Mastering Cooling Soup: Feynman's Explanation of Evaporation and Heat Generation

[Aleoa]

As Feynman says, talking about a liquid in a container :
"So the liquid gradually cools if it evaporates. Of course, when a molecule of vapor comes from the air to the water below there is a sudden great attraction as the molecule approaches the surface. This speeds up the incoming molecule and results in generation of heat.
So when they leave they take away heat; when they come back they generate heat."

And then:
"If we blow on the water so as to maintain a
continuous preponderance in the number evaporating, then the water is cooled.".

I haven't understood this last sentence. It seems counter-intuitive because, as previous said, a molecule that go from the air to the liquid get accellerated ( so become more hot)

 

[A.T.]

a molecule that go from the air to the liquid get accellerated ( so become more hot)

Temperature is not defined for a single molecule based on some arbitrary reference frame.

 

[Aleoa]

Temperature is not defined for a single molecule based on some arbitrary reference frame.

Hi. What i meant is that a molecule that go from the air to the liquid results in generation of heat

 

[hilbert2]

There's more molecules transferring energy away from the liquid than transferring energy back into it, if the liquid has higher temperature than the air. This is just the 2nd law of thermodynamics. The blowing of air on the liquid removes a surface layer of saturated water vapor, allowing faster evaporation.

 

[Chestermiller]

The evaporating molecules are being blown away in the air stream, so they can't recondense.

 

[russ_watters]

"If we blow on the water so as to maintain a
continuous preponderance in the number evaporating, then the water is cooled.".

I haven't understood this last sentence. It seems counter-intuitive because, as previous said, a molecule that go from the air to the liquid get accellerated ( so become more hot)

The velocity with which you blow on the soup is way too low to noticeably increase the temperature of the air or soup.

 

[lokibear0803]

As Feynman says, talking about a liquid in a container :
"So the liquid gradually cools if it evaporates. Of course, when a molecule of vapor comes from the air to the water below there is a sudden great attraction as the molecule approaches the surface. This speeds up the incoming molecule and results in generation of heat.
So when they leave they take away heat; when they come back they generate heat."

And then:
"If we blow on the water so as to maintain a
continuous preponderance in the number evaporating, then the water is cooled.".

I haven't understood this last sentence. It seems counter-intuitive because, as previous said, a molecule that go from the air to the liquid get accellerated ( so become more hot)

Blowing across the water introduces a temperature gradient between the water and its environment. Now, assuming your breath is cooler than the water, thermodynamics tells us that the system will move towards equilibrium via more water molecules escaping the water and as such removing heat from that water. Eventually the temperature of your breath matches the water's temperature and the system is in equilibrium, i.e. the evaporation stops. Here's probably a better explanation than mine: https://physics.stackexchange.com/questions/127309/why-does-blowing-on-hot-coffee-cool-it-down.

 

[Aleoa]

Feynman procedes, saying that:
"Not only does the water go into the air, but also, from time to time, one of the oxygen or nitrogen molecules will come in and
“get lost” in the mass of water molecules.

And so:
"If we suddenly take the air away from the vessel, then the air molecules will leave more rapidly than they come in, and in doing so will make bubbles."

Even this phenomenon isn't explained in the book. Do you know why it happens (in particular the bold sentence) ?

 

[A.T.]

so:
"If we suddenly take the air away from the vessel, then the air molecules will leave more rapidly than they come in, and in doing so will make bubbles."

Do you know why it happens (in particular the bold sentence) ?

If there are no air molecules then they cannot enter the water.

 

[jbriggs444]

"If we suddenly take the air away from the vessel, then the air molecules will leave more rapidly than they come in, and in doing so will make bubbles."

This phenomenon is similar to boiling. It is responsible for the condition that divers call "the bends".

Suppose that we have a small air bubble in the middle of some liquid already (to avoid dealing with the complexities of surface tension and nucleation sites). At the gas/water interface of this bubble there will be a constant exchange of air molecules going into solution and air molecules emerging from solution. For any given air pressure, there will be an equilibrium concentration of air in solution so that the two rates are equal.

If you reduce or eliminate the outside pressure on the liquid, the pressure within any such micro-bubbles will decrease to match. This affects the equilibrium. The rate at which gas emerges will be greater than the rate at which it goes into solution. The bubbles will grow.

 

[Aleoa]

At the gas/water interface of this bubble there will be a constant exchange of air molecules going into solution and air molecules emerging from solution. For any given air pressure, there will be an equilibrium concentration of air in solution so that the two rates are equal.

Hi, which principle regulates this ?

 

[jbriggs444]

Hi, which principle regulates this ?

Boyle's law? PV=nRT. Lower pressure = fewer air molecules in a given volume = fewer air molecules impacting the surface and being absorbed. By contrast, the rate at which molecules come out of solution is determined by the concentration in the water which is not (at least not immediately) a function of pressure.

Think of it as the basic rule of dart throwing: The more darts you throw, the more that will stick.