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Conduction of heat: Transport of heat through bodies

A piece of a metal bar is inserted into water in a glass. The copper rod is heated by a candle at the other end (Movie 1). Over time, the temperature of the water rises.

A long, thin copper bar is stuck in ice water at one end, and heated electrically at the other end. The bar is not insulated from the surrounding air. The temperature of the bar is measured at eight points along its length (Fig. 1). Temperature values first drop a little. Soon after the heater is turned on, the temperatures rise. The parts of the copper bar near the end that is heated become hot first. For the other points there is a more or less prolonged delay in response (Investigation 14).

Interpretation

Heat (*) is produced by the candle and transferred through the copper rod into the water. The flame is very hot, the copper rod is pretty hot at the heated end and cooler at the end stuck in the water; the water is cooler still. Heat flows from hot bodies to colder ones. Heat accumulates in the water, as a result the temperature rises.

First, some heat slowly flows out of the copper bar (which is at room temperature) into the ice water. Since the heat has to flow from points farther away through the part immersed in the cold water, those parts get colder more slowly.

When the electric heater is turned on, heat (*) is produced and it travels through the length of the bar toward the end stuck in the water. The parts of the bar closer to the heater receive the heat first, so their temperature rises most quickly. The rise of temperature is delayed in the sections of the bar farther away from the heated end. As time progresses, a fixed temperature gradient from hot to cold will be established.

The process of migration of heat through a material is called conduction. It is analogous to the conduction of electricity in metals. As the curves in Fig. 1 suggest, it is also similar to the flow of water through a chain of tanks (see Chapter 1: Phenomena) or the flow of electricity through a series of RC-elements (see Chapter 2: Phenomena).

(*) Note: The technical term for quantities of heat is entropy. What we call heat, and what is officially called entropy by engineers and physicsist is closely related to what scientisits before 1850 called caloric. When presenting phenomena in terms of standard language, it makes sense to keep using the word heat for what we normally call entropy.