Why hot air rises up?

Answer 1

Because it's less dense. Temperature affects density. And the less dense a gas is, the lighter it becomes, so to speak. Therefore it rises over cold air.

Answer 2

Well, let us think of a balloon, as our system.
he gas inside it has a random movement and
is less dense than the air outside our balloon.

Let it free to move. The balloon flows upward. Why ??

Well, as you know, the gravity force, acting downward,
results in a force from the bottom to the top, and the
balloon goes up !! Over the superior face of the balloon
you have a great mass of air, acting upon it. But,
against it, under the inferior face of the balloon there
will act a bigger mass of air, that weights more. The
difference in weights results in a force upward.
And the balloon goes up.

If there were no gravity force, there will be no forces
acting upon the balloon.

The same happens with our hotter air surrounded
by the colder air. The random movement of the hotter
air - considered as a system - has no effect upon the
resulting force. And the colder air flows down, to the
place where the hotter air was before ( otherwise
a vacuum will be produced...what impels the colder
air to flow and fill that empty place ). I did not mention
here the action of winds or other atmospheric phenomena
just to make things easier.

Answer 3

When a volume of air is heated its molecular motion increases and the molecules move further apart thus increasing the volume.
Therefore, there will be less molecules in an equal volume of the hot air than the original volume of cold air.

The density (weight) of the hotter air will be less than that of the cold air - it therefore rises - convection currents.

It is this principle that causes a hot air balloon to rise.

In the atmosphere, the rising air is called a 'Thermal' which is used by gliders, hang-gliders and soaring birds to get the lift.

This principle is also used in domestic heating where the convection currents circulate the warm air around a room.

(Convection currents are also set up in liquids)

Answer 4

PV=nRT

as temperature rises - for a constant number of molecules and a constant pressure - the volume increases. A larger volume for the same number of molecules means lower density. Thus a pocket of hot air "floats" in the cold air - or rises up.

Answer 5

When air is hot, the heat energy causes the molecules to move further apart and so the density of hot air is lower than that of colder air. the lower density results in it rising up.

Answer 6

It doesn't!

As my old Physics teacher used to say (usually just after throwing something at us!)...

"Hot air rises *above cooler air*"

As has been said, it is because less dense gasses will rise above more dense gasses. So, since warm air is less dense than cold air it will rise above it until it finds air of the same density, at which point it stops rising.

Answer 7

lighter than cold air...

Answer 8

Because the molecules are further apart, so they need more room.

Answer 9

as cold particles are heavier then hot ones (more CO2)

Answer 10

cos can u imagine what would happen if it didnt