Once even a bit of air hisses in it's not really a very good vacuum anymore, no better than can be achieved with a cheap mechanical vacuum pump. The difference between the vacuum you can pull with something like a repurposed refrigeration compressor and the hard vacuum in a CRT is very little in terms of mechanical forces. It's a huge number of gas molecules but we're talking a few thousandths of an atmosphere of pressure. It's not gonna feel any different to stick your finger into outer space than to stick it into a vacuum chamber connected to a cheap mechanical pump here on earth. If you're trying to make a vacuum tube it makes a big difference though, the residual gas has to be removed.

[Beamin]

Another silly topic opened by Beamin 

If I didn't have silly topics what would I have? Some of these silly topics have answered long running questions that can't just be typed into google. Like how does the dust form in space? Through a few of the posts the answer could be deduced: electrostatic attraction brings the dust together where UV light ionizes it to make chemistry happen without liquid(solutions) and pressure and heat. Most people just said "gravity" but if you think about it that was wrong and gravity is too weak to clump little sand grains together. The dust in the universe is the consistency of cigarette smoke and the dense gas clouds are still less dense then our best vacuums here on earth.

Also the side discussions lead me to think of new things I would have never considered, or bring obscure aspects to light on the topic. Being intelligent is lonely I realized that at a young age so if you have a whole group of intelligent people it is wise to use that rare opportunity to pick their brains. Imagine the conversations that went on at Los Alamos?  Or we could talk about sports, TV shows, or what ever drivel normal people talk about around the water cooler.

[German_EE]

A helium balloon occupies a volume of V cm³. It floats upwards because V cm³ of air weighs more than the balloon and the helium.

If you want a container containing a vacuum to float upwards then it must meet the same condition. It must weigh less than the equivalent volume of air and this is a BIG challenge because of the wall thickness required to stop the container from imploding inwards.

I do not think that this is possible but I am willing to be proved wrong.

[IanB]

Most people just said "gravity" but if you think about it that was wrong and gravity is too weak to clump little sand grains together.

If this is true, how did the first stars form? The first stars formed only from hydrogen, which is the least dense substance known, and which is spread out very thinly in space. And yet, hydrogen was compressed into balls so dense and hot that nuclear fusion started. If not gravity, then what?

[apis]

Certainly sticking a damp finger over the contact gave a good impression of what it would feel like to be being exposed the the vacuum of space.

Interestingly it's not the vacuum that is making the impression, it's the pressure of the atmosphere you feel (or the pressure differential really). If you are suddenly exposed to the vacuum of space nothing super dramatic happens. It is still bad, because you would suffocate and at low pressures water boils at body temperature, etc. But you wouldn't blow up.

[Vtile]

Most people just said "gravity" but if you think about it that was wrong and gravity is too weak to clump little sand grains together.

If this is true, how did the first stars form? The first stars formed only from hydrogen, which is the least dense substance known, and which is spread out very thinly in space. And yet, hydrogen was compressed into balls so dense and hot that nuclear fusion started. If not gravity, then what?

Ether vortexes you silly.

[NiHaoMike]

Being intelligent is lonely I realized that at a young age so if you have a whole group of intelligent people it is wise to use that rare opportunity to pick their brains.

That seems to be a common experience for a lot of young engineers in (elementary to middle) school - hard to find friends. Smart girls have the additional problem of mean guys who can't accept the idea of a girl being smarter than them.

[apis]

Exercise for the reader: What compressive strength and other properties would a material have in order for it to be able to create a buoyant vacuum balloon?

I give up. Some insights can be found here though:
http://eprints.keele.ac.uk/1563/1/Stability.pdf

Because of the thinness of the structures, buckling is the most common failure mode. The classical buckling pressure of a complete elastic thin spherical shell was obtained by R. Zoelly [1], for more details see also [2]
$$q_c = \frac{2 E}{\beta} \frac{h^2}{R^2}$$
Here E is the Young modulus, h and R are the thickness and radius of the shell’s mid-surface, respectively, and \( \beta = \sqrt{3(1-\nu^2)} \) with ν denoting the Poisson’s ratio.

