pressure of liquid and evapouration of water

sgstudent

When we suck a straw, we produce a vacuum in our mouths hence the atmospheric pressure on the outside is greater than of the liquid above the straw hence pushing the liquid up. But what if we arrest underwater and we open our mouth, why would water flow into it? Is it because our mouth has a smaller pressure than the surrounding water, so it seeks to reach equilibrium so it flows into it?

Also, what allows fluid to flow about at constant and random motions? Is there a pressure explanation for this? When we use thee formula P=hpg at a certain point is that theft pressure exerted on whatever is below it? Eg I have a 10m water tank and if I use hpg at 5m, then is that the pressure exerted on the water molecules below it? Lastly, if I have a box placed into the water, the pressure at the base (parallel to the bottom) is acting upwards right, after calculating the P=hpg at that point. However, if I have a larger surface area box placed instead it will still have the same pressure at that point right? So would the force acting also be larger?

As for evaporation, since the water absorbs heat energy from the surrounding our body would be cooler right? But when the enter molecules at the surface of the water body gains enough energy then it would break free its bonds and escape as a gas, so does it mean that the water at the surface is at 100 degrees celcius? So if there is some gas in the air won't it be very hot?

Thanks for the help guys!

jfizzix

The water at the surface is not 100 degrees C. Where a cup of water in a room is made up of septillions of water molecules, the temperature of the water as a whole is just room temperature. But with the random collisions of water molecules with one another and with the air, the energy of every individual molecule is not the same. Some may be faster and others slower. Some may be fast enough to escape the surface of the water altogether.

Evaporation cools the water because all that's happening in evaporation is that the highest energy molecules are escaping the cup. When this happens, the average energy and temperature is lowered as a result.

In fact, you can use this to freeze water by putting the cup in a vacuum. There's a nice video showing how to do this with liquid nitrogen.. That's right. You can freeze liquid nitrogen by putting it into a vacuum.

http://www.youtube.com/watch?v=FReIs6rQbPM

Hope this helps,

-James

sgstudent

Hi James, but I thought that the surface molecules need to gain enough energy to escape from the bottom stuff; so isn't that energy equivalent to the amount of boiling water? So if we have a single water molecule m, and we calculate the energy to get to 100 degree it would be Q=mc(dt) then to get to the gaseous state its Q=ml. So won't it be at 100 degrees? Thanks

jfizzix

The temperature of the surface of water is not at 100 degrees. It's only those few molecules who have so much energy (mc dt + ml) compared to the average molecule and which happen to be at the surface, that can go into the gas phase.

It wouldn't be right to say that the molecules at the surface are all 100 degrees. Only those few which happen to evaporate can be considered to be that hot. Even then, the temperature of a single molecule is ill defined, as it is a bulk property of material, like pressure, or density.

Hope this helps,

-James