Pressure experiment with a burning straw protruding from a hole in a bottle

[jaumzaum]

Consider the following experiment: we have an open bottle, with a hole where we put a straw. We burn the outer side of the straw. We see smoke ascending outside and descending inside. Why is that?

I would say the smoke has lower density than air outside, that's why it goes up. Inside the bottle we would have a smoke with lower temperature, so higher density, as it's made from heavier molecules, that would go down.

But I had some doubts. Is the pressure near the fire lower than the atmospheric pressure? I would say so, because that would make the smoke goes up, but why would that be, I mean, which transformation are we seeing in the air near the fire? If it's an adiabatic, an increase in temperature would increase pressure, the opposite of what we want. The same for an isovolumetric. We had to have something between an isobaric and an isothermal. If so, why is the process that way?

Also, will air flow from the outside to inside through the lid and then to the outside through the straw or the opposite?

Thank you very much!
I appreciate any help, thanks

 

[etotheipi]

I would say the smoke has lower density than air outside, that's why it goes up. Inside the bottle we would have a smoke with lower temperature, so higher density, as it's made from heavier molecules, that would go down.

Hmm, smoke consists of solid carbon, particulates of which get 'carried' by convection currents of hot air. So it's probably better to think about the density of the air which would change noticeably with temperature, rather than the density of the actual carbon that makes up the smoke [which you can take to be constant, here!]. Also, air being denser doesn't mean the molecules are any heavier, it just means there are more if them in the same volume!

 

[kuruman]

Particulate smoke and CO₂ are actually denser than air and sink. You need to focus on the apparent contradiction that smoke rises outside but sinks inside.

An additional observation to explain is how the fumaça generated by the fogo on the outside finds its way through the canudo de papel and gets inside the garrafa plastica.

 

[jaumzaum]

Particulate smoke and CO₂ are actually denser than air and sink. You need to focus on the apparent contradiction that smoke rises outside but sinks inside.

An additional observation to explain is how the fumaça generated by the fogo on the outside finds its way through the canudo de papel and gets inside the garrafa plastica.

Thanks kuruman!
I appreciate the translation! rsrsrsrs

I have a final doubt. Where are the points of high and low pressure? Is the pressure high near the outer side of the straw? What about the inside of the bottle? Why does the smoke goes in the bottle? Does air comes in or out by the bottle opening that is above?

I would say the pressure is higher near the outer side because the temperature is higher, but the temperature increases more than the pressure, so the density outside is lower. That way the air goes up, carrying the smoke outside. Inside we would have a lower pressure, so air comes from outside to inside, carrying the smoke. But if that happens the air inside must go out by the upper opening, again, carrying the smoke upwards, not downwards. If the pressure inside was lower, the air would flow from the outside upper opening to the inside, and then to the outside through the straw, that could explain the fact that the smoke goes down inside the bottle, but it does not explain how the smoke comes from the outside though the straw.

@kuruman

Help me please! I'm stucked.
Thank you very much!

 

[kuruman]

I would say the pressure is higher near the outer side because the temperature is higher, but the temperature increases more than the pressure, so the density outside is lower. That way the air goes up, carrying the smoke outside. Inside we would have a lower pressure, so air comes from outside to inside, carrying the smoke. But if that happens the air inside must go out by the upper opening, again, carrying the smoke upwards, not downwards. If the pressure inside was lower, the air would flow from the outside upper opening to the inside, and then to the outside through the straw, that could explain the fact that the smoke goes down inside the bottle, but it does not explain how the smoke comes from the outside though the straw.

Your explanation is missing one important point. When the hot air rises and leaves the vicinity of the flame, cold air must come from all sides to take its place. Some of that cold air is heated by the flame and rises outside. What happens to the rest of the moving air that does not rise? Hint: Moving air is known as "wind". How do you imagine this wind to "blow"?

I would agree that as smoke enters the bottle, air gets pushed out the top much like getting a bottle to overflow when it is connected to a water source beneath the level of the liquid.

By the way, do you have a video of this or is it a "thought experiment"? I would like to see it if there is one.

 

[jaumzaum]

Your explanation is missing one important point. When the hot air rises and leaves the vicinity of the flame, cold air must come from all sides to take its place. Some of that cold air is heated by the flame and rises outside. What happens to the rest of the moving air that does not rise? Hint: Moving air is known as "wind". How do you imagine this wind to "blow"?

I would agree that as smoke enters the bottle, air gets pushed out the top much like getting a bottle to overflow when it is connected to a water source beneath the level of the liquid.

By the way, do you have a video of this or is it a "thought experiment"? I would like to see it if there is one.

This is a question from a high school test. In Brazil every university applies some entrance exam. This was one of the questions.

 

[jaumzaum]

Your explanation is missing one important point. When the hot air rises and leaves the vicinity of the flame, cold air must come from all sides to take its place. Some of that cold air is heated by the flame and rises outside. What happens to the rest of the moving air that does not rise? Hint: Moving air is known as "wind". How do you imagine this wind to "blow"?

I would agree that as smoke enters the bottle, air gets pushed out the top much like getting a bottle to overflow when it is connected to a water source beneath the level of the liquid.

By the way, do you have a video of this or is it a "thought experiment"? I would like to see it if there is one.

Ok. So air comes from outside to the inside through the straw. If it does that, it's because pressure outside must be higher than inside. In the inside, air must come ou from the straw and leave by the outer opening. So why doesn't it carry the smoke upwards?

 

[kuruman]

So why doesn't it carry the smoke upwards?

Because it isn't hot. If it were hot, it would rise outside the bottle and would not go into the straw. So how does this mixture of relatively cold air and smoke get in the straw? I gave you a clue already of what I think is happening. Reread post #5.

 

[Fred Wright]

Você está familiarizado com o princípio de Bernoulli?