Why Thermal Physics is Hot

Updated on July 26, 2008

Heat is Like Money

The best analogy I've ever heard for heat is money. Absolute zero is the temperature at which you have no money. Imagine you have a system - an ice cube in this case - that has no money in it. To get it to heat up, you have to dump money into it.

One thing that a lot of people don't get is what is known as Heat Capacity. It is how much money is required to get the system to raise in temperature - usually by one degree celsius. If you want to subdivide the system into mass, the specific heat is the heat capacity for one unit of mass - usually grams.

So it might take two dollars to raise the temperature of steel by one degree celsius, but it will take four point eight dollars to make the temperature of water raise by one degree celsius. This is of course if they have the same mass.

Why is Being Wet so Much Colder?

You know all that business about four point eight dollars to raise a gram of water by one degree celsius? There are some points where adding energy into the system will not raise the temperature. These points are phase changes - the point where ice melts and water boils. You can have ice at 32 degrees fahrenheit or cold water at 32 fahrenheit. You can have boiling water at 212 or you can have steam at 212.

When you phase change water into vapor, it draws a ton of energy away from the water. The water loses the money and gets colder. Now when your body is dry, you tend to lose a little heat to the environment. Your body is naturally built to sustain that loss of heat. When you get wet, suddenly the air around you blows across that water and phase changes it to vapor, sucking the heat out of the water.

That vaporization process will cool you down twenty-five times faster than just standing in cold air when dry.

Why Does Water at Sixty Degrees Feel Colder Than Air at Sixty Degrees?

I always wondered why I could be so comfortable jogging in sixty degree weather but it feels so stinging chilly jumping into a pool that is sixty degrees. It all comes back to that heat capacity stuff. You see, the air around you is roughly a thousand times less dense than water (actually it's even less than that, but a thousand is a nice round number).

This means that it takes your body say ten dollars to heat up a certain amount of air around your body by five degrees. Then it takes another fifteen dollars to heat it up the next five degrees. Ten degrees in the air for twenty-five dollars. But when you jump into a pool at sixty degrees, it takes a thousand dollars to raise the same volume of water around your body by five degrees and another fifteen hundred to get the next five degrees. My math isn't exact, but you can see how water will suck out a lot more energy than air will, making it feel cold.

That's why you can be just fine in seventy-degree air and die of hypothermia in seventy-degree water. It's also why after a while being under the surface of the water feels warmer than when you get out of the water.

What is this Phase Change you Speak of?

Phase change is when a solid melts into a liquid or a liquid boils into a vapor. There is an equilibrium temperature between the two. When you dump energy (heat/dollars) into the system, it won't heat up but it will progress the phase change. Mathematically, this energy didn't just vanish. You must account for this energy with the phase change. So when you boil water, all that energy to boil it is transformed into what is known as Entropy. Entropy can loosely be defined as the randomness of the system. Vapor is more random than liquid, liquid more random than solid, hot more random than cold.

In the analogy with dollars, entropy is what is taxed off when paying to heat something up. Oddy enough, you get taxed a second time when cooling it down. Entropy in the overall universe will rise with each individual process involving energy. You can move energy from one place to another and every time you do entropy gets taxed off. Life's a bitch.

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• spryte

10 years ago from Arizona, USA

There is NO fact that is so insignificant, BT, CW and I won't pick it to death. I'm only surpised Eric, Sixty and Shade aren't here yet...

• AUTHOR

Science Guru

10 years ago

I didn't know one detail could create a discussion like this. Wow. Anyway, it all has to do with how fast heat is drawn out of the body. We can sustain 70 degrees in air because heat is drawn out slowly and we replenish it as rapidly as it goes.

• spryte

10 years ago from Arizona, USA

Actually, CW...hypothermia is possible even up to 26C (or around 80F). It might take hours, and you would probably suffer more from dehydration or exhaustion or excessive pruney fingers before hypothermia...but it is possible.

And as for whether or not 70 degrees isn't very cool...brrrr! When the pool temp drops that low, it's time to close it for the season. :) I know, I know...since moving to AZ I've become a major wussy....

• Constant Walker

10 years ago from Springfield, Oregon

70 degree water isn't very cold.

• AUTHOR

Science Guru

10 years ago

I was once told a seventy degree pool could kill you if you didn't stay moving in it. However as I obviously have not experienced this myself, it is hard for me to say for 100% certain.

• B.T. Evilpants

10 years ago from Hell, MI

I've never thought of comparing entropy to taxes. Interesting idea, since it is believed that entropy, when left to its own devices, tends to increase over time.

• Constant Walker

10 years ago from Springfield, Oregon

Interesting take. But, I disagree with dieing of hypothermia in 70 degree water. While in Long Beach during summers, my friends and I spent long days in my uncle's built-in pool, which he kept at a wonderful 70 degrees.

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