Overview and Predicting Expansion
– Thermal expansion is the tendency of matter to change its shape, area, volume, and density in response to a change in temperature.
– Temperature is a function of the average molecular kinetic energy of a substance.
– Molecules vibrate and move more when heated, creating more distance between themselves.
– Substances that contract with increasing temperature are unusual and occur within limited temperature ranges.
– The coefficient of linear thermal expansion varies with temperature and is a measure of the relative expansion divided by the change in temperature.
– An equation of state can be used to predict thermal expansion values at different temperatures and pressures.
– This equation can also predict other state functions.
– Predicting thermal expansion is useful in various applications and industries.
Contraction Effects and Factors Affecting Thermal Expansion
– Some materials contract on heating within certain temperature ranges, known as negative thermal expansion.
– Water exhibits negative thermal expansion below 3.983°C, resulting in maximum density at this temperature.
– Fairly pure silicon has a negative coefficient of thermal expansion for temperatures between 18 and 120 kelvin.
– ALLVAR Alloy 30, a titanium alloy, shows anisotropic negative thermal expansion across a wide range of temperatures.
– Solid materials tend to maintain their shape during thermal expansion, unlike gases or liquids.
– Thermal expansion decreases with increasing bond energy and affects the melting point of solids.
– Liquids generally expand slightly more than solids, and glasses have slightly higher thermal expansion compared to crystals.
– The glass transition temperature leads to characteristic discontinuities in coefficient of thermal expansion and specific heat.
– Absorption or desorption of water can significantly change the size of many common materials.
Effect on Density
– Thermal expansion changes the space between particles of a substance, altering its volume and density.
– Changes in density affect buoyant forces and play a role in convection of unevenly heated fluid masses.
– Thermal expansion contributes to wind and ocean currents.
– The change in density due to thermal expansion is crucial in various natural phenomena.
– The effect of thermal expansion on density can be observed in everyday life.
Linear Expansion, Area Expansion, and Volume Expansion
– Linear expansion is the change in length of a material due to thermal expansion.
– The rate of change of linear dimension per unit change in temperature is represented by dL/dT.
– Change in linear dimension can be estimated as ΔL/L = αLΔT, where αL is the linear coefficient of thermal expansion.
– Area expansion is the change in area dimensions of a material due to thermal expansion.
– The area thermal expansion coefficient αA relates change in area to change in temperature.
– Change in area can be estimated as ΔA/A = αAΔT.
– Volume expansion is the change in volume of a material due to thermal expansion.
– The volumetric thermal expansion coefficient αV relates change in volume to change in temperature.
– Change in volume can be calculated as ΔV/V = αVΔT.
– Isotropic materials have equal expansion in three mutually orthogonal directions, while anisotropic materials have different expansion coefficients in different directions.
Applications of Thermal Expansion and Control of Thermal Expansion
– Thermal expansion is used in bimetallic strips, thermometers, and various mechanical applications.
– Expansion and contraction of materials must be considered in designing structures, land surveys, and casting molds.
– Alloys with small linear expansion coefficients, like Invar 36, are used in aerospace applications and where temperature swings may occur.
– Thermal expansion is utilized in engineering applications such as metal-framed windows, rubber tires, and precision engineering.
– Control of thermal expansion is important in brittle materials like glass and ceramics, as well as in gasoline storage and various thermometers. Source: https://en.wikipedia.org/wiki/Coefficient_of_thermal_expansion
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Thermal expansion is the tendency of matter to change its shape, area, volume, and density in response to a change in temperature, usually not including phase transitions.
Temperature is a monotonic function of the average molecular kinetic energy of a substance. When a substance is heated, molecules begin to vibrate and move more, usually creating more distance between themselves. Substances which contract with increasing temperature are unusual, and only occur within limited temperature ranges (see examples below). The relative expansion (also called strain) divided by the change in temperature is called the material's coefficient of linear thermal expansion and generally varies with temperature. As energy in particles increases, they start moving faster and faster, weakening the intermolecular forces between them and therefore expanding the substance.
