Vacuum question!

Question 1) Would it float? - depends if the amount of glass makes its density heavier than water.

Question 2) Would there be any bubbles? - bubbles of what? if its a perfect vacuum then there would be no bubbles as there is nothing to create bubbles...

The space inside the sphere will be occupied by the surrounding medium, in this case water... If it were in air then it would be air!

The space doesn't go anywhere.

John.

1. Yes it will float - the density is less than water.

2. Yes some bubbles (for the fun of it)

The air dissolved in the water will undisolve when in contact with the vacuum and will rise as bubbles.

Some steam will be formed because of the contact with the vacuum and will also be visible as bubbles but will soon collapse.

The big bubble will disappear with a bang.

Hendrik, how do you know it will float? you don't know the mass of glass do you?

The density could be much more than the density of water and it will sink!

In real life I don't. But it is light enough by (my) definition in fun world.

The glass must be thin enough to break.

wait a minute Hendrik,

If the glass is FLOATING...uh... the bubbles could be observed the water above the glass?

or maybe vacuum non substance is always surrounded bubble magic.

sorry, couldn't resist.

Yeah for question 1 it is the Archimedes Principle. Chances are that it would float but it depends on the ratio of the mass of glass and the volume inside the sphere.

For question two it may help to think of the problem in reverse. Imagine a balloon which is tied off, is completely deflated and it has a device inside which can expand with equal force in all directions. If it expands but there is nothing filling the volume then you get a vacuum. You are just pushing the water away. Now colapse it, there was nothing there to start with so there will be no bubbles or gas coming out.

Relative to where the displaced volume from the sphere would go: If the sphere sinks or was pushed under the wter if it float, it would displace the volume of water in the container holding the water, thereby raising the water level. Once the bubble broke, the water would rush into the vacuum and the water level would retun to normal (except of course for the effect of the volume of glass left in the container).

Must of not been too tired if that kept you awake.

hum glass is made from silica which is the make up of sand it don't float at the beach or there would be no beach. In the center of this glass ball is a complete vacuum. No air to give it buoyancy so it would sink faster than a rock. The shape and all. If there is nothing in the ball a vacuum then there would be no bubbles. The space does not go any where it was there filled with water until the ball move it to it and occupied it. When the glass ball imploded the space that was a vacuum is replaced with water again.

Hope you get a better nights rest now.

Hi OZZB,

You said:-In the center of this glass ball is a complete vacuum. No air to give it buoyancy so it would sink faster than a rock.

Then as air has weight, having a vacuum would actually make it lighter!! Naturally for the same wall thickness/size etc.

( a common classroom experiment of 30 years ago was to take an empty gallon tin can, add a tire pump valve to the cap, weight it carefully, pump it up some way with a tire pump and weigh it again. It would be significantly heavier. The experiment is repeatable,)

Whether it will sink or not is depending upon both the weight and the size, if the amount of displaced water is heavier than the globe, it will float, if less, it will sink. Just like a ship or boat...

If you have two spheres the same size and they both float then the one with the vacuum will float higher in the water because you will have removed the weight of the water!

Glass spheres are very hard to break as long as the pressure is the same on all points, as in the bottom of the deepest ocean trenches. I don't believe it would break at any depth!

What about the effect of strength of the materials utilized in the formation of the sphere. I would suspect that at some point the glass would crush under the forces involved.

If you have two spheres the same size and they both float then the one with the vacuum will float higher in the water because you will have removed the weight of the air! (sorry)!

Glass spheres are very hard to break as long as the pressure is the same on all points, as in the bottom of the deepest ocean trenches. I don't believe it would break at any depth!

Ozzb writes: "In the center of this glass ball is a complete vacuum. No air to give it buoyancy so it would sink faster than a rock."

-----

Ozzb, try this simple experiment: Place a lightbulb in a container of water and see what happens.

Didn't know that the inside of a light bulb was vacuum always thought they where filled with inert gas.

Some are, some aren't. But in no case are conventional light bulbs filled with a gas anywhere close to atmospheric pressure. We're closer to a vacuum in this case than not. And would not the presence of an inert gas increase the light bulb's net mass - making it ride lower in the water - than if the bulb were completely evacuated? The suggested experiment stands.

What?! Are we talking about a laden or unladen light bulb?

The buoyant force is equal to the weight of the water displaced, so whether or not it floats is strictly dependent on the density of the mass of glass. A light bulb surly floats and it is in a state of vacuum, albeit far from complete.

There would definitely be some bubbles on collapse. As previously mentioned, the immediate break of vacuum would cause the release of air and entrained gasses in the water, similar to the reverse effects of cavitation in pumps and such.

Still not satisfied!

Lets imagine a 1m diameter globe with a wall thickness of 10mm, this would displace alot of water and float (or not)(the glass would weigh maybe 30kg) Are we saying that even though there is all that free space inside, it would collapse into nothing?

As I said in my first post...

The vacuum would be filled by the surrounding medium, in this case water...

The free space inside will still exist but as a filled space!

In a small bath the overall water level would go down by the volume of the filled space, so in effect the space is released to atmosphere...

You've got me wondering now.... !!

The free space inside will still exist but as a filled space!

