candle in a balloon

[j_rankin]

A question for the physics buffs:

If you were to put a candle inside a balloon, would the balloon get smaller or bigger?

Would the balloon get smaller as the candle burns up the gas inside,

or

would the balloon get bigger as the heat from the candle energises the gas inside?

Does anyone know the answer to this? I guess it depends on the type of gas and the amount of heat emitted from the candle.

Makes me ponder about dark energy.

 

[MeteorWayne]

Please tie this a little closer to Space Science and Astronomy, or this thread will be moved elsewhere.

 

[j_rankin]

im trying to tie this in with the expansion of the universe and perhaps the acceleration of the expansion.

What happens to heat as it escapes to intergalactic space? As heat affects the atoms floating around between galaxies, might it make more of an effect on them than gravity at such distances? Meaning that the space between galaxies gets bigger and bigger as more heat is poured into the cosmos.

Also, where would all this energy eventually go? Maybe more of it gets built up than gets dissipated, thus making the particles between space get more and more energetic over time.

Could heat cause localised expansions in space?

 

[DrRocket]

A question for the physics buffs:

If you were to put a candle inside a balloon, would the balloon get smaller or bigger?

Would the balloon get smaller as the candle burns up the gas inside,

or

would the balloon get bigger as the heat from the candle energises the gas inside?

Does anyone know the answer to this? I guess it depends on the type of gas and the amount of heat emitted from the candle.

Makes me ponder about dark energy.

Wayne -- your point is well taken. You should consider moving this to Physics.

As to the original question -- if you put a candle in a real balloon the heat will probably just melt the balloon.

But here is a different situation that I think is more in line with what you really had in mind It illustrates some real physics, and is perhaps an interesting story.

Once upon a time, in a land faraway, I was sitting in a technical meeting with quite a well known aeromauticl engineer, a no-kidding world-renowned expert. We were waiting for the next presenter when this expert, Gene, said to me something along the lines of "Ok smart ass. It is pretty easy to describe the physics involved in drinking through a straw, since you have the notion of a fluid head available, but how do you describe the act of sucking spaghetti?"

Quite some time after that meeting I worked out the mathematics, providing a derivation quite similar to that used to derive Archimedes principle, showing that a simple pressure differential ought to do the trick. I also noted that my description was purely mathematical, and I did not have a simple freshman-level intuitive physical description. He looked at the derivation, and agreed that he understood what I had done, but also suggested that perhaps some action of the lips was really required in addition to just a pressure differential. I did not buy his counter-argument.

So, after another period of time I sent him a video tape. The tape showed an experiment. I had a glass canning jar, with a hole in the side just the diameter of a piece of spaghetti. I had a cooked, limp strand of spaghetti that was partially inserted through the hole. Then, using my daughter to conduct the experiment while filmed it, a match was ignited and dropped into the jar, and the cap quickly put on to seal it. The match burned the oxygen in the jar, lowering the pressure, and the spaghetti strand was completely sucked into the jar -- clearly no lip action. That experiment convinced Gene of the appicability of the mathematics that I had sent him. He had only one question after seeing the tape : "How did you put the hole in that jar ?"

 

[j_rankin]

nice analogy drrocket.

I guess that answers my original question, but i still wonder if it would be the same if the fuel for the candle was a thermonuclear explosion and the gas was mostly hydrogen, and there was little to no pressure forcing the balloon in. (using an infinitely stretchy balloon).

If you put a star inside a bubble which had no mass, then would the bubble expand? if so i would imagine that the expansion rate would depend on the amount of energy being released by the star inside. If more and more energy was released over time, then the rate of expansion would increase over time.

If not then i still ponder as to where all the heat goes from galaxies.

 

[j_rankin]

If it is the case that heat is expanding intergalactic space faster than gravity is pulling it in, then this means that the nuclear forces are at work, which are much more powerful than gravity and on such a scale might have to ability to push galaxies apart.

This could also make some galaxies appear to be moving away from us faster than the speed of light as the expansion is happening in all directions.

 

[DrRocket]

nice analogy drrocket.

I guess that answers my original question, but i still wonder if it would be the same if the fuel for the candle was a thermonuclear explosion and the gas was mostly hydrogen, and there was little to no pressure forcing the balloon in. (using an infinitely stretchy balloon).

If you put a star inside a bubble which had no mass, then would the bubble expand? if so i would imagine that the expansion rate would depend on the amount of energy being released by the star inside. If more and more energy was released over time, then the rate of expansion would increase over time.

If not then i still ponder as to where all the heat goes from galaxies.

There are a couple of points that might be pertinent for you.

Heat is simply energy contained in the random motion of atoms and molecules. Heat is released from an atom by the emission of a photon. The rate of emission is proportional to the fouth power of temperature. http://en.wikipedia.org/wiki/Heat_radiation

Objects in a vacuum loose and gain heat by the process of radiation, and that is the case for galaxies. Since galaxies are considerably warmer than the background temperature of the universe (which is a very cold 2.73 K) they are net emitters of heat, That heat leaves in the form of photons -- light.

