[At' Pressure] Glass of water held upsidedown

[BitterX]

Hey,
I'm sure you are all familiar with the experiment where you take a glass of water , put a some seal on it and then turn it upside down.
Due to the (atmospheric pressure) - (the air pressure in the glass) exerting a force on the seal upwards greater than the [mg] of the water, the seal stays in place.

here's a demonstration - physicscentral.com

What I don't understand is why there's a pressure difference in the first place.
As far as I know - if I seal the cup then the air pressure is the same there as it is outside (with minor difference - ρgh ) what makes it change when I turn it upside down?

Thanks.

 

[Nugatory]

What I don't understand is why there's a pressure difference in the first place.
As far as I know - if I seal the cup then the air pressure is the same there as it is outside (with minor difference - ρgh ) what makes it change when I turn it upside down?

The pressure difference develops because gravity is pulling the mass of the water down in the inverted glass, trying to open up a bubble of vacuum at the top. Air pressure resists the formation of that bubble, pushing the water back up against the force of gravity.

This doesn't happen when the glass is right-side-up, because then gravity is holding the water down in the bottom of the glass.