Ahhh, errr, ummm, can you say that another way? I am now confused.

Absolutely!

To check this experimentally, get an ordinary plastic syringe (no needle, not glass - it's too likely to break) or an industrial air cylinder (single-acting, no spring return). Make sure the piston moves freely - lubricate if necessary. Push the piston all the way in and plug the opening (your wet or oiled finger tip will do quite nicely). Now pull out on the handle or on a nut on the threaded end of the air cylinder. You have just created a pretty good, but hardly perfect, vacuum. Let go. If your plug and/or the piston has not leaked, the piston will quite rapidly return to the original closed position with a significant noise (releasing the energy you added when you moved the piston out against atmospheric pressure).

As several others have said in different words, any bubbles that might come up from your imploding sphere would come not from the volume inside the sphere, but rather from the surrounding water. The solubility of gases in water increases with pressure, so a sudden decrease in pressure of the water surrounding the sphere will cause gases to come out of solution, forming tiny bubbles. Since the greatest decrease in pressure would be right next to the imploding sphere, the bubbles would probably appear to have come from the sphere.

And of course a sphere can withstand greater pressure than other shapes made of the same thickness material, but most round glass bottles can withstand a vacuum at atmospheric pressure plus the pressure of a good amount of water, so you can do your experiment. Find a clear glass (only so you can watch inside it - colored would also work) wine bottle (empty). Get one of those rubber stoppers (one brand I believe is Vacu-Vin) with a slit that lets air out, but not back in until you squeeze it in the right direction. Place the stopper on the bottle, but don't take any air out yet. Check it - it will float. Now add sand or pebbles a little at a time to weight the bottle down, until it just barely sinks or just barely floats (stopper in place each test so no water enters). Now use the Vacu-Vin to pump as much air as possible out of the bottle. Test again. If you had it close enough to zero buoyancy before pumping the air out, it will now float, since it has essentially the same volume, but now has less weight.

Now add some more sand so it will sink even with the vacuum, and pump the air out again. Lower it to the bottom of an aquarium so you can see it (no fish please), use a hammer to tap on the end of a small metal rod to break the glass (or better have someone else do that so you can watch from the side). I don't know what fraction of the air can be removed by the Vacu-Vin, but I'll bet that the bubble of air that does come up will be way smaller than the volume of the bottle. I have not actually done this YET. Of course if you could do this with a real vacuum pump, the results would be more dramatic - your local high school or college probably has one.

BTW you can create a pretty good vacuum by vigorously boiling water (heat slowly at first to avoid breaking the glass) in the bottle and sealing just as you remove the heat from it. The bottle is then filled with water vapor, most of which will condense back into liquid when the bottle cools. The remaining water vapor will almost instantly condense into liquid due to the sudden increase of pressure when the bottle is broken, so there should be no bubble except as described in paragraph 2.

Dick

Dear Mr. Truman brain,

Imagine a slightly different scenario where, instead of allowing the glass globe to break, you evacuate the sphere through a hole, submerge the sphere in water, uncover the hole, allowing water to enter.

How much water would enter the sphere? Would it fill the sphere completely? If not, why not?

Then shatter the sphere. Where does the "space" go now?

For making Mr Truman sleep tonight, answer his next question for him in advance.

If the ball with air floats with the top just on the surface.

What will happen if the air is pumped out?

What will happen when the inside pressure is raised?

Have a nice night.

It will lose density so it will rise out of the water slightly, to compensate for the lost air mass...

If the pressure is raised it will sink slightly due to the increased mass of air inside....

For goodness sake this is school physics and nothing complex!!!

John.

last I heard was that air floats!!

Yes! air floats but it still weighs something.... at atmospheric pressure temperature etc... it weighs 1.26 grams per cubic metre....

its the combination of glass and air which determines the overall density of the sphere.

John.

Probably a typo, but air density is about 1.26 kg per m³ (depending on exact pressure and temperature)

Think I will need a heavy dose of sleepers tonight Hendrik!

Air will float but a vacuum will float better. The molecules with weight are removed.

If you take a balloon and fill it with helium it will float (in the air) . If you could stiffen the balloon and pump the helium out without collapsing the balloon it would still float.

If all the helium are expelled (= vacuum) and the balloon is punctured air will be sucked in but no helium will be available to rise in the air.

Putting it in different words – an object will float when the total weight of the object is less than the water it displaces.

If you take a solid birdcage with some budgies in it, the weight is Cage + Budgies. If you open the door the budgies will fly away.

If the budgies are removed the weight will be Cage but the volume will stay the same. If the door is opened no budgies will fly away.

Just to be difficult.

.

If you take the birdcage with budgies and hit it with a stick the birds will fly up and the weight will decrease. The question is now - How can gas in a container but bot in contact with the container add to the weight of it.

How do you keep a man with a beard awake - you ask him if he sleeps with the beard above or below the blanket.

The question is now - How can gas in a container but bot in contact with the container add to the weight of it.

