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How a hot cup of tea cools after a few minutes or how ice melts as the temperature rises? In Physics, these phenomenon occur owing to heat or energy transfer. The study of energy transfer in atoms or particles is known as thermodynamics. 

Thermodynamics is the branch of physics that deals with the relationships between heat and other forms of energy. 

Thermal Properties of Materials:

 The properties  exhibited by a material when the heat is passed through it ,are called thermal    properties of materials.

The major thermal properties are:

  • Heat capacity 
  • Thermal Expansion
  • Thermal conductivity
  • Thermal stress

Heat Capacity:

The heat capacity of a material is defined as the amount of heat required to change the temperature of the material by one degree. 

The amount of heat is generally expressed in terms of joules or calories and the temperature in Celsius or Kelvin.

  • The specific heat of a substance is the amount of heat energy required to raise the temperature of 1 gram of the substance by 

The S.I. unit is joule per kilogram per kelvin (Jkg-1K-1).

Thermal Expansion :

When heat is passed through a material then it expands. This property of a material is called thermal expansion. There can be a change in the area, volume, and shape of the material.

Thermal conductivity :

Thermal conductivity is the property of a material to conduct heat through itself. 
Materials with high thermal conductivity will conduct more heat than the ones with low conductivity.

Thermal stress :

The stress experienced by a body due to either thermal expansion or contraction is called thermal stress. It is destructive in nature as it can make the material explode/broken out.

Thermal Equilibrium :

Thermal equilibrium is the physical state of two bodies when they are connected , there is no heat transfer between the bodies and both the bodies have the same temperature. 

Two objects in contact naturally move towards thermal equilibrium by transferring heat between the higher temperature object to the lower temperature object.

Zeroth law of thermodynamics 

The zeroth law of thermodynamics states that if two bodies are each in thermal equilibrium with some third body, then they are also in equilibrium with each other.

Heat, internal energy and work


Heat in thermodynamics is defined as energy in motion. It is the kinetic energy of the substance’s molecules held. Heat moves from higher to lower temperatures.
The heat entering the system is considered as positive, while the heat leaving the system is considered as negative.

Internal energy:

Internal energy is equal to the sum of internal kinetic energy and internal potential energy caused by molecular attraction forces. A heated body has more internal energy than a cold one of the same size.


Work done in thermodynamics, is the total amount of energy that the system and its surroundings exchange within itself. 
Work is done by the gas during expansion and work is done on the gas during compression. Work depends upon both path and initial and final state.
The work done by the system is considered as positive (+ve) and work done on the system is taken as negative (-ve)

Systems and surroundings

To understand system and surrounding in thermodynamics, take an example of heating a pot of water on the stove, the system is the stove, pot, and water, while the surroundings would be everything else: the rest of the kitchen, house, neighborhood, country, planet, galaxy, and universe. 
In thermodynamics, a system is the selected quantity of matter or a region in space where observations are taken.
The surroundings contain everything other than the system. The system and the surroundings together make up the universe.
Thermodynamic systems are classified as:
  • Open systems
  • Closed systems
  • Isolated Systems

In an open system, exchange energy and matter takes place with the surroundings.

In a closed system, only the exchange of energy is allowed but the exchange of matter is not allowed with the surroundings.  

An isolated system cannot exchange any heat, work, or matter with the surroundings.

States of system 

A thermodynamic system’s state is defined by changes in its state variables such as P (pressure), V (volume), T (temperature), and n (number of moles or mass). The state of a system will change if even one of them changes. 

The values of variables like p, V, and T depend only on the state of the system and not on how it is arrived at, these variables are also known as state variables or state functions.

Properties of system

Any measurable characteristics of the system by which physical condition of system may be described. Example: pressure, temperature, mass, volume, density, internal energy

  • Intensive Property: Properties whose value is not dependent on the mass or size but depends on concentration. Example- density, concentration, viscosity ,thermal conductivity  etc.
  • Extensive Property: Property whose value depends on the mass and the total number of particles. Example - volume, pressure, energy  etc.
Note: Ratio of the extensive properties are intensive property. Ex: density 

Thermodynamic processes

Any change that occurs in a thermodynamic system, during which its properties, such as temperature, pressure, volume, or energy, are changed is called thermodynamic process. There are several types of thermodynamic processes, each characterized by how certain properties of the system change over time. Some common types of thermodynamic processes include:

Isothermal Process:

 A thermodynamic process in which the temperature of the system remains constant. For an ideal gas, an isothermal process is typically achieved by allowing heat exchange with a reservoir at a constant temperature.

Adiabatic Process

A thermodynamic process in which there is no heat exchange between the system and its surroundings. In an adiabatic process, the change in internal energy is entirely due to work done on or by the system.

Isobaric Process

A thermodynamic process that occurs at constant pressure. During an isobaric process, the system may exchange heat with its surroundings, but the pressure remains constant throughout.

Isochoric Process (Isometric or Constant Volume Process)

A thermodynamic process that occurs at constant volume. In an isochoric process, the work done by the system is typically zero, as there is no change in volume.

Reversible Process

A thermodynamic process that occurs in such a way that the system and its surroundings can be returned to their original states without leaving any trace on the surroundings. Reversible processes are idealized and serve as a theoretical benchmark for analyzing actual processes.

Irreversible Process

A thermodynamic process that cannot be reversed, meaning that the system and its surroundings cannot be returned to their original states without leaving a permanent change. Most real-world processes are irreversible to some extent due to factors such as friction, heat transfer across finite temperature differences, and other forms of irreversibility.




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