PDA

View Full Version : Question 2


King_Ghidra
05-04-2005, 11:06:54
Why does heat rise?
(Lansing State Journal, April 24, 1996)



--------------------------------------------------------------------------------

Because heat is not matter, it isnít correct to say it rises or falls. A better way to ask the question is: "Why do hot things (like hot air) rise above cold things (like cold air)?"
To answer this question, it is important to understand what heat is.

Heat is energy. When we say something is hot, we really are saying it has a lot of stored energy.

What does this energy do?

Matter is made up of incredibly small pieces called atoms and molecules. Atoms and molecules are always moving.

The more energy these atoms and molecules have, the faster they move. The faster they move, the more space they take up.

If something is hot, it weighs the same as if it were cold, but it just takes up more space.

This leads us to the concept of density. Because hat air takes up more space than cold air, hot aur has a smaller density.

Less dense hot air will "float" above the more dense cold air.

I understand all that apart from the last bit. It seems to be saying that hot air should take up more space because it is full of energy, but then it says it is less dense and so rises above cold air. Don't understand that bit at all.

Resource Consumer
05-04-2005, 11:10:16
It weighs the same but takes up more space as it moves around so much = less dense

Resource Consumer
05-04-2005, 11:10:51
I think :)

King_Ghidra
05-04-2005, 11:13:35
ah...ok maybe i wasn't imagining it right, that does kind of make sense, in the same way that it would take a fucking big cloud to weigh the same as a smaller amount of water

MoSe
05-04-2005, 11:17:16
if you really are that dense, there's no chance you'll ever learn to fly

Funko
05-04-2005, 11:20:25
Yeah... that's not a great explanation. Not sure this will be much better but...

Air is just a bunch of different molecules whizzing around and bashing into each other.

The hotter the air is the faster they move and the harder they bash into each other.

Imagine you had a table top with a circle in the middle and covered it with marbles. If you jostled the marbles gently you could keep loads of marbles in the circle but if you bashed them all around really fast only a few would stay in the circle and the others would go shooting off around the table. Effectively you need more space for the same number of marbles.

It's kind of the same in air. If the air is hotter you need more space to contain the same amount of air. That also means that if you have two spaces both with the same volume (for example a balloon) but at different temperatures the one with the hotter air in it will need fewer air molecules inside it to inflate the balloon. I think you got all that...

Because the hot balloon has got fewer air molecules in it weighs less than the cold balloon.

Weight is just a measure of how much gravity is pulling an object.

So gravity from the earth will be pulling the cold balloon a bit harder than the hot balloon.

In fact, what happens is not that the heat rises, the cold sinks because it's being pulled down harder by gravity and this forces the warmer stuff up.

Funko
05-04-2005, 11:26:36
For another example - it's exactly the same thing as happens when you get in the bath, you are heavier than the bathwater so as you get in you push the water up (this is Archimedes principle, you know the whole Eureka thing).

If you were in space (no gravity) the heat wouldn't 'rise' or 'fall' the air would just slowly even out in temperature.

King_Ghidra
05-04-2005, 11:27:16
yeah that makes sense. Well at least no one turned up and tried to call a 100-0 on me with these last couple of questions

Funko
05-04-2005, 11:28:00
They weren't simple or obvious questions. :)

MoSe
05-04-2005, 11:36:38
Originally posted by Funko
In fact, what happens is not that the heat rises, the cold sinks because it's being pulled down harder by gravity and this forces the warmer stuff up. In fact, I think that's still simplistic... I waited for you to come to say it better, but you failed to satisfy me :p
I didn't want to mention it, but it's all related to buoyancy.
Of course that's caused by gravity to begin with.
But then, the effect of gravity on a fluid is to generate a pressure gradient. A body immersed in a fluid (and this works for a ballon in air just the same way as a plastic ball in water) is pushed by forces from the fluid from all directions, perpendicular to its surface. Summing all those forces up, the net resultant is an upward force. If the object weights more than that force, it sinks, if it weighs less, it "buoys".

For a balloon in air, you could simplify it by saying that it receives stronger pushes from the air from the bottom side upwards than from the top side downwards. Those differences in the push must overcome its weight to lift it up.

The same can be imagined for unbound portions of the same gas

___
EDIT: why did you go on posting? didn't you see I was replying? :rolleyes:

Funko
05-04-2005, 11:52:47
Well... you could say that but I think it's a bit more accurate to say that it's sinking and displacing the warmer air upwards.

Funko
05-04-2005, 11:54:50
Technically there are two forces at work. Gravity is pulling everything down*. The force pushing the warm air upwards is the pressure difference between the cold dense air and the warm air and this can only happen if the warm air is already in the cold air. If the warm air starts above the cold air it can only be displaced upwards as cold air moves downwards...

*towards the earth's center.