Interesting question. Other thinkers have already considered this, and offered an answer based on kinetic-molecular theory (the basis on the Ideal Gas Laws). The gas molecules add to the weight (gravitational force) of the container by colliding with its inner walls. Wait! -- now someone will object that these the gas molecules collide with the top of the container just as often as they collide with the bottom of the container. True, but due to gravity the upward-moving gas molecules decelerate on their way up, while the downward-moving gas molecules accelerate on their way down. Since the downward-moving gas molecules strike with more force that the upward-moving gas molecules, a net downward force results, equal in magnitude (averaged over time) to the total weight of the contained gas. By the way, violation of this scenario (i.e., somehow canceling out some of the gravitational effect of the gas molecules) would lead to violation of the First Law of Thermodynamics (energy conservation), and thus to the possibility of a perpetuum mobile of the first kind. So if anyone can accomplish such a violation, please let us know!

Wow, that really over-complicates things! Gas molecules are in as much contact with the container as any other substance. Just because they are spread out more in a gas than a liquid or a solid doesn't make any difference. At a molecular level, nothing really touches anything anyway, so no difference in solids, liquids or solids.

One trap you are falling into is that of weight vs. mass. Weight is an indirect measurement of mass, and is imperfect. Think of buoyancy. A He balloon certainly has mass, but if "weighed" without correction in the atmosphere, it would have a negative "weight".

This is also the problem with the budgie example. Even when the birds are flying, there is no change in the mass of the closed cage system, although a scale may register a loss in "weight". Actually, I doubt that this really happens either. In fact, the birds must exert a force downward equal to their weight to fly, so overall, the apparent "weight" of the cage is likely to be the same whether the birds are flying in the cage or resting on a perch!

Tad

Wow, that really over-complicates things!

Strange to hear you say that. The explanation I gave is very simple and a direct statement of widely accepted physics. This is very basic -- it should not seem complicated to anyone with engineering or scientific training.

Gas molecules are in as much contact with the container as any other substance.

The molecules making up solids and liquids are in constant intimate contact, but not gases. Gases consist of mostly *empty space* (i.e., the mean free path between collisions is much larger than the molecules). The only time gas molecules contact anything is when they collide with the container walls, or with each other. But each gas molecules spends most of its time in free ballistic flight (not in contact with anything).

One trap you are falling into is that of weight vs. mass.

By saying that I fell into a "trap", do you mean to say that I wrote something untrue? If so, I challenge you to point out exactly which sentence.

Weight is an indirect measurement of mass, and is imperfect.

Incorrect. Weight is a measure of the force caused by gravity acting on a mass. Weight and mass actually have different units. The common misconception that they are interchangeable terms originates from an inconsistency with the system of English units (this problem does not occur with metric units). I'm afraid that if you are not an engineer or physical scientist, you probably have no idea what I mean. But if in doubt, just ask the other knowledgeable folks here. Or just see the Wikipedia page

http://en.wikipedia.org/wiki/Weight

In fact, the birds must exert a force downward equal to their weight to fly, so overall, the apparent "weight" of the cage is likely to be the same whether the birds are flying in the cage or resting on a perch!

Correct -- if averaged over time. Yes, while the birds' wings flap they exert a downward force via air pressure waves. But what happens if they all simultaneously stop flapping and go into free-fall? For a brief instant the total weight of the system does not include the weight of the birds (except for a slight fraction of their weight relayed via air friction). Once the birds either resume flapping (or collide with the floor), the time-averaged weight will include the contribution made by the birds' weight.

Nicely put.

I fully understand your explanation, it is just overly complicated. Gases have mass, are subject to gravity and therefore have weight. Pretty simple.

How can gas in a container but bot in contact with the container add to the weight of it.

This statement is incorrect. The molecules are in "contact" with the container, as you state in your explanation.

"Contact" at the atomic/molecular level is a complicated thing, which I won't get into.

Gases consist of mostly *empty space*

All matter consists of mostly "empty space". True, molecules or atoms in gases have a greater mobility than in liquids or solids, but it is a matter of degree, not a totally different situation.

For a brief instant the total weight of the system does not include the weight of the birds.

This statement is totally incorrect. If the system is the cage + birds, the weight of the "system" does not change. The mass of the system remains constant. Since weight is defined as the measure of the force of the mass due to the acceleration due to gravity, the weight of the system does not change, by definition.

This is why I called the weight vs. mass issue a trap. If you have the cage on a scale, it's apparent "weight" changes when the birds fly, you are simply making a bad instantaneous measurement.

Hey, we are really just talking semantics. Your explanation of the weight of gases is fine. It is just that atoms/molecules are always in constant motion (except at absolute zero) so you could use the same explanation for all matter, so why make a special case for gases? Molecules/atoms in liquids are moving around pretty good too!

Tad

I fully understand your explanation, it is just overly complicated.

Doesn't seem complicated to me, nor apparently to Andy Germany either (nor most of the people here since you are the only one claiming my explanation is too complicated). Weight is a force. Gas mases exert the force known as "weight" via collisions with the bottom of the container. This force occurs because gravity accelerate the molecules according to F=m*a, just as occurs for liquids and solids. Simple.

How can gas in a container but [not] in contact with the container add to the weight of it.

This statement is incorrect.

First of all, the above "statement" (in bold font) is actually a question. You may not like the wording or the implied meaning you attribute to it, or you may consider it nonsensical (I don't), but a question that includes no assertion can be neither correct nor incorrect. Secondly, Hendrik wrote that question (message 16), so you should mention this to avoid the impression that I wrote it.

