Истории и востоковедения
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- Thermal Expansion at the atomic level
| Thermal Expansion
When a material is heated or cooled, it changes its dimensions. Generally, it expands when heated and contracts when cooled although there can be exceptions to this rule. Examples: If water is gradually cooled, it shrinks in size as expected. But at 3.98 ºC it begins to expand again until it turns to ice at 0 ºC. This expansion is peculiar to water and is associated with the unusual shape of the water molecule. This behavior explains why lakes freeze from the top downwards in winter. The colder water is at the top of the lake because it expands and becomes less dense. When this water freezes it insulates the water below it from the outside cold air like a blanket. It is because of this property many fish can survive the winter rather than becoming part of a giant Popsicle. When you first turn on a hot water tap, the water rushes out but is still cold. When it starts to become hot, the flow of water starts to become less and in some cases it stops. This can be explained as follows: the hot water heats the metal valve inside the tap which expands to block off any more flow of water. The change in length of a solid is related to the original length and the change in temperature. , where ∆ L is the change in length, L 0 is the original length before the change, ∆T is the change in temperature, and is the linear thermal coefficient of expansion, which is different for different materials. 44 Thermal Expansion at the atomic level At the atomic level, thermal expansion means there is an increase in the average spacing between atoms. As a particular atom oscillates about its equilibrium position, it experiences an asymmetric potential energy as shown in figure 2. If it moves towards another atom it experiences a very steep rise in the potential energy. Whereas if it moves away from the other atom, it experiences a relatively slow increase in the potential energy and so travels much further. The asymmetry in the potential energy curve leads to a shift in the average position of the atom. (Adopted from жүктеу/скачать 1,14 Mb. Достарыңызбен бөлісу:
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