Guys, I've been looking around the internet and my school textbook and I still can't figure out the difference and connection between evaporation and boiling. I know evaporation happens at any temperature and only on the surface and boiling has a specific boiling temperature and happens everywhere in the liquid. Does evaporation has to happen first and then boiling, what is the connection?
@NovaliumCompany Boiling is what happens when the liquid reaches a temperature above which it cannot exist as a liquid at the current pressure. Evaporation is what happens to liquids below boiling temperature all the time.
@NovaliumCompany No, not really. Evaporation happens always - by random chance some molecules in the liquid gain enough energy from collisions to overcome the environmental pressure and escape into the surroinding air as gas.
@NovaliumCompany Yes, as the difference between the liquid's vapor pressure and the environmental pressure becomes smaller, evaporation becomes more rapid.
E.g. put a bucket of 20* C water into a vacuum. It instantly boils, until the steam reaches a certain pressure, then it stops boiling. That pressure at which it stops/starts boiling is the vapor pressureat 20° C
The pressure the vapor has when the boiling stops - i.e. when equilibrium between the amount of gas and the amount of liquid is reached.
If you have a system consisting of a liquid and the gaseous form of a substance, then the vapor pressure is the pressure of the gaseous form when the system is in equilibrium, i.e. the ratio of liquid to gas doesn't change anymore
Thanks for the help man, but I think I'll understand better with an animation, I found a video, let me watch it. :)
If I have a full glass of water and cover it with something on the top. After some time, I should have half with water and the other half with vapor. And vapor pressure is the pressure of that vapor?
If you cover it, the water will evaporate until the pressure the water vapor alone would exert is equal to the vapor pressure. That's what one calls the air being "saturated" with water vapor, or 100% relative humidity.
If you put water into a vacuum it will become gaseous until the water gas above the liquid reaches a certain pressure. That pressure is the vapor pressure.
Now, if you cover a glass, there's air above the water, also having a certain pressure. As long as that pressure is above the vapor pressure for that temperature, the liquid will not boil, just evaporate
Evaporation stops when there's enough vapor in the mixed air that if you took away all the air, the remaining vapor would still exert the vapor pressure.
(i.e. evaporation stops when the partial pressure of the gas is equal to the vapor pressure.
Trying to derive $\delta e = \frac{d}{d \tau} (\xi e)$, the infinitesimal form of $e'(\tau') = \frac{d \tau}{d \tau'} e(\tau)$ for einbein $e = e(\tau)$ and parameter $\xi(\tau)$. From $e'(\tau') = \frac{d \tau}{d \tau'} e(\tau)$ we have $e'(\tau + \xi) = [\frac{d}{d\tau'}(\tau' - \xi)]e(\tau) = (1 - \frac{d \xi}{d \tau'})e(\tau)$ we have $e'(\tau) + \xi \frac{d}{d \tau'}e = e(\tau) - \frac{d \xi}{d \tau'}e(\tau)$
so that $\delta e = e'(\tau) - e(\tau) = - \xi \frac{d}{d \tau'}e - \frac{d \xi}{d \tau'}e(\tau) = - \frac{d}{d\tau} (\xi e)$, clearly messed up signs and primes, how do I clean it up? :(
@enumaris Yeah. In large parts a legacy of Germany needing to stock up its workforce in the 60s and a lot of Turkish people moving here to fulfill that need
I..e. many Turkish people living here today are second or third generation immigrants
@ACuriousMind I think I finally understood it. So boiling occurs when the pressure of the vapor (the water molecules with high enough kinetic energy to escape the water surface) is equal to the atmospheric pressure, right?
In a mixture of gases, each gas has a partial pressure which is the hypothetical pressure of that gas if it alone occupied the entire volume of the original mixture at the same temperature.
@enumaris Yup. There's an authentic Chinese restaurant here I visit regularly with some of my friends - we never really know what we're ordering because the menu is only in Chinese with bad English Google translations but it's always a nice experience and the staff is always very nice :D
@ACuriousMind the part where it says "pressure of that gas if it alone occupied the entire volume of the original mixture". That part confuses me of the partial pressure definition.
Well, a gas filling up only a part of the total mixture will have different pressure if it filled the whole mixtures as if the other gasses didn't exist.
@NovaliumCompany The pressure of a gas only being a part of a mixture is not well-defined. That's why we define partial pressure the way we do
I mean, for ideal gases, it turns out that partial pressure is really "the pressure of that part", but you can't really talk about the pressure being due to a part of the mixture
Ok I have 2 gases in a closed container. The first gas has 50kPa pressure and is filling half of the container, the other one is 10kPa and filling the other half. So the total pressure is 60kPa. (I know it's weird to imagine it that way, but what's the right one?)
@NovaliumCompany No, it doesn't work that way. IF the two gases are really filling different halves of the container, you have a non-equilibrium system where the pressure is not uniform until they have fully mixed.
Once they have mixed, the total pressure will be the pressure of one of them filling the whole container plus the pressure of the other filling the whole container
Again, for ideal gases. The real world is more difficult but it's often a good enough approximation
@NovaliumCompany Because boiling only occurs when the total pressure is below the vapor pressure, while evaporation occurs as long as the partial pressure is below the vapor pressure.
You have an open container filled with water. Evaporation constantly occurs and vapor is constantly given out to the air. Now if we keep supplying the water with heat, more vapor will form and the total pressure (air + vapor) will increase. That's where I cut off.
In a typical kitchen, the gas pressure will effectively not change.
But e.g. in a pressure cooker, it will. That's why pressure cookers allow you to heat the liquid to higher temperatures than its boiling point at standard pressure.
So overall, why boiling occurs. When the atmospheric pressure and the vapor pressure are equal (or vapor pressure is higher). Hmm, but I thought we looked at them as one?
Total pressure (air pressure + vapor pressure), Partial pressure (can be of the air or the vapor)?
(I can't imagine how annoying it can be, explaining something to me, over and over again, and I still can't understand it) Should I have some sleep and try again tommorow?
@NovaliumCompany No. Vapor pressure is the theoretical pressure in some closed system consisting onloy of the liquid and its vapor at fixed volume - e.g. a hypothetical pressure cooker that was initially evacuated - at which no further evaporation will occur.
@NovaliumCompany Probably. As I said, I didn't understand these concepts at my first go, either :)
Much like mathematics, sometimes physics is something you slowly get used to rather than understanding everything in a flash
@enumaris am thinking may be forced to install some ML libraries soon because many months of linear regression is starting to run into discernable limits...
@enumaris lol yeah have done that quite a bit... musing on it again... started coding in ruby many years ago & didnt want to switch to python as it ascended, its clearly superior in this area now... have dabbled )( in python... my day job is enterprise java + linux etc...
GSW say that for branes higher than strings, the action is non-renormalizable and you can't constrain all the components of the metric with symmetries, has anything changed in 30 years?
@bolbteppa Yes, the brane scan is better understood now, i.e.. we know for which combinations of spacetime dimensions and brane dimensions consistent actions exists, although they have not become any more renormalizable
Renormalizability is the god of those pursuing "truly" fundamental physics, not of the phenomenologists.
I get defensive about string theory because I think there's a lot of good arguments one can make against it but most people don't really bother and go for stuff based on a misunderstanding of it :P
@bolbteppa youre overthinking it. its an information based theory just like Wheeler espoused/ laid out. strongly related to digital physics, holographic principle, 't hoofts + wolfram cellular automata etc
