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Author Topic: evaporation and condensation of water  (Read 28396 times)
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mtsr
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on: March 27, 2010, 07:00:05 pm » posted from:Ioánnina,Ioannina,Greece

Dear sir
My name is Maria and I am a physics teacher in Greece. (My English are not very good, so I am sorry for my mistakes).
I am trying to find a qualitative simulation about evaporation and condensation of water from an open and closed container.
I want my students to see what is going on (microscopically) in an open and closed container.

More specifically:

OPEN CONTAINER
Let’s say that there is an open container with water at temperature T. There are some molecules of water that have enough kinetic energy to escape from the liquid and became water vapor (gas). This procedure (the conversion from liquid state to gas state) is called evaporation and goes on at all temperatures. Due to evaporation the level of the liquid in the open container decreases.
The rate of evaporation depends only by the temperature of water.
At high temperature lots of molecules have enough energy to escape from the liquid. At low temperature fewer molecules have sufficient energy to escape from the liquid.
The wind, the air temperature, the humidity etc play no role at this procedure! The rate of evaporation [the number of the molecules that convert from liquid to vapor is constant at a given (and constant) temperature].
All the simulations / videos etc that I have found, except of temperature, claim that the rate of evaporation depends on wind, humidity etc. This is only true when we talk about net evaporation.

CLOSED CONTAINER
If there is a close container with some water, the following procedures take place.
Initially, the system contains only liquid, and air above it. As evaporation starts (the rate of evaporation is constant for the specific temperature of the water), the molecules from the surface of the liquid escape into vapour state, in the confined space above. Therefore, the level of liquid falls.
Then starts the process of condensation. This is the conversion of vapour into liquid. Initially, escaped molecules (from liquid state) move randomly in all directions and collide with one another. As more and more molecules enter the confined space, some slow-moving molecules are pushed back. They collide with the surface of the liquid to reconvert into liquid.
In the initial stages, the rate of evaporation (constant) is more than the rate of condensation because only small number of molecules are present in the gaseous state. The rate of condensation thereafter gradually increases as the number of molecules in the gaseous phase increases. Finally, a stage is reached when the rate of the two opposing processes is the same.
The state where the rate of evaporation becomes equal to the rate of condensation is called a state of dynamic equilibrium. In such a state, although the amount of liquid level in the container does not change, evaporation has not stopped and the system is not at rest. In fact, the number of molecules, which escape from the liquid to the gaseous phase (due to evaporation), becomes equal to the number of vapour molecules that return to the liquid.
I want my students to understand the above processes at microscopic level and I believe that a qualitative simulation accompanied with a qualitative diagram of the rate of evaporation and condensation with time would be very helpful.

Can you help me?
Thank you very much!
Maria

 

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Fu-Kwun Hwang
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Reply #1 on: March 27, 2010, 11:01:17 pm » posted from:Taipei,T\'ai-pei,Taiwan

Here is the first version of the evaporation and condensation of water.
This simulation is modified from browian motion (a simplified model).
-*-
The model used are:
There is a fix probability for particle to evaporate into gas state (and the probability is propotional to temperature).
The probability for particle to condense back to liquid state is proportional to the particle density in gas state.

Let me know your suggestion about this simplified model.

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mtsr
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Reply #2 on: March 30, 2010, 06:30:50 pm » posted from:Ioánnina,Ioannina,Greece

Dear sir,
Thank you very much for your kind response to my request.

I have seen the simulation and I have some questions and some comments.

1)   What is “minr” ? I changed its values but I couldn’t see the difference.
2)   I would expect to see all the water of the open container to be evaporated (after some time) while the water of the closed container to lower its level and finally to remain constant. This does not happen. Why?
3)   Is possible to change “vmax” with “temperature” ?
4)   In case of a constant temperature (vmax) in the open container, the relationship between the number of liquid water molecules and time must be linear (constant evaporation rate).

Maria
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Fu-Kwun Hwang
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Reply #3 on: March 30, 2010, 10:06:19 pm » posted from:Taipei,T\'ai-pei,Taiwan

I just modified the code and update with new model.
I hope it is closer to what you need!

I can create a simulation according to what the rule required.
However, I would rather to use a physics model try to simulate it.
Instead of using a set of equetions to animate it.

I can make the evaporation a straight line. But I would rather use random number to simulate it.
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mtsr
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Reply #4 on: March 30, 2010, 11:27:21 pm » posted from:Ioánnina,Ioannina,Greece

YOU ARE GREAT!
YOU ARE THE BEST!
I WILL ADVERTISE YOU!
MARIA
p.s. Please change the label 'vmax' with 'temperature'
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Fu-Kwun Hwang
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Reply #5 on: March 30, 2010, 11:53:57 pm » posted from:Taipei,T\'ai-pei,Taiwan

Do you mean the label for the slider GUI element ?

You can click your mouse with any element , right click , select rename to rename it's label.
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Reply #6 on: March 31, 2010, 12:20:41 am » posted from:Athens,Attiki,Greece

To the right I see "T" which is temperature. This is OK now.
But to the left I see "yc". What is this? In the previous version, I think that it was the number of molecules, which was better.
Maria
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Reply #7 on: March 31, 2010, 01:23:25 am » posted from:Athens,Attiki,Greece

Thank you very much indeed!
Your help was very valuable.
Maria
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Reply #8 on: April 05, 2010, 11:12:12 pm » posted from:Taipei,T'ai-pei,Taiwan

Here is a modified version: (change view) Click eye (image) to view applet.
 

Embed a running copy of this simulation

Embed a running copy link(show simulation in a popuped window)
Full screen applet or Problem viewing java?Add http://www.phy.ntnu.edu.tw/ to exception site list
Press the Alt key and the left mouse button to drag the applet off the browser and onto the desktop. This work is licensed under a Creative Commons Attribution 2.5 Taiwan License
  • Please feel free to post your ideas about how to use the simulation for better teaching and learning.
  • Post questions to be asked to help students to think, to explore.
  • Upload worksheets as attached files to share with more users.
Let's work together. We can help more users understand physics conceptually and enjoy the fun of learning physics!

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Youe can not help men permanently by doing for them what they could and should do for themselves. ..."Abraham Lincoln(1809-1865, US President 1861-1865"
 
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