[svengali wrote:]

Gases consist of mostly *empty space*

[Tad replied:]

All matter consists of mostly "empty space". True, molecules or atoms in gases have a greater mobility than in liquids or solids, but it is a matter of degree, not a totally different situation.

You miss the point. Yes, we all know that atoms consist mostly of empty space in the sense that the vast majority (over 99.9%) of the mass is localized in the nucleus which is only about 1/10000 the size of the surrounding electron cloud. But I referred to the empty space *between atoms*. In condensed phases (solids and liquids), the intra-atomic distance is essentially zero (i.e., less than an atomic diameter). By contrast, the average distance between gas molecules is hundreds or thousands of times larger than the atomic diameter. Furthermore (and very important to the Ideal Gas Laws), gas molecules have very little attraction for each other, while the molecules in liquids and especially solids have strong attraction for each other. These two aspects of kinetic molecular theory explain why walking through air is much easier that walking through water or a brick wall. The difference between the gas phase and the condensed phase is not just "a matter of degree". Gases really do exhibit *qualitatively* different behavior. Hence the existence of Ideal Gas Laws, but no "Ideal Liquid Laws" or "Ideal Solid Laws". Also, the Carnot energy efficiency limit applies to heat engines and heat pumps employing gases, but not to any energy transducers using only liquids and/or solids. Wow, this discussion reminds me of teaching college chemistry years ago.

[svengali wrote:]

For a brief instant the total weight of the system does not include the weight of the birds.

[Tad replied:]

This statement is totally incorrect. If the system is the cage + birds, the weight of the "system" does not change. The mass of the system remains constant. Since weight is defined as the measure of the force of the mass due to the acceleration due to gravity, the weight of the system does not change, by definition.

No, I spoke correctly. The measured weight of the system certainly does fluctuate, but the time-average remains constant if measured over time-scales that are long compared to the motions of the objects inside the container (i.e, over intervals of at least several seconds to minutes). I think this is the source of our misunderstanding. You speak in terms of time-averages, while I focus on instantaneous events (kinetics). Weight *does* vary. Weight is a force which obeys F=m*a. The mass remains constant (if nothing is allowed to escape). But acceleration changes, either by varying the gravitational field strength (an object weighs 1/6 as much on the Moon), or by forcing the motion of the inertial frame to deviate from a straight line (drive over a speed bump very fast and you will feel first heavier, then lighter, then heavier again, then finally back to normal). Or place a cardboard box on your bathroom scale, climb in, and jump up and down while noting the weight reading -- not very constant because you are accelerating chaotically inside of the box (constantly changing the "a" in F=m*a).

If you have the cage on a scale, it's apparent "weight" changes when the birds fly, you are simply making a bad instantaneous measurement.

Then you do see my point about the difference between instantaneous and time-averaged weight of an object (or collection of objects) in motion. But I disagree with your wording "bad instantaneous measurement". An instantaneous measurement is bad only if your measuring device is inaccurate. Perhaps the type of error that you really had in mind is that of mistaking an instantaneous weight for a time-averaged weight.

Hey, we are really just talking semantics. Your explanation of the weight of gases is fine.

Semantics can get in the way, especially for very abstract topics. This is why I try to use precise mechanistic language (that, and I'm a physical chemist). I think everything I wrote is self-consistent, therefore if I described gases and weight (force) correctly, than my comments about the flapping birds should hold true too.

Okay, enough said. Anyone who wants to can have the last word.

Hey, let's call a truce. I apologize if my original post struck you the wrong way. Your reply struck me the wrong way, and we were off to the races.

I say a gas is in contact with the vessel, you say there are only collisions. We only differ in that you object to my use of the word "contact". Fair enough.

I never commented one way or another about your kinetic explanation, which I fully understand, and is not complicated. It simply is more complicated than stating that gasses have a mass, and therefore a weight, by definition.

Thanks for the English and logic lessons. I really do know the difference between a question and a statement. I did not realize that Henrik originally posed the question.

I understand the properties of gases, and that they are a unique form of matter. We agree on this.

You entirely missed my point about the birds. Had you said that the apparent weight or measured weight of the system changed, I would agree. Of course the true weight of the system does not change, because the mass does not change. I thought I was pretty clear that I was nit-picking your wording.

My comment about the measurement is really referring to the fact that most instruments do not measure the actual physical property you are looking for. The scale measures force, not actual weight or mass. So even though the total weight of the system does not change, the force on the scale does.

I do have one question: If your explanation is correct, why don't gases have a higher pressure at the bottom of vessel than at the top? The molecules exert a greater force at the bottom and pressure is a measure of force. I'm not trying to rekindle the flame war, but this is one of the problems I have with the explanation.

Peace,

Tad

I could be wrong but I think the pressure will be slightly higher at the bottom of the container. Sorry not logged in.

Troy 36

Seems reasonable. The weight of a 1" column of air extending to the "top" of the atmosphere from sea level is 14.7 lbs. Go to Denver, and that weight is lower, as is air pressure. Of course, in a small vessel, the differences in pressure are hard to measure. But imagine water as an analog of a dense gas. Dive to the bottom of a swimming pool, and your ears hurt. Gravity causes pressure gradients -- some are a little hard to measure, though.

