Question

Air pressure, water in a glass and cardboard experiment

We all know this famous experiment where u take a glass full of water, and put a light cardboard over it, then invert the glass and gwall the board doesn't fall

well.. i know the principle, and the reason is, in that small area (filled with water), from above i have pressure only due to water of the height of glass (rho g h), and from below, i have air pressure, which is higher, and thus the cardboard stays intact.. BUT!

Answer 1

My doubt is, this should only work, as long as there is no air in the glass right? cause if air enters the glass, then that air has 1 atm air pressure, and therefore now the pressure on the top part of the board is gonna be higher than the bottom so it shoudl fall

but it holds. i don't understand why? I know there is only little air, but the air DOES have 1atm pressure right?

Answer 2

it depends on how many air molecules does it have and the temperature and of course volume.. if we assume the air molecule is ideal then

PV=nRT

but we know air molecules are not ideal gasses, but we can do that kind of approximation ..

Answer 3

@oksuz_ Good point about the temperature - just tried it with hot water and my counter tops are now dripping :P

Answer 4

:)
There are small fraction of air molecules in the emptiness of the glass and temperature is not high, the summing of water hydrostatic pressure and air pressure is smaller than atmosphere pressure. So cardboard does not fall.

Answer 5

if i take a small cube of cardboard.. extremely small and close it completely trapping air in it

the air pressure inside is exactly equal to the air pressure outside right? .. cause if it wasn't, then the cube would collapse on itself.. ya ?

so that makes me think, that unless u compress air or anything, the air pressure remains mostly atmospheric, regardless of how much air molecules u take.. but i know something is wrong.. can u tell me what? :-P

Answer 6

think about opposite of compressing. when you pull the injector while your finger is on the tip, you feel suck on your finger because the pressure decreases in the injector tube. That is because the volume of the tube increases while the air molecules' number in of it is same.

Answer 7

yea. so whats the relation here? i am not compressing or expanding the air

the air bubble that enters the glass, is just normal air. so it should have the same pressure right :-/