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Thermodynamics basic concepts

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shreder
#1
Mar19-14, 08:59 AM
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I) is it physically correct to say that something is hot or cold

II) if something is cold do you say that it has a high heat capacity or high thermal conductivity
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dauto
#2
Mar19-14, 09:23 AM
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I) Hot and cold are relative terms. You must compare the temperature of two objects. The one with higher temperature will be warm while the other one is cold.

II) Neither. Cold means low temperature. It has nothing to do with high heat capacity or high thermal conductivity
shreder
#3
Mar19-14, 09:30 AM
P: 9
what if you touch two object that are in thermal equilibrium with each other and one of them feels like its colder than the other. does that body have a high thermal conductivity or a high heat capacity
(the object feels cold because your hands are at a higher temperature than the object)

Andrew Mason
#4
Mar19-14, 10:39 AM
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Thermodynamics basic concepts

Quote Quote by shreder View Post
what if you touch two object that are in thermal equilibrium with each other and one of them feels like its colder than the other. does that body have a high thermal conductivity or a high heat capacity
(the object feels cold because your hands are at a higher temperature than the object)
If they are in thermal equilibrium with each other they are at the same temperature. Temperature is a measure of the average translational kinetic energy of molecules. So the molecules of both substances have the same translational kinetic energy.

"Feeling colder" refers to the way your body senses temperature. It senses heat flow, not molecular kinetic energy directly. If your body loses heat rapidly, it senses "cold". If you were to touch a very cold piece of wool it would not feel very cold because heat will not flow out of your skin to the wool very rapidly. If, however, you touched a piece of metal, heat would flow quickly and you would sense that it is very cold.

This is why we use thermometers rather than our fingers to measure temperature.

AM
Chestermiller
#5
Mar19-14, 02:47 PM
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If you bring together two flat slabs of different materials at different starting temperatures, and only heat conduction is involved, then the interface between the two materials will almost instantly come to a temperature intermediate between the bulk temperatures of the two materials (and remain at that temperature for a long period). The interface temperature will depend on the thermal conductivities, the heat capacities, and the densities of the materials. Something similar happens when you touch an object with your hand, although conduction will not be the only mode of heat transfer on your side of the interface. See Carslaw and Jaeger, Conduction of Heat in Solids for the solution to the conductive slab problem.

Chet


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