[ianhutt (OP)]

ian hutt  asked the Naked Scientists:

Once a bicycle tyre is inflated and under pressure will it retain a higher temperature as compared to an identical tyre which is flat. I presume that as the force i.e. the pump has been removed there is no additional
available energy to be transmitted as heat although presumably the inflated tyre must retain some potential energy.

The newbielink:http://www.thenakedscientists.com/HTML/podcasts/ [nonactive] always is stimulating and thought provoking- Keep up the excellent work!

Regards and best wishes

Ian Hutt (Little Chalfont, Bucks)

What do you think?

[graham.d]

No. The temperature will equalise to the same as that of the surrounding environment. You are right that the tyre will have some potential enery if under pressure. If you deflate a tyre with your finger over a valve in can get quite hot!

A related point that is interesting is that if using a tyre that is underinflated it will tend to get hot because of the wasted energy in continually distorting the tyre as it is moving over the road; the extra energy in riding a vehicle with a poorly inflated tyre has to go somewhere. A fully inflated tyre gives the lowest resistance to motion and will dissipate least heat. I suppose over-inflation only has the disadvantage in reducing comfort and roadholding (and probably the tyre life).

[lyner]

If you deflate a tyre with your finger over a valve in can get quite hot!

What would get hot? The air in the tyre would drop in temperature because the gas laws apply and there would be an energy transfer. The air blowing against your finger would also be cooler, for the same reason (it had expanded against the atmosphere).

The converse is definitely true; compressing air into a diver's cylinder can make it too hot to hold and they have to be filled under cold water. If you don't cool them when filling them you will end up with a lower pressure than you wanted when they cool down- less dive time.

You are definitely right about underinflated tyres getting hot and damaging themselves, though.

[graham.d]

Your finger, if you let the air out whilst restricting the flow with said digit. The potential energy of the compressed air moving at velocity passed your finger will heat your finger. If some distance away from the valve it will feel cool because the forced air convection wins in cooling your finger.

[lyner]

I'm not so sure about that. Remember, the emerging air is already colder and although its bulk velocity is fairly high, the mean velocity (aka kinetic energy of each molecule aka temperature) will be lower so I wonder whether there would be energy available for warming up your finger.
If you have actually experienced this then I would reluctantly agree that there is something in what you say but it doesn't seem likely.

I would agree, however, that you could use the KE of the emerging air to drive a turbine which could then produce some heat for you - either by friction or an electrical generator / heater.

[Bored chemist]

Just for a start, it's not convection.
If you put your finger on the valve so the escaping air pushes it away and it bounces back with an amusing rude noise then the internal friction will convert some of that mechanical energy into heat.
In the absense of such a mechanism the air cools by the so called Joule-Thomson effect.
http://en.wikipedia.org/wiki/Joule-Thomson_effect

[graham.d]

You mean you don't pump up your tyre with an ideal gas? Oh dear :-)

Do you know what temperature drop to expect from a pressure change from a max of about 4 atmosheres (I think that is a typical max level for a bike tyre) to 1 atmosphere with air (probably can assume nitrogen)?

I think I can remember getting a hot finger letting down a tyre but it would have been a while ago. It seems reasonable that the friction of air whizzing past at speed could have that effect. Certainly when it makes the rude noises that is dissipating a fair bit of energy. I suggest a practical experiment for the energetic and interested.

[lyner]

Just for a start, it's not convection

BC it is, actually convection which transfers the heat from finger to gas or the other way round - it's forced convection. To be fair, convection is the least well defined transfer process because it is not as fundamental as radiation and conduction.

PV = RT
 is the equation to use to give you the change in temperature (of an ideal gas, that is).

[graham.d]

The Joule-Thomson effect does not apply to an ideal gas though. At normal temperatures and pressures all gases except helium and Neon (I think, from reading up on it) cool when forced through a constriction. Helium and Neon heat up. An ideal gas is unaffected.

I was just wondering though whether forced air convection is really the dominant effect here though. My guess is it is but I have not done any calculations - there must be a simple graph somewhere.

[lyner]

With or without Joule Thompson, won't the temperature of the uncompressed gas  be lower than that of the compressed gas?  If work has been done by the air jet (if it is not completely 'free expansion'), energy has to come from somewhere. Or does it come from the remaining air, compressed in the tyre?
I found that bit about Helium and Neon having low inversion temperatures was interesting - good old Wikkers.

[graham.d]

Hmm, thermodynamics (or what kT did next) is not my strongest subject. As far as I can tell the JT effect assumes "free expansion" and the methods of achieving this are through a constriction and the contricting valve must not conduct so as to allow much heat tranference. The JT effect does not apply to an ideal gas but it would suggest that the criterion of use of an insulated constricting valve is classed as a free expansion. If this is the case, and no work is done by the escaping gas, an ideal gas does not change in temperature.

[lyner]

I wonder how 'free' is free. It would need to be venting into a low pressure and not be doing any work on the surrounding air, I suppose.
It's an interesting question - very TNS.

[chris]

All I can say is that the last time I held a gas-propelled deodourant too close to my armpit for too long, despite the noise and the resistance imposed on the gas flow by my skin, it was bloody cold!

[lyner]

I think that was droplets of the propellant evaporating from your skin.

[chris]

No, that's not correct. It's most definitely the expansion of the gas because a) the cannister itself becomes cold and b) I have breathed from a free-flowing scuba regulator underwater (it's part of the open-water certificate) and the gas hitting your teeth gives you the world's worst ice-cream headache - it is seriously cold.

Chris

[Bored chemist]

Chris,
what, apart from the propellant boiling away in the can, do you think drives the stuff out of an aerosol?
Since it's boiling it has to get the latent heat from somewhere so it gets cold.

[lyner]

The diving scenario is a bit different. I think that the air from a demand valve feels very cold partly because it is extremely dry and causes an unusual level of evaporation. (The air is dried in the compressor, I believe.) There is also a fair amount of work done (against the hydrostatic pressure at depth); it is definitely not free expansion, so that the energy transferred will result in a (gas law) drop in temperature. It feels cool right down in your lungs - I seem to remember.

What happens in the can is not the same as what happens to the spray jet. Inside the can you can get ordinary gas-law cooling plus cooling due to change of state.
You will get Joule thompson cooling of the escaping gas plus further evaporative cooling of the propellant which emerges as droplets.