UCSB Science Line
 Let's say you have a copper plate. The copper plate is at 30 degrees celsius. That's 86 degrees fahrenheit. Why is it that air at 86 degrees fahrenheit doesn't feel as hot to your skin as copper at 86 degrees fahrenheit? Another aspect of my question: Heat is the average kinetic energy of a substance, average translational energy I believe, ie the motion of the particles, in a gas the particles have kinetic energy, and move about chaotically, and collectively this energy is the substance's heat energy/internal energy. In air, the particles are free to move as described above. In the copper, the total heat energy of the particle is described by the average energy of the particles, which i suppose are vibrating, not moving about translationally. So let's say you have your copper plate at 86 degrees fahrenheit, and you have a certain volume of air at say 80. The metal plate will warm the air, transferring heat, until the air reaches 86. The whole system will probably settle at a common temperature of like 83 or whatever, and in reality it would eventually just reach a lower equilibrium as more and more heat dissipated. But I don't know if I am correct in this. It seems like the copper plate at 86 degrees is much hotter than the air at 86 degrees, and the plate will continue to dump heat into the air. Maybe I'm wrong. What about water? What if you have an 86 degree copper plate in contact with water at say 84 degrees. It just seems to me like the plate feels much hotter than the water, yet it would seem thermal equilibrium would tell us that the copper plate will only heat the water two more degrees, and then no more heat transfer will occur. Is this correct? Thank you very much for your help. Question Date: 2004-02-07 Answer 1:It's a very good question and I will try to do it bit by bit. The answer to your first question involves the ease at which heat is transferred by air vs. copper. Air it turns out is a horrible conductor of heat. Engineers often take advantage of this. If you go out by a decent thermos bottle for coffee and you saw it in half you will find two walls separated by air with a few joints for stability. Because the air can't conduct heat well, the thermos can keep the coffee hot for hours. The same is true for double panned windows for houses in which two sheets of glass are separated by a small amount of air. This design can save a significant amount of money in heating costs. Copper on the other hand is an incredible good thermal conductor. We actually use in my research on LEDs and Lasers. Both types of devices heat up as they work which reduces the efficiency of the devices and decreases their reliability. By placing the devices on small pieces of copper the heat is removed rapidly. your are quite right that heat is the average kinetic energy of a substance, average translational energy IE the motion of the particles, in a gas the particles have kinetic energy, and move about chaotically, and collectively this energy is the substance's heat energy/internal energy. In air, the particles are free to move as described above. In the copper, the total heat energy of the particle is described by the average energy of the particles, which I suppose are vibrating, not moving about translationally. The answer to the next part of your question really depends on your experimental set up. For instance, how much air is there, is the hot plate still being heated. The best way to think about the problem is how much thermal energy is there at the very beginning. Unless you add to that energy, change its form, or remove it, you will always have that energy. No more and no less. So if you had a 100 gram plate, and 1 gram of air. Given enough time the air and plate would eventually come to equilibrium in which they have the same amount of thermal energy, i.e. the same temperature. In this case the plate wouldn't change temperature very much. As a back of the envelope calculation: (100*86 + 1*80)/101 = 85.94. But if you had your whole house of air it would go the other way and the plate would cool to the temperature of the air. This happens all the time when cooking and you withdraw a pot from the stove. It all comes down to thermal conductivity. An example of this, say it was really cold last night and you leave a plastic bucket, and a metal bucket outside. Eventually they both reach the air's temperature. If you pick one up which feels colder? The metal bucket, because it is a good thermal conductor it can suck your thermal energy away much faster. But eventually it you held onto them both, each would draw the same amount of thermal energy. (Assuming right balance of sizes etc.) Just one does it faster than the other. In relation to the last part of the question - the same thing applies for water as air except for two things. It is denser which means that it can absorb more energy per volume, and it has a higher thermal conductivity, which means it will suck the heat faster. But thermal equilibrium will still be reached. Very good question and I hope this helped.Click Here to return to the search form.

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