## What do you mean by Thermal Properties of Matter?

Thermal energy is described as one of the form of heat energy which flows from one body of higher temperature to the other with the lower temperature when these two bodies are placed in contact to each other. Heat is described as the form of energy which is transferred between the two systems or in between the systems and their surrounding by the virtue of difference in temperature. Calorimetry is that branch of science which helps in measuring the changes which are taking place in the heat energy of a given body.

Thermal property in a material are influenced by chemical properties of the body, they share indirect relation which will be discussed with specific property. When a body is under thermal process there is significant physical changes in it. Thermal properties are the intensive property of a material and does not depend on the mass. The properties of matter which are in relation to the conductivity of heat.  These are considered as the physical properties of matter. They generally tell how the material will respond when it is provided with various amount of heat, which can be high or low.

• Thermal expansion
• Thermal stress
• Thermal conductivity
• Heat capacity

### Thermal Expansion

It is that tendency of matter which causes the body to physical change such as area, shape, volume or density due to the change in temperature.  In a solid body, various types of expansion takes place which includes linear, superficial and volume expansion. when there is restriction to these physical change in body a thermal stress is developed in the body in the direction of restriction. There is no chemical change.

• Linear expansion: When the length of a solid increases when the heat is provided to the system, such type of expansion is termed as linear expansion. For ex- If the body of length L is provided with a temperature ΔT, its length expands by ΔL. And the expression of linear expansion is given by:
• Superficial expansion: When the area of a solid increases when heat of certain amount is provided to it, such type of expansion is termed as superficial expansion. If A0 is the area of solid at 00C then the expression for area at t0C is given as:

Where β is referred as the coefficient of superficial expansion.

• Volume Expansion: When the volume of the solid changes, when the heat of certain amount is provided to the body, such type of expansion is termed as volume expansion. The expression can be given in the form as:

Where γ is referred as the coefficient of volume expansion.

The relation between all of these coefficients is given as,

For any solid body, the values of coefficient of expansions can vary, they are not supposed to be constant.

### Thermal Stress

It is generally defined as the mechanical stress which is generally created due to some change in the temperature of material. These types of stresses can leads to fracture or plastic deformation in a body depending upon the variety of material and other constraints. Thermal temperature, thermal expansion and the thermal gradients are the factors which can cause thermal expansion. Thermal expansion coefficient helps in such type of stresses, and their values varies for each and every material. Generally what happens is that, as much the temperature changes, more high the stress level can occur. While on the other side, thermal shocks can sometimes leads to cracking or shattering of material due to the sudden change in temperature.

### Thermal Conductivity

It can be described as that ability of a material of conducting heat. It is defined as how well a material will conduct heat energy. It is generally denoted by k or λ. Conductivity can be of different type in different materials as the heat transfers with slow rate in low thermal conductivity materials while the heat transfers is at high rate in the materials having high thermal conductivity. Generally, these types of materials with higher thermal conductivity are used in the applications of heat sink while those of low thermal conductivity are typically used for thermal insulation. Thermal resistivity is considered as the reciprocal of thermal resistivity. And the most basic form of thermal conductivity is a second rank tensor. In case of anisotropic materials, this tensorial description becomes very important.

Temperature, impurities have different effects to thermal properties like, for liquids impurities increases thermal conductivity, in gases with increase in temperature the thermal conductivity increases attributed to decrease in intermolecular forces. In case of pure metals one will observe drop of thermal conductivity when temperature is increases.

There are two important techniques for the measurement of thermal conductivity, one of which is Steady state while the other one is transient state techniques. Transient techniques works with the instantaneous state of the system to get the state to the steady condition while the steady-state techniques results in the thermal conductivity when the measurements of steady state temperature on a material are taken when the stead state profile is obtained. Steady state techniques does not needs any kind of complicated signals. While one drawback is that, it requires a proper engineered experimental setup. Rapid measurements is prohibited to reach the steady state condition.  The factors influencing the thermal conductivity includes thermal anisotropy, temperature, chemical phase, magnetic field, gaseous phases, electrical conductivity and isotopic purity.

### Heat Capacity

It can be described as the amount of heat required by a material of a given mass so that it will show a unit change in its temperature. It is considered as an extensive property while the specific heat capacity is considered as specific heat capacity. We can obtain the molar heat capacity by dividing heat capacity to the quantity of substance which is in moles.  While the heat capacity for a volume is taken as volumetric heat capacity. We also refer the heat capacity with thermal mass in case of civil engineering and in architecture as well.

Different type of heat capacities when they are going through the different kind of thermodynamic processes. These includes:

• At constant temperature (dQ= dU + Pdv)- As the first law of thermodynamics states, when a system is undergoing constant pressure, the heat supplied to the system will help in work done as well as in the change in internal energy. The heat capacity for such type of system is called as Cp and this is called as isobaric process.
• At constant volume (dv= 0 and dQ= dU)- From this, we understood that, when a system is performing at constant volume, it would show that no work have been done. And the heat provided to the system will only help in changing the internal energy. The heat capacity for such type of system is called as Cv

The value of Cis always greater than the Cv

## Context and Applications

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for

• Bachelors in Science Physics
• Masters in Science Physics

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