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Physics Question

 
 
Reply Tue 30 Nov, 2004 09:07 am
If you create a vacuum in a bottle half filled with cold water:

1. Will the water get colder?
2. Will the water get warmer?
3. Will the water stay the same?
4. Will the water first get warmer and then colder?
5. Will the water first get colder and then warmer?


Explain your answer.
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Type: Discussion • Score: 1 • Views: 1,367 • Replies: 11
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carditel
 
  1  
Reply Tue 30 Nov, 2004 10:07 am
All of these will depend on the ambient temperature outside the bottle.
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whimsical
 
  1  
Reply Tue 30 Nov, 2004 10:14 am
Pretend that it doesn't depend on the ambient temperature outside the bottle.

Whim
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Equus
 
  1  
Reply Tue 30 Nov, 2004 10:17 am
The water will stay the same temperature as it was before, but it will boil and at least partially become vapor.

On Earth, water boils at 212 degrees Fahrenheit (100 degrees Centigrade) AT SEA LEVEL. Campers know that in high elevations, water boils at slightly lower temperatures. This is because there is lower air pressure at high altitudes than at sea level. The lower the air pressure, the lower the temperature at which boiling occurs. In outer space, a near-perfect vaccuum, water boils instantly.

So in a sealed container with no air pressure, the water will boil, but remain at room temperature.
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Thomas
 
  1  
Reply Tue 30 Nov, 2004 10:40 am
whimsical wrote:
Pretent that it doesn't depend on the ambient temperature outside the bottle.

But it does. There's no way of pretending it doesn't and still reach a correct conclusion. Here are three scenarios that can occur.

(1) If the inside of the bottle doesn't exchange heat with the outside, the water will cool down to some temperature and stay there. I'm afraid I can't back this up though. The proof of this is to start with the "real gas equation" and see what happens when you apply an adiabatic expansion -- meaning energy conservation inside. This is too complicated to discuss in this forum, and I can't think of a way to demonstrate it without the formulas.

(2) If the inside of the bottle does exchange heat with the outside, and the exchange happens fast compared to the time it takes for you to pump out the air (i.e., "create the vacuum"), then the temperature stays constant, and stays the same as the outside of the bottle.

(3) The in-between scenario is that the bottle exchanges heat with the outside, and the exchange happens slowly compared to the time it takes to create the vacuum. In this case, which probably will be the most common one in real life, the temperature first drops to almost the same level as in (1), then warms up again until it reaches the outside temperature as in (2).
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Tryagain
 
  1  
Reply Tue 30 Nov, 2004 12:16 pm
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Reyshak
 
  1  
Reply Tue 30 Nov, 2004 03:44 pm
Tryagain wrote:


What would happen if a person were sucked into space from a spaceship and he wasn't wearing a pressurized suit.

Would he explode or implode Question



First, I would like to say hello to everyone on the forum. I'm new and I hope this answer will help me get started on a long career of answering riddles, if such a thing exists...

The person would explode, because the pressure of the water and air inside his body would attempt to diffuse with the outside vacuum of space, therefore causing the skin to burst, much like an overstuffed balloon. The diffusion of the water and air, if slow enough, could cause the person so implode. However, the amount of pressure that is ACTUALLY inside the human body would correct itself much too quickly with the vacuum of space to allow the material to slowly seep out and the gravity of the human body itself (assuming you are in space away from any celestial bodies) to implode, much the theoretical black hole.
0 Replies
 
Tryagain
 
  1  
Reply Tue 30 Nov, 2004 06:11 pm
Hi Reyshak, and welcome to the Riddles Forum. Good first post, keep them coming. Have a good time. I will give a full answer as soon a Whim has finished with his question.
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whimsical
 
  1  
Reply Wed 1 Dec, 2004 06:53 am
Quote:
Therefore, the boys in the gang choose answer 4.


And that's the right answer.
But why does it get warmer first?


Whim
0 Replies
 
Tryagain
 
  1  
Reply Wed 1 Dec, 2004 08:54 am
I think you can say it is due to the vacuum and 'ambient temperature' outside the bottle.


