Microscopic interpretation of conduction

  Fig. 1-3 Schematic illustration of heat conduction through a rod. Particles on the hot side vibrate more rapidly than that of the cold side.

But what really happens in the process of conduction? Let's examine the process in the microscopic point of view. Recall that the internal energy of an object leads to the random vibrations of its particles (atoms or molecules), and that temperature is actually a measure of the average kinetic energy of the vibrating particles. When one end of an object is heated, its temperature rises, the kinetic energies of the heated particles increases and the particles vibrate more rapidly and with larger amplitudes. These rapidly vibrating particles set the neighbouring particles to vibrate faster, transferring some vibration energy to them. This in turn affects other neighbouring particles, and energy is thus transferred from the hotter side to the colder side along the object.

The animation below illustrates schematically the conduction of heat through a rod. It is seen that the temperature, as well as the degree of vibration of its constituent particles, varies continuously from the hot side to the cold side of the rod.

Flash animation: Conduction of Heat

Conduction also happens when a hot object is placed in direct contact with a cold object. The more energetic particles of the hot object transfer some energy to the less energetic particles of the cold object.

Conduction is especially important in solids where the particles are held together by strong intermolecular forces. In liquids, the intermolecular forces between the particles are weaker than that in solids. Vibration energy of particles is less easily transmitted through a liquid and therefore liquids are usually poorer conductors of heat. Gases are very poor conductors of heat because their particles are widely separated so that collisions do not occur frequently to enable efficient energy transfer.

  Fig. 1-4 This experiment demonstrates that water is a poor conductor of heat. The ice at the lower part of the test tube does not melt quickly even when the water boils at the top.
The video below shows an interesting experiment demonstrating that water is a poor conductor of heat. A test tube filled with water has a piece of ice fixed at the bottom by a wire gauze. The upper part of test tube is heated with a Bunsen burner until the water boils there. Interestingly, you will discover that the ice at the lower part of the test tube does not melt quickly even when the water boils at the top. Can you explain why?

Water is a poor conductor of heat
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