Are we talking about a laden or an unladen ear?

I'm not sure. Is an ear considered virgin until is gets laden?

Are we talking about a laden or unladen gas?

Are we talking about a laden or unladen Andy Gremany?

Are we talking about a laden or unladen emoticon?

Bridgekeeper: What... is the air-speed velocity of an unladen emoticon?
Arthur: What do you mean? An African or European emoticon?
Bridgekeeper: Huh? I-- I don't know that! Auuuuuuuugh!
Bedevere: How do know so much about emoticons?
Arthur: Well, you have to know these things when you're a king, you know.

Yes! You get the joke! It's the simplest of questions, but these guys are changing it into a royal circle-jerk!!!

How can they get this much mileage out of such a simple question!!! I personally am for the weird, but these guys go way beyond weird... I think it's called "stupid?"

Silly bunts, yes?

I read about it in the bolor supplement...

The real answer to your question 2: If you sank, there would almost certainly be a few bubbles released before you were fully drowned!

Hello Mr. Brain,

Everybody got the Archimedes stuff about floating. But, the bubble...

Well, there will be a bubble, but not of air, and looking kind of funny. You could probably see this best by shining a laser through the side. At the moment the sphere breaks, water will rush into the vacuum. There should be localized boiling, producing small air bubbles, but they are tiny and likely redissolve. But, for a short time there is a large volume of low density water, i.e., a mixture of water and vacuum and the external water pressure on the bottom of that is higher than that on the top and it "bubbles" up. Whether anybody ever sees this depends on how deep it started since the pressures will rapidly equalize.

Not such a stupid question after all!

My wife has offered my head to test this, which she claims is a perfect vacuum and quite sure there will be no bubbles (from my head at least) claiming bubbles are at least 'something' and she is quite sure there is absolutely nothing inside my head. I'll ask her mother this question as she knows bloody everything.

If you read all these postings, I guarantee you'll be to sleep in no time. Nightie night!

1, If you could manufacture it, then No, it wont float it will sink. why? glass is heavier than water. There is a point where it will float, and that is where the degree of potential displacement plus the surface tension of the water combined with the buoyancy tension of the water is greater than the weight of the glass.

your next question is also interesting point;

2, the thing about it is, that there is space but its void space. basically, all that will happen is that the void space is replaced with matter. and the displacement that was created will be reduced to the total mass of the glass

****

Far out, I should have read Hendriks reply, but my points are still valid. and he brings up an interesting thing about our atmosphere being contained by outer space which is near enough a vacumm. and what's it doing?, floating on top of the atmosphere.

A bit off the topic (macro) but if space (vacuum) is "floating" on the earth's atmosphere, as you say; is the earth "sinking" in space? (ignoring gravitational effects) and, if so, are all the planets "decending" in space to..... where....?

Ignore the rotational effects, just see the whole lot (planets) heavier than space!

Have to think about this one... any more for sleepless nights?

jt.

http://www.mfnb.at

1.WRONG! Of course glass is more dense than water, as long as it is solid. But this sphere was defined as hollow, although unfortunately no wall thickness was given. Ever hear of the notes in bottles that have traveled thousands of miles (km) and then washed up on some beach? They definitely do float. I'm assuming that the spheres are large enough so surface tension is a negligible force. In a subsequent post, the OP mentioned a 1-meter sphere. A 1-meter sphere has over half a tonne of buoyancy with virtually no surface tension, and could easily be made out of a few kilograms (or pounds) of glass. The difficulty would be forcing it under water without breaking it.

Pumice is natural glass with many inclusions of gas bubbles; much of it floats quite easily.

check post 8! wall thickness 10mm

Benthos Co. in the USA makes such evacuatable glass spheres for oceanagraphic research. They are available in different sizes (boyancies). They can also tell you from first hand experience what occurs when one implodes. They are sold as hemispheres which you assemble with your instrumentaion inside.

hmm, on the surface, this seems pretty simple, but then so is the concept of breathing - we don't even have to think about that . . . Perhaps the answer may be that it will float or sink depending on more information being supplied about the original 'sphere' -

Isn't it really the final DENSITY of the sphere - area and mass versus the area in contact with the water and it's specific gravity that will determine the float/sink relationship (regardless of the VACUUM [emptiness]) inside, and by the way, it would be a near vacuum not a perfect vacuum - perfect implies that there is no space between glass molecules)

1) you mention a 'sphere' - example - a 10" sphere with a .5 in wall (9" opening inside that is empty) has a much lighter weight hence density versus it's displacement area than a same sized 10" sphere with a 4.5" wall (1" opening inside that is empty - vacuum is vacuum)

2) as a check, let's imagine we cut the spheres in half as above and 'fill both with air' they both should have about the same displacement area. Place both round outside surface down on water of sufficient depth and it seems like the half sphere with a smaller opening in the center is very likely to 'drop like a rock', where as the half sphere that is open like a boat hull will float -

this may be a bad example because the wall thickness in the end will determine the final density versus displacement area -

that's my guess and I'm stickin with it -

cheers

Jim Wilson

You said:-"it would be a near vacuum not a perfect vacuum - perfect implies that there is no space between glass molecules)"

I suppose its what you define as perfect, as far as I am aware, they can get to "space" standards nowadays....is that perfect?

