# Thermodynamics – Definition, Equations, Laws, Meaning

Thermodynamics

The laws of thermodynamics are three principles that describe how energy moves in a system, as well as being transferred among groups of objects. The first law of thermodynamics links the different forms of kinetic and potential energy within a system with the jobs that a system can do, and with heat transfer. The first law states that energy may be transferred among physical systems in terms of heat, work, and transfer of matter. It states that heat is one form of energy, and thus is subject to conservation principles.

The First Law states that there are two types of processes, heat and work, which may cause changes to a system’s internal energy. The first law of thermodynamics is sometimes taken to define the intrinsic energy, and it introduces the additional variable state, enthalpy. Internal energy is a fundamental property of a thermodynamic state, whereas heat and work are modes of transferring energy through which processes can modify that state. A system is in a thermodynamic process when some change of energy occurs in the system, associated with changes in pressure, volume, and internal energy.

Equilibrium thermodynamics is the study of transfers of matter and energy within systems or bodies which, through agents in their surroundings, maybe driven from a thermodynamic equilibrium state into another system or body. Thermodynamics studies not only the relationships between heat and mechanical work, but also the roles that temperature, volume, and pressure play in energy exchange. More specifically, thermodynamics studies the effects that changes in temperature, pressure and volume have on a physical system on the macroscale, by analysing the collective movement of its particles using statistics. Thermodynamics is the branch of physics concerned with heat, work, and temperature, as well as the relationships between heat and energy, radiation, and physical properties of matter.

The laws of thermodynamics originated from studies of heat and machines centuries ago, but since then they have become basic statements about how the universe works on a basic level. The so-called Helmholtz Free Energy of a thermodynamic system describes how much useful work can be produced by a closed thermodynamic system with constant temperature and volume. Gibbs’s free energy, in contrast, describes the maximum amount of reversible work a thermodynamic system can do at a constant temperature and pressure. Thermodynamic equilibrium leads to the large-scale determination of temperature, in contrast with the smaller-scale determinations related to the kinetic energy of molecules.

The third law of thermodynamics states that entropy in a system approaches a constant value when the temperature approaches absolute zero. The entire scope of this section of Thermodynamics depends upon the consideration of the intrinsic energy of the body systems, depending on temperature and the state of physical conditions, and the shapes, movements, and relative positions of those bodies. The First Law states that the amount of energy added to the system is the sum of its increased heat energy, and the work done in the system. Key Takeaways The first law, also known as the law of conservation of energy, states that energy cannot be created or destroyed in an isolated system.

## 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.

To be specific, it explains how thermal energy is converted to or from other forms of energy and how matter is affected by this process. Thermal energy is the energy that comes from heat. This heat is generated by the movement of tiny particles within an object, and the faster these particles move, the more heat is generated.

Thermodynamics is not concerned about how and at what rate these energy transformations are carried out. It is based on the initial and final states undergoing the change. It should also be noted that Thermodynamics is a macroscopic science. This means that it deals with the bulk system and does not deal with the molecular constitution 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 analysed 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 characterise 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 in changes of states.

#### Equilibrium Thermodynamics

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

## Basic Concepts of Thermodynamics – Thermodynamic Terms

Thermodynamics has its own unique vocabulary associated with it. A good understanding of the basic concepts forms a sound understanding of various topics discussed in thermodynamics preventing possible misunderstandings.

### Thermodynamic Systems

#### System

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

There are three types of systems:

• Isolated System – An isolated system cannot exchange 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. Refrigerators and 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.
 Interactions of thermodynamic systems Type of system Mass flow Work Heat Isolated System ☓ ☓ ☓ Open System ✓ ✓ ✓ Closed 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 processes 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.

A thermodynamic cycle is a process or a combination of processes conducted such that the initial and final states of the system are the same. A thermodynamic cycle is also known as cyclic operation or cyclic processes.

### Thermodynamic Equilibrium

At a given state, all properties of a system have fixed values. Thus, if the value of even one property changes, the system’s state changes to a different one. In a system that is in equilibrium, no changes in the value of properties occur when it is isolated from its surroundings.

• When the temperature is the same throughout the entire system, we consider the system to be in thermal equilibrium.
• When there is no change in pressure at any point of the system, we consider the system to be in mechanical equilibrium.
• When the chemical composition of a system does not vary with time, we consider the system to be in chemical equilibrium.
• Phase equilibrium in a two-phase system is when the mass of each phase reaches an equilibrium level.

A thermodynamic system is said to be in thermodynamic equilibrium if it is in chemical equilibrium, mechanical equilibrium and thermal equilibrium and the relevant parameters cease to vary with time.

## Thermodynamic Properties

Thermodynamic properties are defined as characteristic features of a system, capable of specifying the system’s state. Thermodynamic properties may be extensive or intensive.

• Intensive properties are properties that do not depend on the quantity of matter. Pressure and temperature are intensive properties.
• In the case of extensive properties, their values depend on the mass of the system. Volume, energy, and enthalpy are extensive properties.

### What is Enthalpy?

Enthalpy is the measurement of energy in a thermodynamic system. The quantity of enthalpy equals the total heat content of a system, equivalent to the system’s internal energy plus the product of volume and pressure.

Mathematically, the enthalpy, H, equals the sum of the internal energy, E, and the product of the pressure, P, and volume, V, of the system.

 H = E + PV

### What is Entropy?

Entropy is a thermodynamic quantity whose value depends on the physical state or condition of a system. In other words, it is a thermodynamic function used to measure the randomness or disorder.

For example, the entropy of a solid, where the particles are not free to move, is less than the entropy of a gas, where the particles will fill the container.

### Thermodynamic Potentials

Thermodynamic potentials are quantitative measures of the stored energy in a system. Potentials measure the energy changes in a system as they evolve from the initial state to the final state. Different potentials are used based on the system constraints, such as temperature and pressure.

Different forms of thermodynamic potentials along with their formula are tabulated below:

 Internal Energy Helmholtz free energy F = U – TS Enthalpy H = U + PV Gibbs Free Energy G = U + PV – TS