What is Thermodynamics?

Thermodynamics in physics is a branch that deals with heat, work and temperature, and their relation to energy, radiation and physical properties of matter.

Distinction Between Mechanics and Thermodynamics

The distinction between mechanics and thermodynamics is worth noting. In mechanics, we solely concentrate on the motion of particles or bodies under the action of forces and torques. On the other hand, thermodynamics is not concerned with the motion of the system as a whole. It is only concerned with the internal macroscopic state of the body.

Different Branches of Thermodynamics

Thermodynamics is classified into the following four branches:

• Classical Thermodynamics

• Statistical Thermodynamics

• Chemical Thermodynamics

• Equilibrium Thermodynamics

Classical Thermodynamics

In classical thermodynamics, the behaviour of matter is analyzed with a macroscopic approach. Units such as temperature and pressure are taken into consideration, which helps the individuals calculate other properties and predict the characteristics of the matter undergoing the process.

Statistical Thermodynamics

In statistical thermodynamics, every molecule is under the spotlight, i.e. the properties of every molecule and how they interact are taken into consideration to characterize the behaviour of a group of molecules.

Chemical Thermodynamics

Chemical thermodynamics is the study of how work and heat relate to each other in chemical reactions and changes of states.

Equilibrium Thermodynamics

Equilibrium thermodynamics is the study of transformations of energy and matter as they approach the state of equilibrium.

System

A thermodynamic system is a specific portion of matter with a definite boundary on which our attention is focussed. The system boundary may be real or imaginary, fixed or deformable.

There are three types of systems:

• Isolated System – An isolated system cannot exchange both energy and mass with its surroundings. The universe is considered an isolated system.

• Closed System – Across the boundary of the closed system, the transfer of energy takes place but the transfer of mass doesn’t take place. Refrigerator, compression of gas in the piston-cylinder assembly are examples of closed systems.

• Open System – In an open system, the mass and energy both may be transferred between the system and surroundings. A steam turbine is an example of an open system.

Surrounding

Everything outside the system that has a direct influence on the behaviour of the system is known as a surrounding.

Thermodynamic Process

A system undergoes a thermodynamic process when there is some energetic change within the system that is associated with changes in pressure, volume and internal energy.

There are four types of thermodynamic process that have their unique properties, and they are:

• Adiabatic Process – A process where no heat transfer into or out of the system occurs.

• Isochoric Process – A process where no change in volume occurs and the system does no work.

• Isobaric Process – A process in which no change in pressure occurs.

• Isothermal Process – A process in which no change in temperature occurs.

• What is Intensive Property?
  An intensive property is one that does not depend on the mass of the substance or system.
  Temperature (T), pressure (P) and density (r) are examples of intensive properties.

• Intensive Property Examples
  The properties of matter that do not depend on the size or quantity of matter in any way are referred to as an intensive property of matter. Temperatures, density, color, melting and boiling point, etc., all are intensive property as they will not change with a change in size or quantity of matter. The density of 1 liter of water or 100 liters of water will remain the same as it is an intensive property.
  What is Extensive property?
  An extensive property of a system depends on the system size or the amount of matter in the system.
  If the value of the property of a system is equal to the sum of the values for the parts of the system then such a property is called extensive property. Volume, energy, and mass are examples of extensive properties.
  
  Extensive Property Examples
  There are properties such as length, mass, volume, weight, etc. that depend on the quantity or size of the matter, these properties are called an extensive property of matter and their value changes if the size or quantity of matter changes. Suppose we have two boxes made up of the same material, one has a capacity of four litres while the other has a capacity of ten litres. The box with ten litres capacity will have more amount of matter as compared to that of a four-liter box.

• Other Examples of Properties
  Thermodynamics deals with the flow of heat energy. This flow of heat energy and its transformation into different forms is governed by the principles of thermodynamics. It depends on the matter and the factors that determine the state of a matter. The thermodynamic properties of a system depend on certain parameters. The parameters or variables are classified as state functions and path functions as defined below:

  2. State functions or state variables are those parameters that depend only on the current state of the system and not on the path that they have taken to reach this state.
     For e.g: Temperature of the system.

  3. A path function is a parameter that depends on the path taken by the system to reach the current state.
     For e.g: Work is done by frictional force.

• A state function depends only on the initial and final conditions while a path function depends on the path taken to reach the final condition from the initial condition. The thermodynamic properties of matter are also classified as intensive and extensive properties. This classification is based on the dependence of property on the size or quantity of matter under consideration.

• Frequently Asked Questions – FAQs
  Which is the intensive property?
  An intensive property, is a physical property of a system that does not depend on the system size or the amount of material in the system. According to the definitions, density, pressure and temperature are intensive properties and volume, internal energy are extensive properties.
  What is the difference between intensive property and extensive property?
  An extensive property is a property that depends on the amount of matter in a sample. Mass and volume are examples of extensive properties. An intensive property is a property of matter that depends only on the type of matter in a sample and not on the amount.
  Is density a extensive property?
  Density is an intensive property because there is a narrow range of densities across the samples. No matter what the initial mass was, densities were essentially the same. … Density is an intensive property of matter that illustrates how much mass a substance has in a given amount of volume.
  Is work intensive or extensive?
  Work is the product of Force (which is intensive) times distance (which is extensive). There are several distinct forms of ‘energy’ that are treated in thermodynamics. Pressure (an intensive property) times volume (an extensive property) is a form of energy.
  Is weight an intensive or extensive property?
  Extensive properties vary with the amount of the substance and include mass, weight, and volume. Intensive properties, in contrast, do not depend on the amount of the substance; they include color, melting point, boiling point, electrical conductivity, and physical state at a given temperature.

Adiabatic Process Examples

• The vertical flow of air in the atmosphere

• When the interstellar gas cloud expands or contracts.

• The turbine is an example of the adiabatic process as it uses the heat as a source to produce work.

Adiabatic Process Derivation

The work done in adiabatic process derivation can be derived from the first law of thermodynamics relating to the change in internal energy dU to the work dW done by the system and the heat dQ added to it.

dU=dQ-dW

dQ=0 by definition

Therefore, 0=dQ=dU+dW

The word done dW for the change in volume V by dV is given as PdV.

The first term is specific heat which is defined as the heat added per unit temperature change per mole of a substance. The heat that is added increases the internal energy U such that it justifies the definition of specific heat at constant volume is given as:

Explain the work done in an isothermal process.

• Second Law of Thermodynamics
  Second law of thermodynamics states that the entropy in an isolated system always increases. Any isolated system spontaneously evolves towards thermal equilibrium—the state of maximum entropy of the system.

• Third Law of Thermodynamics
  Third law of thermodynamics states that the entropy of a system approaches a constant value as the temperature approaches absolute zero.

  • Zeroth law of thermodynamics: If two thermodynamic systems are in thermal equilibrium with a third system separately are in thermal equilibrium with each other.