Air pressure at sea level is around 14 lbs per Square inch, or thereabouts, that is not really much, so a glass hemisphere of reasonable proportions should hold that back easily....

I would be interested in more infos from somebody "in the know" myself on that score.....

Andy,

You are correct. It is possible to draw a hard vacuum in laboratory glassware. A perfect sphere would be ideal, requiring thinner glass. I am sure that you could make a vacuum sphere that floats.

The comment :-"it would be a near vacuum not a perfect vacuum - perfect implies that there is no space between glass molecules)" makes no sense at all.

Tad

air has nothing to do with floating objects. if the object displaces water of a greater weight (mass) than the mass of the object, it will float. Air inside a beach ball causes the beach ball to float because the air has far less density than the water. If you put helium or hydrogen in the beach ball, it would float even higher in the water (or possibly even float up into the air, since helium and hydrogen are less dense than air (mostly nitrogen plus about 20% oxygen and some other gases, water vapor). If you had a gas less dense than hydrogen inside the beach ball, it would float higher still in the water. There is no gas less dense than hydrogen, but let's imagine a gas 1% of the density of hydrogen. If you filled your beach ball with that, it would float. Keep imagining gases less dense than hydrogen until you get a gas 1 quintillionth as dense as hydrogen. It would cause a beach ball to float. A vacuum is even less dense than that. You'll note the trend--the less dense the material inside, the better the object will float because its weight will be less and less relative to the water it displaces. The problem with putting a vacuum in a beach ball is that the structure of the ball isn't rigid enough to remain spherical with a complete vacuum inside. A vacuum in a beach ball would reduce the ball to its skin. Then it would depend on the density of the skin whether the ball would float. But if the ball were rigid, like your glass sphere (a light bulb is what I'm visualizing), it would float like crazy.

But there's no reason to imagine it. You probably have a light bulb somewhere in your house. Unscrew it, take it to the bathtub, fill the bathtub with water, place the bulb gently in the water and see if the light bulb floats. My money is on floating. Now take a very fine file and file away at the glass on the neck of the bulb until you puncture the skin, letting air into the lightbulb. See if it floats. I believe the bulb will still float, but somewhat lower in the water.

Try it out. Go to an antique store and buy an old tv vacuum tube (valve). Immerse it in a bucket of water and break it with pliers.

Although they were called vacuum tubes, some weren't truly vacuum, they either had inert gas at low pressure, or the oxygen was burnt up using a small piece of magnesium after sealing.....God knows what was in there then, co2 maybe?

The real reason was that they wanted no oxygen in there....

I have to disagree Andy, a vacuum of a very high nature is in a valve (tube) the 'getter' is there to clean out any remaining gas particles after the tube has been evacuated and sealed.

The getter is fired using radio waves and remains actively gathering gas atoms as long for the whole of the valve's life...

A good vacuum is needed to stop electrons colliding with gas atoms and so improve the valve's efficiency.

John.

...but it can only remove stuff that burns, oxygen for example, and what is left as a result of the burning......

I agree that the tubes were evacuated as well as possible before sealing, but I am not convinced that there was a vacuum in there........any gas of any sort is still not a vacuum....no matter how close!!

It most certainly does remove all gas atoms that come within reach of the material in the getter...

It doesn't remove the gas by burning, as I said it continues removing gas particles for many years, that is why a valve will usually fail because it has gone 'soft' which means the getter has used up its ability to remove gas atoms and has turned a whitish colour.

I'm not a chemist so I don't know exactly what chemical combinations are in the getter mixture, but it does continue collecting all gas atoms for many years after its activated after evacuation and assembly...

This is why fault finding a valve instrument used to be, sometimes, easy, the first thing you look for is a soft valve (tube) by looking for the getter material not being silvery colour but whitish in colour.

I think maybe you're getting confused between a light bulb and a valve? a light bulb is filled with an inert gas whereas a valve is a vacuum.

John.

OK. Many thanks for your kind comments...

Andy.

Vacuum is a relative term. In common terms, vacuum means less than atmospheric pressure. An absolute vacuum is the absence of everything.

By your definition, nothing is a vacuum. Even in deep space, an atom/molecule may come floating by. See:

http://en.wikipedia.org/wiki/Vacuum

I don't think that Wikipedia is a defining resource, but this is a good explanation.

Tad

The oxygen is not "burnt up". The metal introduced into the tube is known as a getter. It actually absorbs the O2 into the metal matrix.

Tad

Andy, you weren't thinking this time! If the getter is Mg, then after it does its job, you have a combination of Mg and MgO. MgO (and all other metallic oxides I'm aware of) is a solid, so the previously gaseous molecules are now trapped in the oxidized metal.

Responding to someone else's objection to your use of the word burnt, the chemical meaning of the word 'burn' is to combine a fuel with oxygen. That is exactly what is happening here, so you were correct in saying the oxygen was 'burnt up'.

Dick

Since it is a "perfect" vacuum, there is nothing there and there is nothing to get rid of hence no residue and no bubble.

