4 of 15
edited tags
Qmechanic
• 154.2k
• 28
• 372
• 1827

Water evaporation problem. Homework from Feynman's lectures

I read lectures of Richard Feynman and try to solve problems for more deep understanding themes.
And I have some problem with calculating water evaporation.

There is a sum:
A glass full of water is left standing on an average outdoor window in California.
a) How long do you think it would take to evaporate completely?
b) How many molecules $$cm^{-2} s^{-1}$$ would be leaving the water glass at this rate?

I think more easy way for solving this sum is calculate the velocity of evaporation first. So I take a glass and break it into layers, every layer have a high of one water molecule ($$2.8\times10^{-8}cm$$). I know how many molecules are in 1 $$cm^3$$ in water ($$3\times10^{22}$$). I found average diameter my glass (6.75 $$cm$$) and suppose that it is surface. Next I look at surface layer in 2 dimensional view. So molecules move in 4 main directions. I suppose that in $$\frac{1}{4}$$ ways molecule goes into the air. I take normal humidity of air like 60%. But I don't know velocity of this molecule that I need for calculating rate of evaporation. This velocity is equal the temperature (because temperature is only movement of molecules), but there is some trouble with commensurability. And I don't know how can I find it.

Maruska
• 117
• 6