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Introduction
What is Thermal Expansion
The ability of matter to change its form, area, volume and density in reaction to a change in temperature is known as thermal expansion. Transitions between phases are not included.
The temperature has a monotonic effect on a material’s average molecular kinetic energy. When a solid is heated, the atoms begin to vibrate and move more, increasing the space between them. Materials that contract as the temperature rises are rare and only occur in a few temperature ranges. The material’s coefficient of linear thermal expansion, which varies with temperature, is the relative expansion (also known as strain) divided by the change in temperature.. Particles travel faster as their energy rises, eliminating intermolecular interactions and allowing the material to expand.
Types of Thermal Expansion
Thermal expansion can manifest in three different ways: in the form of linear, areal and volume expansion.
LINEAR EXPANSION
In solid, thermal expansion takes place in the form of an increase in length. For example, if we consider the rod of length l to increase due to increased temperature. The linear expansion is given by,
ΔL = 𝛼L LΔT where,
- ΔL illustrates the difference in length
- L is defined as the linear coefficient of thermal expansion
- In the CGS unit, the linear coefficient is represented by celsius.
- In the SI unit, the linear coefficient ‘a’ is represented by kelvin.
Area Expansion
The thermal area expansion coefficient links a change in temperature to a change in the area dimensions of a substance. The fractional change in area per degree of temperature change is what this term refers to. Without regard for the pressure, we may write:
Coefficient
Areal expansions are expressed through coefficients, which is the fractional change in area per degree of temperature change. Therefore, the coefficient denotes the rate of change of area per unit degree change in temperature.
Thermal Expansion Formula is expressed as ΔA = 2αAAΔT
Volumetric Expansion Coefficient
We may express the volumetric (or cubical) thermal volume expansion coefficient as follows, ignoring the effects of pressure on a solid:
The formula is expressed as ΔV = βV1 ΔT
Wherein:
- V1 is the initial volume,
- ΔT is the temperature change,
- ΔV is the increase in volume,
- β is the coefficient of volumetric expansion.
THERMAL EXPANSION IN METALS
Thermal expansion is all about expansion on heating and contraction on cooling. On heating, the dimension of the objects will change. Based on the changes that occur in the body, these are the following observations:-
- If the length of the objects gets expanded, it is called linear expansion.
- On heating, if the length and breadth of the object get changed, it is called area expansion.
- If any objects expand on both sides on heating and lead to an increase in volume, it is called volume expansion.
- Some of the examples of thermal expansion in solids are molten rods, metal-framed windows, thermometers, and railway works where expansion is needed in joints of the structure.
Thermal Expansion of Liquids
Because the intermolecular interactions in liquids are relatively weak and the component molecules are more mobile, their thermal expansion is generally greater than that of solids. The coefficient of linear expansion can theoretically be calculated from the coefficient of volumetric expansion (αV ≈ 3αL). Unlike solids, liquids have no distinct form and take on the shape of the container. As a result, because liquids have no fixed length or area, linear and areal expansions of liquids are meaningless. However, the experimental value of αL is occasionally used to determine αV.
Heating expands liquids in general. Water, on the other hand, is an exception to this rule: below 4 °C, it contracts when heated, resulting in a negative thermal expansion coefficient. Water exhibits more usual behaviour at higher temperatures, with a positive thermal expansion coefficient.
Solids’ thermal expansion is normally unaffected by temperature, particularly at very low temperatures, but liquids’ thermal expansion is at various temperatures, they can grow at varying speeds.
Coefficient of thermal expansion
The volumetric coefficient of thermal expansion of a gas, liquid or solid in general is given by
Where v is the volumetric coefficient of thermal expansion.
The derivative’s subscript “p” denotes that the pressure is maintained constant during the expansion. The fact that the pressure is kept constant is critical in the case of a gas, because the volume of a gas varies significantly with pressure and temperature. This may be observed in the ideal gas for a low density gas.
Thermal Expansion in Gases
Gases are more heat expandable than matter in the other two states of liquid and solid. The reason for this is because air or gas particles, unlike solids or liquids, are propelled by a significant repulsion for one another. The presence of heat amplifies this repulsive response and induces significant dilation. The rate of expansion of air and gases as a function of temperature has long been a source of controversy. This resulted from the early experimenters’ failure to dry the air or gas in which they worked. The existence of a little amount of water, which is rising in the state of the introduction of steam into the gas, along with the application of heat, resulted in large and uneven expansions. However, independently of each other, M. Gay-Lussac of Paris and Dr. Dalton of England established the law of dilitation of gases in 1801. These thinkers found that applying the same amount of heat to all gases results in the same increase in volume, and that the rate of expansion remains uniform at all temperatures. Dr. Dalton used a graduated tube to contain a little amount of dry air over mercury. He then put the whole thing in a situation where it was unnaturally heated to a specific temperature and saw it expand. The apparatus used by Gay-Lussac was more sophisticated, but it was calculated to produce exceedingly exact results
Conclusion:
We learned in this article what thermal expansion of solids, liquids and gases is.
When a solid is heated, the atoms around its fixed points vibrate faster. As a result, the comparative enlargement of solids when heated is small.
Because the intermolecular interactions in liquids are relatively weak and the component molecules are more mobile, their thermal expansion is generally greater than that of solids.
Gases are more heat expandable than matter in the other two states of liquid and solid. The reason for this is because air or gas particles, unlike solids or liquids, are propelled by a significant repulsion for one another.
