# Heat Energy II: Thermal Expansion

Expansivity or Expansion is the amount a material expands or contracts per unit length due to a one-degree change in temperature.

CONTENT

CONCEPT OF EXPANSIVITY

THERMAL EXPANSIVITY

CONSEQUENCES AND APPLICATION OF EXPANSION

CONCEPT OF EXPANSIVITY

Thermal expansivity is the tendency of matter to change in volume in response to a change in temperature. It is the tendency of matter to change in volume in response to a change in temperature. (An example of this is the buckling of railroad track, as seen in. ) Atoms and molecules in a solid, for instance, constantly oscillate around its equilibrium point. This kind of excitation is called thermal motion. When a substance is heated, its constituent particles begin moving more, thus maintaining a greater average separation with their neighboring particles. The degree of expansion divided by the change in temperature is called the material’s coefficient of thermal expansivity; it generally varies with temperature.

THERMAL EXPANSIVITY

Linear expansivity, Area expansivity and Volume or cubic expansivity are all parameters that are being governed by heat energy. All three parameter is dependent on temperature rise which is primarily in Celsius unit of temperature.

• Linear expansivity (α)

This is an increase in the fraction of the length of the solid sample and an increase in the unit temperature. If the length of the sample increases from l1 to l2 when the temperature increases θ°, the expansion rate (α) is given by: l2=l1 (1+αθ). This relationship assumes that α has nothing to do with temperature.

α =    l2 – l1      = e__

l1 (Q2-Q1)      l1

• Area and Volume expansivity ()

This is a fractional increase in the surface area of the solid caused by an increase in temperature per unit, i.e. A2=A1(1+βθ), where β is the surface expansion rate. To a good approximation β = 2α.

= A2 – A1 or  = 2a

A1 x

• Cubic expansivity (r)

This is the fractional increase in the volume of a solid, liquid or gas when the unit temperature rises, i.e. V2=V1(1+γθ), where γ is the cubic expansion rate and γ=3α. For liquids, the increase observed directly is called apparent expansion, because the container will also expand with the increase of temperature. Absolute expansion is the apparent increase plus the volume increase of the container.

r = V2 – V1 or r = 3a

V1θ

EFFECT AND APPLICATION OF EXPANSION

EFFECT

• Expansion causes sagging of transmission line
• Expansion cause bridge to bend
• Expansion causes glass cup to crack

IT’S APPLICATION

The idea of expanSIVITY is used in the following:

• Construction of bridges
• Construction of thermostat in electric iron and fridge.
• Is used in the design of some special alloy
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