Stupid question

[TMcD_in_MI]

I remember seeing an article about some guys that built a go cart that ran on compressed air. They compressed the air by freezing a high pressure tank (like a n oxygen tank from a cutting torch) half full of water.

Hard to see how that would give you much air pressure. If the half that is water expands by 9%, then the pressure of the air in the other half will only go up by a shade under 10%. So from 15psi you would get to 16.5 psi. Not a lot, right?

I guess we're getting pretty far away from the original topic, now. Sorry.

Tom

 

[mjarrels]

On a bitter cold day like today, does the refrigerator in the unheated garage become a heater?

mark

 

[tallyho8]

On a bitter cold day like today, does the refrigerator in the unheated garage become a heater?
mark

Actually, it does. (sort of/sometimes) The refrigerant pressure gets so low in cold temperatures that if the compressor comes on it will not build up enough pressure to cool. (Like having a unit that is out of freon). Many times the thermostat sensor will get so cold that it will not turn on the compressor and the insulation prevents the colder outside air from getting into the refrigerator thus keeping it warmer in the refrigerator than in the garage. In my 37 years of owning a refrigeration business I have found this to be true many times.

 

[hazmat]

What happends if you have a steel "ball" filled completely with water and so strong that the freezing pressure cant crack it, and you put it in a cold place?
Will the water remain liquid? No matter how cold it is?

( not much on TV tonight)

Check out this phase diagram (msn)

Increasing the pressure will decrease the freezing point, but it will eventually freeze. The steel will strech some. You'll end up with a ball of high pressure ice (assuming the ball is strong enough that it won't rupture...)

 

[SPYDERLK]

Doc - I was taught this and you were taught this, but it seems to be one of those things that when people actually got around to investigating it seriously, they found that it just wasn't true. It turns out that the pressure of a skate blade is enough to lower the freezing point by less than one degree C - not enough to melt the ice unless it is already really close to its melting temperature. If you would like, I'd be happy to email a classroom handout to you and you can do the calculation yourself.

It's too bad, isn't it? It made such a clever explanation.

Tom

It is true. You cant skate very well on very cold ice. Too much friction. Normal ice on a lake will not be much colder than 0C if it isnt pretty thick. Right under that ice is water that is 0 and that holds the ice T near 0. Also the continuous forming of new ice as it thickens holds the ice sheet near the freezing point because the latent heat of fusion is given up as the new ice forms at 0C - and it is given up directly to the ice sheet. This keeps the sheet "warm" wrt air.
Larry

 

[TMcD_in_MI]

Normal ice on a lake will not be much colder than 0C if it isnt pretty thick. Right under that ice is water that is 0 and that holds the ice T near 0. Also the continuous forming of new ice as it thickens holds the ice sheet near the freezing point because the latent heat of fusion is given up as the new ice forms at 0C - and it is given up directly to the ice sheet. This keeps the sheet "warm" wrt air.

Larry - That is an interesting point of view that I don't think I have heard before. You seem to be saying that if we have, say, eight inches of ice with a typical winter air temperature of -7 C, that the temperature of the top mm of ice will be at the same temperature as the bottom mm which is next to the 0 C water. Wouldn't you expect to see a temperature gradient through the ice sheet, from 0 C on the bottom to pretty close to -7 C on the top? Ice is not very good at transferring heat, right?

I hear what you are saying about the latent heat of fusion being released to the ice, but (1) that is happening at the bottom of the ice sheet, and (2) some of that heat will certainly be given up to the surrounding water as well as the ice.

Can you give me any references to support your point of view?

Here's one to support mine -http://www.princeton.edu/~lehmann/BadChemistry.html


Regards,

Tom

 

[SPYDERLK]

Tom, good point about the gradient. I did cover that in my message, but not explicitly. I would consider 8 inches "pretty thick" - and the surface would definitely be below 0C, but would definitely not be at -7C because of conductivity in the solid. Im extrapolating downward to reasonable 2-3" ice for iceskating and saying its surface would be in the range of -1 to 0C required in your initial point.
Since Im borderline illiterate in attaching things, Ill leave it to you to look. I will too. Try something like "coefficient of friction of ice".
Gotta go read your link.
larry

Did. I think a contributory issue is the frictional heating, but not in the way the link alludes. The major heat would come from the distortion and compression of the ice by the skate blade. It would be extemely localized so the actual amount of heat would not have to be much.

 

[TMcD_in_MI]

...I think a contributory issue is the frictional heating, but not in the way the link alludes. The major heat would come from the distortion and compression of the ice by the skate blade. It would be extemely localized so the actual amount of heat would not have to be much.

Larry - Thanks for responding. I really can't argue with the new issue you have brought up regarding compressive heat, since I don't know enough about that. My only point was that pressure-induced melting point depression is no longer considered a sufficient explanation for why ice skates slide so easily on ice. Here is another link you may want to read where they mention that ice is actually most slippery at -7 C. Ice skating - Wikipedia, the free encyclopedia

Given that the pressure of an ice skate can lower the melting point by only a few tenths of one degree C, it's hard to see how just that could melt the ice under the blade at -7 C, don't you think?


Regards,
Tom

 

[SPYDERLK]

Tom. No, I dont think just the pressure lowering the freeze point could do it. That interesting link says theres always a film of water tho. I wonder if they mean that occurs at the transition to air only, or all transitions. In the case of the latter I wouldnt expect to see ice be able to stick to a smooth surface - and it can. If it doesnt occur at other transitions than to air I would expect a cold tire to instantly stick to cold ice as the surface water froze. Or for that matter ditto an iceskate. Need more input.

By the way, that -7 sweet point is for active sliding [kinetic min]. I wonder where the static break point is lowest. Im guessing 0C.
larry

 

[TMcD_in_MI]

By the way, that -7 sweet point is for active sliding [kinetic min]. I wonder where the static break point is lowest. Im guessing 0C.
larry

I wonder too.

Good talking to you, Larry.

Tom