So the buckling pressure decrease with R² (makes sense, since the curvature decrease when R increase and a dome is stronger than a flat sheet), it has to be countered by making the shell wall proportionally thicker, which is bad news.

But rather than a solid homogenous shell I would imagine you would use a some kind of space frame truss with only a thin sheet preventing air from entering, or a sandwich composite material, or both. Not so easy to calculate exactly what it would take.

[Beamin]

Most people just said "gravity" but if you think about it that was wrong and gravity is too weak to clump little sand grains together.

If this is true, how did the first stars form? The first stars formed only from hydrogen, which is the least dense substance known, and which is spread out very thinly in space. And yet, hydrogen was compressed into balls so dense and hot that nuclear fusion started. If not gravity, then what?

Electrostatic attraction apparently. The universe was also much denser. Gravity across an atoms size is pretty weak. The first gas clouds would need to be shocked into forming stars like when supernovas compress gas clouds. The universe's non uniformity might cause these waves to form. Not much info on what the universe was like on a practicality level when it went from orange hot to stars around 380ka old after reionization. Why is it _re_ionization? If its re that's the second time, when was the first? Astronomers and Neil deGrasse Tyson will never tell you this.

If you had a sheet of graphene held apart by nano tubes on the outside into some type of fram work would work, problem is how do you make it?

[james_s]

Interestingly it's not the vacuum that is making the impression, it's the pressure of the atmosphere you feel (or the pressure differential really). If you are suddenly exposed to the vacuum of space nothing super dramatic happens. It is still bad, because you would suffocate and at low pressures water boils at body temperature, etc. But you wouldn't blow up.

Given how painful it is (for me anyway) in my sinuses and ears when the pressure changes even just taking off and landing in an airliner which is pressurized so not even a huge pressure change, I would imagine that suddenly stepping into the vacuum of space would feel pretty darn dramatic even if my body didn't blow up. I would bet all that atmospheric pressure in various bodily cavities would not feel very good at all.

[Rick Law]

A very interesting thing to try :

A free floating helium balloon in the middle of the car with all windows closed, and the brake is applied to stop the car quickly.  Where would that balloon go?

(best if someone else is driving and the observer is alert and ready to prevent the balloon from obstructing the driver)

The air density will be higher towards the front of the car because the acceleration will do that.  The He balloon will then move backwards in the opposite direction.  Gravity is indistinguishable from acceleration and, according to general relativity, gravity and acceleration are the same thing.  So, if an He balloon moves up against the accelerating force of gravity then it will move backwards against the accelerating force of braking!

...

Brian

Yup - it is really fun to watch / play with.  Very counter-intuitive to have everything come flying towards the front while the He balloon rush to the back.

As said before - best done when someone else is driving and ensure nothing will obstruct the driver - else it could be danagerous.  Do this at your own risk.

[apis]

Given how painful it is (for me anyway) in my sinuses and ears when the pressure changes even just taking off and landing in an airliner which is pressurized so not even a huge pressure change, I would imagine that suddenly stepping into the vacuum of space would feel pretty darn dramatic even if my body didn't blow up. I would bet all that atmospheric pressure in various bodily cavities would not feel very good at all.

Yeah, and how big a change in pressure you experience and how quickly you decompress probably has a large effect as well. Astronauts doing EVA have lower pressures than what we have a sea level, some as low as 20.7 kPa (3.0 psi), they use a pure oxygen atmosphere inside the spacesuits to make sure you get enough oxygen. The sea level pressure is about 101.3 kPa (14.7 psi). The ISS apparently use sea level pressure and a roughly standard atmosphere though. But most movies/tv-shows depict it overly dramatic.

[Red Squirrel]

I always kinda wondered about that too, but I guess the issue with a vacuum is the practicality of it.  You need a very thick/strong chamber and the material will end up being too heavy for it to be practical.

[hamster_nz]

I am surprised that nobody has looked at this the other other way... trying to build a 1m^3 vacuum balloon is like building something that holds a cubic meter of air at atmospheric pressure, weighs less than 1.2 kg (the weight of 1m^3 of air), and can be submerged to  10 meters in water without crushing.

At 10 meters the water pressure is about 1 bar.

Likewise silly question. Does a jar with a lid on it weigh more when it is in a vacuum?