MoSe
05-04-2005, 11:56:11
No it's not.
It's more simplistic and less accurate :p

Resource Consumer
05-04-2005, 12:01:09
what's wrong with that

Funko
05-04-2005, 12:03:29
Originally posted by MoSe
No it's not.
It's more simplistic and less accurate :p

Ok, well I didn't read what you said but it's basically exactly the same except I said pressure difference and displacement and you said push up. :p

MoSe
05-04-2005, 12:17:44
Originally posted by Funko
The force pushing the warm air upwards is the pressure difference between the cold dense air and the warm air
that's what I was saying- actually, there are gravity induced pressure differences even in a thermally uniform fluid, and THOSE are what generate the pushing force. The difference in pressure between the hot and the cold fluid part only accounts for the weight of the hot part being lighter than the same volume of cold fluid and thus than the buoyancy force, but it doesn't "generate" any buoyancy in itself

Originally posted by Funko
this can only happen if the warm air is already in the cold air. If the warm air starts above the cold air it can only be displaced upwards as cold air moves downwards...

this I don't quite understand...
I'd say, "of course" :)

We need to have
- a fluid
- gravity, generating a pressure gradient

- a body in the fluid, OR
- differences in temperature within the fluid making hotter parts less dense and thus behaving as a separate buoyant body with respect to the cold parts

There is a pressure gradient also in the hotter parts. As that gradient is related to gravity, hotter=lighter gases generate less buoyancy.
I would even get to say that
"the hot parts push up the cold parts LESS than the cold parts push up the hot parts"
is as accurate as saying
"cold parts are pulled down by gravity MORE than cold parts"
if not even more accurate.

we could find a compromise by combining the two

HOT air: LESS downforce (weight), MORE buoyancy (applied by surrounding cold air)
COLD air: MORE downforce (weight), LESS buoyancy (applied by surrounding hot air)

the system, being free to move, spontaneously positions itself in the state with less potential energy, i.e. with hot air up.

Funko
05-04-2005, 12:22:05
Yes, I don't disagree with you. I just didn't read what you said carefully enough. :)

MoSe
05-04-2005, 12:23:37
Originally posted by Resource Consumer
what's wrong with that

hey, he began to say "this is more accurate".

we were basically saying the same thing, only we were striving to find a dumb enough example to make KG understand :p
And I didn't want KG to lull himself in the illusion he had been able to understand an accurate explanation: if he managed to get it it was because it was more simplified than accurate.
Which is good.

IF you have KG as a goal, and you admit it
:p
;)

MoSe
05-04-2005, 12:24:20
Originally posted by Funko
Yes, I don't disagree with you. I just didn't read what you said carefully enough. :)
and we're out of synch with our posting times, my fault :lol:

Oerdin
05-04-2005, 12:26:35
Why does heat rise? It's density driven; everybody knows that. Molecules with more energy (heat) move and bounce around more so they bump into each other more and they end up having a lower density then cooler molecules. The denser heavier molecules sink below the lighter less dense molecules.

MoSe
05-04-2005, 12:28:27
you're offending KG, Oerdin: he didn't know that!

MoSe
05-04-2005, 12:29:25
and, are you implying, dense=cool?
something must be wromg here? ;) :lol:

miester gandertak
05-04-2005, 12:34:01
yaaay very wromg

MoSe
05-04-2005, 12:36:54
and you're the coolest of us all.
you can sink now

Gary
05-04-2005, 13:59:16
If you have the same quantity of air spread out over a larger volume, it must be less dense.

Now close the door, you're letting the cold in.

Greg W
06-04-2005, 01:58:39
So, KG's brain has less quantity spread out over the normal amount of space?

Darkstar
06-04-2005, 02:10:54
Maybe KG just has less blood spread over the normal amount of space.

King_Ghidra
06-04-2005, 09:23:38
no, no, it's sang froid, that's the english disease

miester gandertak
06-04-2005, 10:57:40
english disease = Supah Kewl.
Yeah right prince Charles.

Venom
06-04-2005, 13:08:00
"we really are saying it has a lot of stored energy"

Is that really correct? Would heat be kinetic energy and not potential? Or has that already been discussed? I didn't want to waste more time in this thread.

Funko
06-04-2005, 13:23:27
In very simple terms - yes. The molecules are moving faster.

In a solid it's more obvious, at absolute zero they don't vibrate at all and as a material gets hotter they vibrate more and more.

Venom
06-04-2005, 13:28:29
So they're wrong or I'm wrong?

Funko
06-04-2005, 13:29:50
Going historically you're probably wrong. I have no idea what your point is or who they are.

Tizzy
06-04-2005, 13:35:15
Everyone's wrong

Venom
06-04-2005, 13:35:54
Well they said heat has more potential energy, I thought that sounded wrong.

Funko
06-04-2005, 13:40:36
If it's not moving up or down and just heating up it'll gain kinetic energy. If it 'rises' it'll increase potential energy and lose a corresponding amount of kinetic energy. Thus slowing down and cooling, if you keep heating at the bottom... well, we just invented convection kids.

Venom
06-04-2005, 13:48:44
Damn if it ain't everything then.

King_Ghidra
06-04-2005, 13:57:17
is wind just a result of air movement as a result of pressure changes then?

and prevailing winds caused by pressure movement between areas that are always hot/cold?

Greg W
06-04-2005, 13:59:28
I believe so.

MoSe
06-04-2005, 14:03:43
I thought it was all a butterfly's fault

Funko
06-04-2005, 14:06:13
Originally posted by King_Ghidra
is wind just a result of air movement as a result of pressure changes then?

and prevailing winds caused by pressure movement between areas that are always hot/cold?

Pretty much yes, hence the mentions of high pressure and low pressure areas in the weather and the use of barometers (pressure gagues) to predict weather.

Venom
06-04-2005, 14:16:26
Low pressure bad. High Pressure good.