Secondly, I think your questions with regard to temperature and pressure when couched in terms of galaxies may be hinting at some hypothetical connection with the expansion of the universe, and perhaps even the accelerated rate of expansion. It is true that pressure is one of the parameters in the stress-energy tensor that determines curvature of space-time in the general theory of relativity. It is also true that quantum field theory predicts a pressure, in this case associated with the vacuum state, that can be shown to be equivalent to a positive cosmological constant which results in an accelerated rate of expansion. Unfortunately that pressure is negative. A negative pressure is a purely quantum phenomena, but the point is that a normal positive pressure in the stress-energy tensor would serve to actuall increase the normal attractive effect of gravity.

Finally the question ragarding a "bubble" begs the questioin of what consitutes a bubble. Do you mean a real physical bubble, with some sort of an elastic boundary ? If so, that will take a pretty fancy source of rubber. If not, then what would permit a bubble to exist in a vacuum, and in a vacuum what does "bubble" mean ?

 

[j_rankin]

Now i understand a bit more, thankyou.

But it does raise another question for me....

What determines the distance between atoms within intergalactic space?
If photonic energy can be perceived as both particles and as a wave, then any atoms within intergalactic space would be getting energised to some degree by the passing photons, thus creating more pressure between galaxies.

I will come back to the bubble when i get the time for another post.

 

[DrRocket]

Now i understand a bit more, thankyou.

But it does raise another question for me....

What determines the distance between atoms within intergalactic space?
If photonic energy can be perceived as both particles and as a wave, then any atoms within intergalactic space would be getting energised to some degree by the passing photons, thus creating more pressure between galaxies.

I will come back to the bubble when i get the time for another post.

There are a couple more points to be made here;

1. The "wave" associated with a photon is the quantum state. It is NOT an electromagnetic wave. A electron is either absorbed by an atom or it goes on with no interaction and with all of its energy. There is no in between. Photons are also emitted by atoms, in the process of thermal radiation, so there is mechanism for cooling as well as heating in this process. Hotter bodies radiate and cooler bodies absorb leading to a state of equilibrium. The background temperature of the universe is a very cold 2.73 K as a result of expansion, and the tendance of most things is to radiate and to become cooler.

2. Pressure does not in general cause things to move at all. Think about a submarine submerged and at neutral buoyancy in a sea with no currents. It will stay in one place. The pressure forces are ballanced. They are trying to "crush" the submarine, but not to move it. This is what is called a state of hydrostatic stress. There is just as much force on one side of the submarine as on the other and those forces cancel. In order to get things to move you need a pressure difference or gradient.

In fact, when you start looking at problems in structural analysis, under ordinary engineering circumstances (we are not talking about black holes or neutron stars here) materials do not fail underhydrostatic stress. What is commonly done is to subtract from the overall stress tensor the hydrostatic stress, resulting in what is called "Von Mises" stress, and only the Von Mises stress is then considered in determining material failure. A solid ball does not fail under hydrostatic compression (or tension) it just changes diameter, and the center remains stationary.

3. I haven't thought about this a great deal, but the atoms in interstellar space should behave as a rarefied gas. The distance between them would be determined by the local density (that is pretty much a definition of density). Pressure associated with them would be determined by their energy, or, what is the same thing, temperature. Temperature and density together determine pressure.

 

[emperor_of_localgroup]

A question for the physics buffs:

Once upon a time, in a land faraway, I was sitting in a technical meeting with quite a well known aeromauticl engineer, a no-kidding world-renowned expert. We were waiting for the next presenter when this expert, Gene, said to me something along the lines of "Ok smart ass. It is pretty easy to describe the physics involved in drinking through a straw, since you have the notion of a fluid head available, but how do you describe the act of sucking spaghetti?"

Quite some time after that meeting I worked out the mathematics, providing a derivation quite similar to that used to derive Archimedes principle, showing that a simple pressure differential ought to do the trick. I also noted that my description was purely mathematical, and I did not have a simple freshman-level intuitive physical description. He looked at the derivation, and agreed that he understood what I had done, but also suggested that perhaps some action of the lips was really required in addition to just a pressure differential. I did not buy his counter-argument.

So, after another period of time I sent him a video tape. The tape showed an experiment. I had a glass canning jar, with a hole in the side just the diameter of a piece of spaghetti. I had a cooked, limp strand of spaghetti that was partially inserted through the hole. Then, using my daughter to conduct the experiment while filmed it, a match was ignited and dropped into the jar, and the cap quickly put on to seal it. The match burned the oxygen in the jar, lowering the pressure, and the spaghetti strand was completely sucked into the jar -- clearly no lip action. That experiment convinced Gene of the appicability of the mathematics that I had sent him. He had only one question after seeing the tape : "How did you put the hole in that jar ?"