Take a 500-mL flat bottom flask and fill it about half with water. Bring water to boil and let it boil for 3-4 minutes to flush out air replacing air with steam. Close the flask mouth with a tight rubber stopper entrapping steam inside. Let it cool down completely. May be next day invert the flask over a ring stand. Pour cold water over flask bottom, water would be boiling at room temperature.

Because the flask inside is cut off from outside air and its pressure, water boils under reduced pressure (partial vacuum) created by condensing water vapours when cold water was poured over the bottom. When water is open to air, it has to be heated to normal boiling temperature before it boils (100 deg C). However, under reduced air pressure it does not have to be heated to high temperature to bring it boil.

Use of pressure cookers at higher elevations because of water boiling at temperatures lower than normal boiling point signifies air pressure is lower than normal at mountain tops.
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whimsical
 
  1  
Reply Thu 2 Dec, 2004 02:21 am
Quote:
What would happen if a person were sucked into space from a spaceship and he wasn't wearing a pressurized suit.



I think the water inside the body will start boiling because of the vacuum, and escape from the body via the lungs and then he freezes. The difference in pressure (1 atm in the body and zero outside) isn't big enough to cause an explosion.

whim
0 Replies
 
Tryagain
 
  1  
Reply Thu 2 Dec, 2004 06:27 am
Because the human body is something like 85% water, the behaviour of water in space is the key to understanding this subject

What does all this mean for an unprotected person in space? The vapour pressure of water at body temperature, 98.6 F or 37 C, is about 0.93 pounds per square inch (PSI), or 48 mm of mercury (Hg). For comparison, the pressure of the atmosphere is 14.7 PSI or 760 mm Hg. Since the surrounding pressure is zero in space, water in the body might be expected to start boiling furiously, except for one important thing: the water in the body is very significantly contained by skin, blood vessels, etc. If the vapour pressure at body temperature were 1 atmosphere, the results would be immediately disastrous, as the skin and blood vessels cannot contain such a high pressure.

The result might well be something like an explosion, certainly severe rupture and haemorrhaging from many points. But since the actual pressure is only about 50 mm, we have to ask how much pressure can the skin and blood vessels really contain? Well, the normal blood pressure in the arteries is 70-150 mm Hg, so there is no way the blood is going to boil in the arterial side of the circulatory system, so long as one has a working heart and normal blood pressure. Furthermore, the arteries can contain at least 200 mm for short periods without rupture. Even the veins have to withstand the pressure differences caused by gravity between head and feet when we stand up, and these must be of the order of 75-150 mm, well above 50 mm.

It appears to me that the main source of water loss will be through the lungs. As soon as one is exposed to the vacuum, almost all of the air in the lungs will escape, since one cannot effectively contain more than a fairly low pressure, much less than 1 atmosphere, by closing the mouth and nose. So taking a deep breath in advance won't help much. After that, the lungs should fill with water vapour at about 50 mm pressure, mixed with some gases that are dissolved in the blood, oxygen and carbon dioxide certainly, and some nitrogen. Because 50 mm is such a low pressure, I believe that a trained person could significantly slow the loss of water vapour and gases from the lungs by simply trying to keep the mouth and nose closed as much as possible.

Then unconsciousness and death would come when the amount of oxygen in the blood was sufficiently depleted. The time for this to occur would probably depend on the oxygen content of the blood immediately prior to the depressurization; it could range from a few seconds if you were caught in the midst of strenuous exercise, to possibly as long as a minute or more if one had some warning and were to hyperventilate in advance.
After death, water and gases would continue to escape slowly, and the interior of the body would cool off. The cooling would cause the vapour pressure to drop even more; at freezing temperatures it is less than 5 mm Hg.

Thus, the escape of water vapour (along with gases and other volatile substances) would be progressively slower and slower. This process is known as "out gassing", and happens whenever anything with a significant content of volatile substances is placed in a high vacuum or in space. In the end, the body would be entirely dried out, desiccated, and reduced to a small fraction of its original mass.
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