How can objects in the same room be different temperatures?

Conductors versus insulators


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How can objects in the same room be different temperatures?

Asked by David Webb of Hayle, Cornwall, UK

The sense of touch allows us to recognise temperature differences of objects in the same room which we could easily assume should all be at that same room temperature. But this is not the case. Some objects feel colder than others. Why?

The temperature of an object is dependant on its heat capacity and the rate at which heat can be removed from its surface. The energy required to heat up a certain volume of material differs.

Temperature is the average kinetic energy of atoms. The heat capacity per unit volume is dependant on the number of atoms per unit volume. Heat conduction is also closely related to the atom number density of a material. Heat conduction also depends on the structure and capability of the atoms to move within the material and pass on their energy to other atoms.

Marble is a dense solid and requires a great deal of energy to pass from your body to warm up. It also has a reasonable rate of heat conduction, so the surface remains cold for some time when in contact with your skin.

Cloth is not a dense solid. Most of its volume is occupied by air. It, therefore, warms up very quickly to the temperature of the skin, at which point it prevents further heat loss and feels warm. This is why cloth makes good clothing. In reality, cloth never reaches skin temperature since it is constantly being cooled by cooler air replacing warmer air. The main cooling process for cloth is convection, not conduction. It is the opposite for marble.

Styrofoam is an insulator and a very poor conductor of heat. When you hold a Styrofoam cup, the heat flows from your hand and warms the Styrofoam surface. The surface of the Styrofoam cup becomes nearly as warm as your hand because the heat is not conducted away quickly, thus little additional heat leaves the skin.

Metal is a good conductor of heat. When you touch metal, heat is carried away from your hand into the metal and away from the metal surface and your skin surface. So metal feels cool.

Stephen Juan, Ph.D. is an anthropologist at the University of Sydney. Email your Odd Body questions to s.juan@edfac.usyd.edu.au


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