Ok quick question...about air pressure?

Question

If you heat water inside a can, then where is there more air pressure...in the inside or outside?

it seems simple but plz explain this i don't get it...

if hot air is less dense, why would there be more air pressure

or if its not that way...

shouldn't hot air have more air pressure because the molecules move faster???

Answer 1

You didn't specify whether or not the can was sealed. If the top is open then the air directly above the water is going to be hot and therefore less dense. The reason for this is because the air molecules are vibrating faster and therefore need more room. This causes the air to expand and you have X amount of air now occupying a greater space. If the can is sealed then the air pressure is going to increase since the original volume of air now has to make room for the water vapor and steam which is being created. That will squeeze the air into a smaller volume. Keep in mind here that when water boils it increases in volume 14,000 times.

Answer 2

There is a little thermodynamic law that says

- hot air is only less dense if it is allowed to expand.

So if your bottle is closed the pressure will rise because it wants to expand into something.

and if the molucules move faster it means it has more energy and it doesnt affect the pressure. ( i am not 100% about this though)

Answer 3

With an open can, the pressure inside will always be equal to that outside. If it 'tended' to be lower, the higher atmospheric pressure will enter the can and, if the pressure in the can 'tended' to be higher, the excess pressure will flow out into the lower pressure of the air. Either way, the internal and external pressures will equalise.

Answer 4

If you heat inside a closed can, pressure will increase due to production of steam. Also water will expand  and occupy more volume pushing the air to build pressure.

If you heat air in open sapce,hot air will go up due to less density and cold air will flow to cover that area .