# Introduction to Thermodynamics definition

## Thermodynamics Definition

• A combination of two words, (THERM = HEAT & DYNAMICS = MOTION).
• Thermodynamics is the branch of physics that deals with energy change involving all types of chemical and physical changes.
• In thermodynamics, we basically deal with the macroscopic bodies or systems i.e., an assembly of a larger number of molecules so that it has certain values of pressure, volume, temperature.
• Heat and Work are the two modes of energy interaction of one body with another.

### Important Thermodynamics Terms

#### System

• It is the part of the universe that is under observation.
• It must be a macroscopic i.e., consisting of an extremely large number of particles so that it has some values (temp., pressure, etc.).
• This system may be a solid, liquid and gas or a combination of these.

#### Surrounding

• Rest part of the universe which interacts with the system.
• In general, surrounding is very large so any change in properties taking place can be neglected.

#### Boundary

• Anything which separates the system from its surrounding.
• It may be imaginary, rigid or non- rigid, conductor of heat (diathermic) or non-conductor (adiabatic), permeable (mass transfer possibility) or non-permeable.

### Types of Thermodynamics Systems

• Depending upon the types of the boundary it is of following types-

#### Open System

A system having the capability to transfer both matter and heat.

#### Close System

This can transfer energy, heat but not mass.

#### Isolated System

It can neither transfer heat nor matter. A universe is an isolated system.

#### Property of a System

• A measurable quantity.

### Thermodynamics Properties

#### Intensive Property

• Values remain the same in the division of a system.
• Examples are- pressure, temperature, density, vapor density, specific heat, surface tension, viscosity, molar mass, melting point, etc.

#### Extensive Property

• Values change in the division of the system.
• Examples are- volume, mass, internal energy, heat capacity, moles, entropy, enthalpy, etc.

Important Points:

• Extensive property defined per mole or gram changes to an intensive property.
• The ratio of two extensive is intensive.
• Extensive properties are additive but intensive are not.

### STATE OF A SYSTEM

• The state of a thermodynamic system is basically described by its macroscopic property, for example, the state of a gas is described by pressure, temperature, volume, etc.

### STATE FUNCTION OR VARIABLES

• Quantities like pressure, volume, entropy, etc. which help us to study the behavior of a thermodynamic system.
• Its value depends on the present state and is independent of the past state and path followed to arrive at that state.

#### Path Function

• When state function moves from point A to B, it follows a path to reach B from A. So, the variables or state functions which not only depend on ‘A’ (initial state) and ‘B’ (final state) but also the path followed.

#### Process

• The path along which change of state takes place or change of thermodynamic variables with time.

#### Spontaneity

• When a reaction is carried or a process having the potential to proceed without any external agency’s assistance.

#### Cyclic Process

• A system after going through a number of different thermodynamic processes returns to its original state is called a cyclic process.
• Change in all state functions must be zero (∆U = 0, etc.)

#### Thermodynamics Reversible Process

• Reversible processes are carried out at infinitesimally slow.
• If the initial state can be achieved at any state by reversing the process.
• It takes infinite time and infinite steps to show any appreciable changes.
• It is a hypothetical process.
• It exists in all states of equilibrium at each state.
• In the case of an ideal gas, the ideal gas (PV = nRT) equation can be applied throughout the process.
• Friction, viscosity, etc. should not be present for the process to be reversible.

#### Irreversible Process

• If the initial state of the process can be restored without any changes in the surrounding or system.
• It takes finite time and finite steps to show any appreciable changes.
• It exists in the state of equilibrium at initial and final states.
• The ideal gas equation can be applied in the initial and final state.

#### Quasi-Static Process

• A process that is extremely slow.
• Quasi means ‘Almost’, so almost a static process when goes from one state to another.
• It should always be in equilibrium and there may be a loss of energy.

NOTE-

• A process may be reversible or irreversible for a system but it is always reversible for the surroundings.
• All reversible process is quasi-static but all quasi-static process is not reversible because energy loss may be possible in quasi-static but not in reversible.