Nice story. LOL. But I also think the 'lips' have some contribution in sucking spaghetti strands. It's more like Bernoulli' work than Archimedes'. Open lips suck in air closer to the spaghetti stands creating a low pressure (air velocity near the spaghetti is higher). Surrounding high pressure air rush to the low pressure area and hold the spaghetti strands in place and 'possibly push them into the mouth'. And of course low pressure inside mouth will definitely help or do the same thing.

yes, the same question, how did you put the hole in the glass jar? Drill bits?

Now the original question. I think the question has two simple answers based on the question, "Are the materials inside the balloon required to burn the candle ?"
1) If yes, inside materials will be consumed, pressure will drop, and the balloon will collapse.
2) If no, temperature of the inside materials will rise creating a high pressure, the balloon will expand.

 

[DrRocket]

Nice story. LOL. But I also think the 'lips' have some contribution in sucking spaghetti strands. It's more like Bernoulli' work than Archimedes'. Open lips suck in air closer to the spaghetti stands creating a low pressure (air velocity near the spaghetti is higher). Surrounding high pressure air rush to the low pressure area and hold the spaghetti strands in place and 'possibly push them into the mouth'. And of course low pressure inside mouth will definitely help or do the same thing.

That glass jar didn't have any lips. All that was at work was a pressure difference. No more, no less. There was no air flow since the spaghetti strand sealed the hole. That doesn't support your theory very well.

yes, the same question, how did you put the hole in the glass jar? Drill bits?

It started as a diamond sytlus (diamond dust in a matrix) but pretty soon the matrix failed. But I had the rpms on drill press so high that the friction got the temperature to point where the glass was melting so I just kept up the pressure and melted my way through the glass.

 

[MeteorWayne]

OK, no real tie to SS&A so off to Physics.

 

[origin]

Ok, I am confused on this. Assume a lit candle is inserted into a jar and sealed. After the jar is sealed there has been no mass transfer into or out of the jar. A lit candle is converting wax and oxygen to carbon dioxide and particulate (smoke). Once all of the oxygen is converted to CO2 the pressure is lower. Why?

Is the partial pressure of CO2 lower than O2?
Or is part of the O2 combined into a particuate form of a C-O complex.

Or is it both or something else?

Seems like I should know this but I can't quite see what is going on here....

 

[DrRocket]

Ok, I am confused on this. Assume a lit candle is inserted into a jar and sealed. After the jar is sealed there has been no mass transfer into or out of the jar. A lit candle is converting wax and oxygen to carbon dioxide and particulate (smoke). Once all of the oxygen is converted to CO2 the pressure is lower. Why?

Is the partial pressure of CO2 lower than O2?
Or is part of the O2 combined into a particuate form of a C-O complex.

Or is it both or something else?

Seems like I should know this but I can't quite see what is going on here....

That is a good question. I am not sure of precisely what is going on, but I know that the experiment does work.

The wax is a hydrocarbon, and the combustion products are not just CO2. There is also, at least, H2O and probably quite a bit of CO. Now the CO2 ought to create one mole of gas from one mole of molecular oxygen, while the H2O is a two for one proposition. But there is likely to be some condensation of water on the sides of the jar as well, and that takes gas out of the system. This trick works as well with just a match as a candle, so there is nothing magic about wax as a fuel. Cellulose also contains hydrogen, so the water mechanism may still be viable.

There might also be other species and some adsorption onto the surface of the carbon (soot) that is produced.

 

[origin]

Thanks, Dr Rocket. I will have to research this a bit. It is funny how the seemingly simple and mundane can get so complicated.

Gotta love physics...

 

[origin]

So I had to run the experiment myself and yes the pressure drops slowly until the candle goes out and then the pressure drops rapidly for a few seconds.

The initial pressure drop is primarily due to the formation of water vapor this can clearly be seen as a fog on the inside of the glass. The rapid pressure drop that occurrs when the candle goes out is due to the temperature drop (PV=nRT). I am so proud that I have been able to reprodue a grade school experiment and even understand it.

 

[DrRocket]

So I had to run the experiment myself and yes the pressure drops slowly until the candle goes out and then the pressure drops rapidly for a few seconds.

The initial pressure drop is primarily due to the formation of water vapor this can clearly be seen as a fog on the inside of the glass. The rapid pressure drop that occurrs when the candle goes out is due to the temperature drop (PV=nRT). I am so proud that I have been able to reprodue a grade school experiment and even understand it.

For a proper grade-school experiment you need an OLD glass milk bottle -- the one with wide mouth -- and a hard-boiled egg. Put the candle (or a match) in the bottle and put the egg (peeled please) in the mouth to seal it. When the fire goes out, the egg goes in.

I did that one LONG before the spaghetti experiment. But I haven't seen a milk bottle, let alone one with the wide mouth, in years. I did that one so long ago that you could do with the strike-anywhere matches that could be purchased almost anywhere.