An excellent questionary.

I am new to this forum, reading simple questions. But Glass Sphere with Vacumm is quite mind boggling, it revives my college time phisics.

Ans 1.

Glass Globe floates in to water if Glass Globe's weight is less then the volume of water it displaces.

If Glass globe weighs say 1 kg and its volume is more then 1 ltr, it will float in water.

If Glass globe weighs say 1 kg and its volume is 1 ltr, it will be in quasi state, it will retain in water where it is placed.

If Glass globe weighs say 1 kg and its volume is less then 1 ltr, it will sink in water.

Above three condition are on approximation for volume at one temperature and say sea level pressure.

Weight of air is quite negligable compared to water so vacuum will have practically no effect in floating of globe.

Ans 2.

Now, comming to vacuum. If Glass sphere has air and breaks underwater then bubble will form of air and will come to surface but if there is vacuum and no air, on breaking of globe, water will rush to vacuum space. Since there is no air, no bubble can form.

Your question is not silly but very interesting for phisics lovers.

Weight of air is quite negligable compared to water so vacuum will have practically no effect in floating of globe.

Correct. The difference in densities between air and water is about 1/830 at sea level, so the effect is small in this case. In another case it may be significant and needs to be taken into account.

Wow! You ask a simple question and these guys are all over it like flies on stink! It's a good question, but it has a very simple answer.

1. If the weight of the glass ball (with or without vacuum) is less than the weight of the water it displaces, the ball will float in water. This is how you get a giant, steel battleship to float in the ocean - the total weight of all that steel is less than the weight of the sea water that it displaces.
2. If you were to break the globe in water, it will implode because of the vacuum. For the small amount of vacuum contained in the ball a small amount of water will "probably" rush in very quickly and momentarily boil - we're talking milliseconds here. So, instead of seeing air bubbles, as with a ball that was filled with air, you will probably see a very short burst of small bubbles, like you do in soda water. It will be sort of a quick fizz.

That's about as straight an answer as you're going to get from this bunch of mental masturbaters!!!

Trouble is Vermin, as is typical with these threads, the answer is given within the first half dozen posts or so and then someone takes it off track so that some others come along and start re-stating the bleedin obvious again!!!

And so it goes on and on until people get tired of stating the same thing and someone ends up re-stating it yet again before we all give up... in this case its you!!

John.

By Archemedes principle. The volums water should be equal to displaced water. So it would sink completely or float depending on the weight of water. So if " d" is dia. and the weight of glass sphere is more than 4/3 x pix (d/2)x x 3 gms. if dia is in cm. It will float accordingly.

If the glass burst, water will take place of vacuum and water level will come down to the extent of spce inside sphere as given above. Water bubbles should not come as there is no air entrapped.

I agree! It's also a little like the "sparrow" bit in the Holy Grail...

"What? Do you mean a laden or unladen sparrow?"

"Why would he be laden?"

"Well, he might be carrying a coconut, he might!"

For the love of God, man! Run!!! Run away before it's too late! Save your sanity!!!

Well, if you can't do that, then do this...

Materials: two gallon bucket, two gallons of water, 40 Watt incandescent bulb, hammer, safety glasses, and pair of gloves.

Experiment:

1. Fill the two gallon bucket most of the way up with water.
2. Drop the 40 Watt bulb into the bucket full of water - Does the bulb float?
3. Put on gloves and safety glasses.
4. Grab hammer.
5. Pull the bulb underwater.
6. smack it with the hammer - do bubbles appear?

There! You have your answer, discovered all by yourself, and you can stop listening to all these dick-weeds!!!

'Dick-weeds'? Hello Vermin, I've sat back and had a good laugh reading some of these replies, we've looked at bird cages, fully laden African swallows, we could say the effect of smashing the sphere would be a reverse played holy hand grenade sketch but I'm sure somebody would point out the squirrel actually has lungs full of air! Most of the stuff we have ranted about is quite easy to picture in my head but when you said, and I quote 'and you can stop listening to all these dick-weeds!!!' I had to draw the line! The last thing I want to think about is a weed infested dick! I will go back to thinking about birds if it's alright with you guys! African or Indian? ARRRRRGGGGHHHHHHHHHHHHHH!!!!!

OK, I understand now... You're having fun with what your question generated. Done that myself!!! In that case, enjoy! It's better than alcohol!!!

It's better than alcohol!!!

-----

No it isn't. You've never tasted my margaritas.

Your experiment is essentially what I suggested back in post#17. Neither my wine bottle nor your light bulb have anywhere near a perfect vacuum, so in both cases there will probably be a small bubble, but in your case the splash of the hammer going through the water will make it nearly impossible to observe. Someone else suggested an old radio vacuum tube (valve). I still have quite a collection of those, so maybe I'll destroy one or two myself. Somewhere I think I still have a socket for say a #37 to hold it under water. I know I have an octal socket somewhere, and several 6L6s or 5U4s.

Now what is this about my weeds?

an old radio vacuum tube (valve). I still have quite a collection of those, so maybe I'll destroy one or two myself

No no no no no no no no no no no no no!

eBay them!!

I KNEW somebody was going to say that!

