After my challenge yesterday to tell me what a car was, I have a new challenge. It proves not all I learn is useless (honest). I will answer later if nobody gives a good answer - it's interesting I think! *hence most won't lol*

The breath that comes out of your mouth is hotter than the ambient temperature.

Prove this by breathing onto the back of your hand, it should feel warmer (if not, 999).

So the air we breath out is hotter, why do we blow on food then to cool it?



Second:

Breath on the back of your hand again, it's hot (again, if not, 999)

now move your hand slightly further away and blow.

It feels cold right?

Hang on, it was hot when blown out before but colder than your hand when blown.

Try doing it at different distances, you should find the further away the colder it is (attempt to keep same flowrate of air).

Why is this?


notes, heat is a form of energy and can only travel "downhill" i.e. hot to cold. This is VERY important in understanding the problem.

Answer Part 1, the food is even hotter than the air from your mouth and therefore cools it.

Part 2 Probably because it has chance to be cooled by the surrounding air as it has a longer distance to travel.

Andy

Not bad but requires a lot more work to part 1 (think, why would you blow on hotter air if the ambient air temp is higher?)

Part 2 is ok but try doing it over the same different and do the two different methods, you will see that the faster velocity still makes it colder. So for a given distance there's still a change.

The Negotiator said:

Not bad but requires a lot more work to part 1 (think, why would you blow on hotter air if the ambient air temp is higher?)

Part 2 is ok but try doing it over the same different and do the two different methods, you will see that the faster velocity still makes it colder. So for a given distance there's still a change.

Click to expand...

Part one - Probably something to do with the air molecules being slowed down when compressed through a smaller gap (When blowing) so thats why its cold when you blow... and something about the surface area of the air being blown onto too

Answer to part one, blowing on food allows water molecules with higher energy to escape from the food, thus the total amount of energy within the food is less thus its cooler.

The answer to part 1 is the answer to part two, by blowing on your hand from a distance, you're blowing off molecules of water molecules which have the most energy, your hand feels colder because its heating up any water on the surface of your skin (swear for example)

It *could* be to do with the area but we decided otherwise as the technical reason is really the over-riding factor.

Again,it could have something to do with water molecules but, well, as I say, heat is energy at it can only flow from hot to cold. I don't think anybody has answered why the higher velocity air is cold and the low velocity air is warm?

All good answers just none (IMO) hit the nail of the head yet, I suppose I am being unfair because it does need a bit of engineering knowledge to use the logic that follows.

To help, renolds number is proportional to the velocity of the air, the higher the renolds, the higher the turbulence.

paul, is this something to do with P=V/T?

No, but the equation you're thinking of is:

Pv = RT => P =RT/v

v is specific, i.e per mole, otherwise PV = nRT

It's very much qualatitive

The Negotiator said:

No, but the equation you're thinking of is:

Pv = RT => P =RT/v

v is specific, i.e per mole, otherwise PV = nRT

It's very much qualatitive

Click to expand...

Hmmm that sounds familiar. Now why didn't I do a degree in physics?

The Negotiator said:

Not bad but requires a lot more work to part 1 (think, why would you blow on hotter air if the ambient air temp is higher?)

Click to expand...

Nothing to do with ambient air, it is the ability of the air to remove heat as a transport medium. Even if the air you blow is warmer than ambient air it is the fact that you are blowing the air which is cooler than the food. This means the air flow has the ability to remove heat quicker than waiting for the ambient air to remove the heat without any air flow.

The Negotiator said:

ThePart 2 is ok but try doing it over the same different and do the two different methods, you will see that the faster velocity still makes it colder. So for a given distance there's still a change.

Click to expand...

Same story as part one.

Andy

The Negotiator said:

No, but the equation you're thinking of is:

Pv = RT => P =RT/v

v is specific, i.e per mole, otherwise PV = nRT

It's very much qualatitive

Click to expand...

Hey, I remember that equation from my GCSEs

No real need for it in my life now, though.

Poggy is right, the moving air over the top (be it warm or cold) aids convection, faster the flow rate the higher the turbulence, higher the turbulence the higher it carries away the heat.

Second thing is similar and the above should help you answer.

Does that mean you can get me a Chemistry degree .

Andy

CHemical Engineering, very different! But still no...!

The Negotiator said:

CHemical Engineering, very different! But still no...!

Click to expand...

Ooooooohhhh. I'll just have to buy one on the internet then.

Andy

You mean the other two you bought aren't enough?!

I thought being a Mpharm and Mchem would have helped your degree, especially with that PhD from Oxford University Tennesee

poggy said:

Ooooooohhhh. I'll just have to buy one on the internet then.

Andy

Click to expand...

Why not, me and Jules got Lord & Lady status off Letsbuyit.com

On top of your acre of moon?! I bought several stars last week, bargain at $40!

I'll have you know Oxford University Tennesee is fine institution, their degrees cost a lot more than the others and come gift wrapped.