## What is thermal property?

In mechanical engineering, the thermal property is represented as the physical quantity that is relevant to the application of heat. The thermal properties are evaluated by variations in any specific material due to low heat or high heat. The quantities that influence the thermal property of the materials are mass, density, temperature, and many others.

## What are the thermal properties of materials?

The different types of thermal properties of substances are listed below:

• Heat capacity
• Thermal expansion
• Thermal conductivity
• Thermal stress
• Thermal diffusivity
• Thermal effusivity

### Heat capacity

Heat capacity is expressed as the thermal property of an element which indicates the capability to retain heat from the external surrounding. In the standard international system, the heat capacity is estimated in the units of joule per kelvin. The heat capacity of a material does rely on its mass. The formula of heat capacity can be addressed as:

$C=\frac{dQ}{dT}$

Here, $dQ$ is the amount of heat supplied to the material and $dT$ is the change in temperature.

In the above relation, it is evident that the heat capacity (C) gets influenced by the variation in temperature of the system (material).

The schematic diagram represents the experiment for measuring specific heat capacity.

The following is the graph between heat capacity and temperature.

The above graph represents the relationship between heat capacity and temperature for metals and ceramics. The diagram has two curves: the bold curve is for metals, and the second light curve is for ceramics. The term ${\theta }_{d}$ indicates the temperature change.

### Thermal expansion

Thermal expansion is the thermal property that emerges because of the presence of heat energy in the subatomic particles as the temperature rises. When the temperature of atoms rises, the kinetic energy also rises, leading to an increase in the distances between molecules. Hence, the material expands. Generally, there are three types of thermal expansion, which are listed below:

• Linear expansion
• Area expansion
• Volumetric expansion

Below listed are two coefficients of thermal expansion:

• Coefficient of thermal expansion
• Coefficient of volume expansion

Coefficient of thermal expansion

The relation of coefficient of thermal expansion can be written as:

$\alpha =\frac{∆L}{{L}_{0}∆T}$

Here, $\alpha$ is the coefficient of linear expansion, $∆L$ is the change in length of the material, ${L}_{0}$ is the initial length of the material, and $∆T$ is the change in temperature.

The linear expansion coefficient indicates the correlation between the alteration in the dimension of a substance per unit length. In the standard international system, it is represented in terms of inverse degree Celsius.

Coefficient of volume expansion

The relation of coefficient of volume expansion can be written as:

${\alpha }_{vol}=\frac{∆V}{{V}_{0}∆T}$

Here, ${\alpha }_{vol}$ is the coefficient of volume expansion, $∆V$ is the change in volume, ${V}_{0}$ is the initial volume of the material and $∆T$ is the change in temperature.

The volume expansion coefficient signifies the difference in the volume of the material along with its temperature. The dilatometer is a device that is extensively employed to estimate the coefficient of thermal expansion. If a certain substance has a high melting point, then its thermal expansion coefficient will be less.

### Thermal conductivity

Thermal conductivity is described as the capability of transfering heat from substances having more temperature to substances having less temperature. The two methods used for transmitting heat energy are the electronic contribution and phonon contribution. The relation of thermal conductivity can be written as:

$Q=\mathrm{kA}\frac{∆T}{∆L}$

Here, Q is the heat transfered, k is the thermal conductivity of the substance, A is the area, $∆T$ is the variation in temperature, and $∆L$ is the variation in length.

The quantities on which thermal conductivity relies are as follows:

• Temperature
• Moisture
• Density of substance
• Path length of heat
• Characteristics of substance

The schematic diagram represents the thermal conductivity through a solid.

### Thermal stress

Thermal stresses are generated on any specific body because of the difference in a temperature gradient. When the temperature is higher, the nature of the body will be compressive, whereas, at low temperature, its nature is tensile. In order to estimate the thermal stress, the stress calculation theory is be applied.

The formula of thermal stress is given by,

$\sigma =\frac{Y\alpha ∆T}{L}$

Here, $\sigma$ is the thermal stress, $Y$ is the young's modulus, L is the initial length, and $∆T$ is the change in temperature.

### Thermal diffusivity

Thermal diffusivity is represented as the quantity which explains the amount of heat spread by an object. The measurement unit of thermal diffusivity is meter square per second. It can also be utilized to measure the time consumed in altering the temperature of the object during heating or cooling. The quantities which affect the thermal diffusivity are,

• Thermal conductivity
• Density
• Heat capacity

The expression of thermal diffusivity is written as,

$h=\frac{k}{{c}_{p}\rho }$

Here, k is the thermal conductivity, ${c}_{p}$ is the heat capacity, and $\rho$ is the density of the object.

### Thermal effusivity

When a particular body comes in contact with another body, there is some exchange of heat among them, which is estimated by thermal effusivity. Thermal effusivity has an essential part in thermal shock as well as thermal fatigue, and in heat conduction, it does affect the thermal strain or stress. One of the applications of thermal effusivity is to determine the warmness or coolness of fabric substances.

The formula of thermal effusivity is given as,

$e=\sqrt{k\rho {c}_{p}}$

Here, k is the thermal conductivity, $\rho$ is the density and ${c}_{p}$ is the specific heat.

## Common Mistakes

• In topic thermal properties, students are often confused between heat capacity and specific heat, and they think that both of them are the same. However, the heat capacity is different from specific heat.
• Misconceptions are created by students in thermal properties that specific heat and specific heat capacity are two different terminologies in mechanical engineering, but both are actually similar.

## Context and Applications

The topic thermal properties of the material are very much significant in the several professional exams and courses for undergraduate, diploma level, graduate, postgraduate. For example:

• Bachelor of Technology in Mechanical Engineering
• Bachelor of Technology in Civil Engineering
• Master of Technology in Mechanical Engineering
• Doctor of Philosophy in Mechanical Engineering
• Bachelor of Science in Physics
• Master of Science in Physics
• Conduction and its property
• Density of metal and density of non metal
• Specific density or relative density
• Physical property and mechanical property
• Application of thermal properties
• Thermophysical properties and melting points
• Thermal resistance, heat flow and latent heat
• Conductance, emissivity and thermal energy
• Thermodynamic property and equation
• SI units of materials and their thermal properties
• Melting points of materials

## Practice Problems

Q1. In polymers, the thermal conductivity rises because of?

1. Reduction in crystallinity
2. Constant crystallinity
3. Both (a) and (b)
4. None of these

Explanation: In polymers, the thermal conductivity increases due to the involvement of covalent bonds. The thermal conductivity varies with the temperature, density, and also moisture content. There is no variation due to decrease and constant crystallinity.

Q2. The addition of thermal energy on a substance produces?

1. No effect
2. Thermal contraction
3. Thermal stress
4. Thermal expansion

Explanation: Due to the addition of heat energy, the expansion takes place. There is an increase in the temperature, and a change in the state of the materials takes place.

Q3. Identify the element that will expand more as compared to others at a similar increase in temperature?

1. Plastic
2. Glass
3. Aluminium
4. Tungsten

Explanation: Generally, metals expand more while increasing in temperature. Aluminum expands more as compared to glass, plastic. Tungsten has the lowest coefficient of thermal expansion.

Q4. Which of the following is a good absorber of heat?

2. Non-emitters
3. None of these
4. Both (a) and (b)

Explanation: The black body can be considered a good absorber and good heat emitter. A good absorber of heat is considered a good emitter of heat energy.

Q5. Which of the following temperature is the correct melting point of water?

3. -273 Kelvin

Explanation: The melting point of water can be taken as zero degrees centigrade, or 32 degrees Fahrenheit, or 273.15 Kelvin.

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