I have an Eimac 450TH, unused, in perfect condition, and still packed in horsehair in the original shipping carton. Mfr'd April, 1957. Here's a pic of a 450TH in action:

I'm forever blowing bubbles.

Pretty bubbles in the air....

<hic>

EVen in the imagination, how could the glass (bubble?) be a bubble containing a perfect vacuum? Whatever that is?

I think the key phrase is glass ball not glass bubble.

"I think the key [??] phrase is glass ball not glass bubble."

Perhaps you were thinking the ball would be, say, a molded ball? or assembled, as was mentioned somewhere above? The point in referring to the "bubble" nature of glass globes was to key back into a question induced in Mr. TrumanBrain's reverie, to wit:

"'If I had a glass sphere which contained a perfect vacuum:-....'"

To begin with, Truman's conception of a perfect vacuum is something which could not be realized in practical reality...much less confined in a glass bubble/ball which is in an un-molten, rigid state. But the misconception underlying the question does not end there. Implicit in the mind game is the notion that the matter--the gas--to be hypothetically evacuated so thoroughly from the ball had--it was seemingly subconsciously assumed--hitherto imparted buoyancy (more buoyancy and not less): as if glass which might have sunk in its pre-blown, pre-spheroid, pre-rigid, globular state became buoyant as a hollow sphere because of the air captured inside...rather than both:

• Because of the glass's increase in external dimension to a point at which the glass globe (and contained gas, however rarefied) weighs less than an equal volume of water, and...
• The physical and chemical/mineral properties of the glass (and contents) itself, which could be conceived thusly:
• Glass composed with heavier constituents will be less buoyant that with lighter--and the addition of more air to the heavier will only make it even heavier, not lighter--less buoyant, not more so.
• A sphere that was made to sink by increasing (say) its wall-thickness (yes, that would also entail chemical modification; and remember, this is only an hypothetical ideation (but of the non-fallacious kind) to make the point) would do so not because of the decrease of gas within, but because of increase in glass weight (proportionately more than the corresponding weight of displaced interior gas) above the unaltered volume and weight of water that can be displaced. And, continuing such imaginary wall thickening, two things become apparent which help unravel the seeming paradox presented in Truman's dream questions:
• With increasing thickness the sphere would eventually "return" to being the un-hollow glass blob (ball of the un-inflatable kind) it started out as, will have (actually and realistically) been evacuated of (all or most) air, and will (normally) sink...only because of greater weight of an unchanged volume...not because it was no longer "inflated" (held at a certain minimally required volume) with air. Further...
• If the thickening of its wall caused the ball to sink, the same thickening ought to render the ball less less likely to shatter...and/or to release any remaining gas as bubbles; while, on the other hand,
• If nothing--no thickening--had been "done" to increase the ball's shatter resistance, it (still) would not shatter (or release anything, or nothing) because it could not strike the bottom because the weight of its unchanged volume was not increased.

Thus is the second question, as is the first, and any seeming paradox, nullified for lack of a valid premise.

...which all leads one to suspect that Mr. Truman's unconscious brain had in mind a deflatable or semi-deflatable ball that was perhaps clear...but but not made of glass.

So you see, it could be argued that neither ball nor ball shaped bubble was a key phrase....

No, I'm sure my conscious mind had a glass rigid ( transparent ) sphere which was void of all forms of gas, liquid, solid, matter in suspension, and any other things which would be able to ruin my perfect hypothetical vacuum! To produce this vacuum, my conscious mind had designed and built (in my conscious mind) the perfect vacuum contained in a glass sphere that had sufficient wall thickness to be able to withstand the forces created by this hypothetical, in my conscious mind, vacuum! I wouldn't say that I had an unconscious mind! I prefer to think I have a sub-conscious mind that can normally operate on its own without too much outside interference! I'm sure some of you will agree to differ on the state of my mind but that is the beauty of having a fully operational subconscious mind!

Back to you guys!

...sufficient wall thickness to be able to withstand the forces created by this hypothetical, in my conscious mind, vacuum!

Of course there is the issue: is the force created by the vacuum? (Or maybe this has been raised before... but I'm too tired and pressed for time to read all this stuff.) Suppose you fill a one meter long test tube with mercury (being careful, of course). Then you invert the test tube into a pool of mercury. You find that the height of the mercury column falls to equal the barometric pressure (in inches of Hg) wherever you are. Above the column is something that is very close to a vacuum, with a couple quartz molecules, and a few mercury molecules... Suppose the test tube is too thin, and it collapses: what caused it to break? The vacuum, or the air pressure, or the difference between the two?

Mr. T. Brain,

Not to run this in the ground too badly, but I awoke this morning at 4 am, thinking about this question. WE GOTTA GET MORE SLEEP!! Anyway, I thought about the reverse: a sphere of pressurized water floating in space. I realized that, when it bursts, you should expect to get little water droplets from surface tension. And, so, it seems we might, in your instance, get little vacuum droplets enclosed by water having too much surface tension to completely collapse the vacuum. Now, just as water droplets, if they are small enough, don't react readily to an acceleration field, neither might the vacuum droplets. In that case, they would remain suspended without rising. I don't think there is any way of this earth to test this idea, so it's perfect.

Tom