# Thermodynamics Definitions

## Introduction to Thermodynamics

### Thermodynamics Definitions

System The part of the universe selected for thermodynamic study.
Surroundings The remaining part of the universe around the system which is not under study.
Isolated System A system which can neither exchange energy nor matter with the surroundings is called an isolated system.
Closed System A System which can exchange energy but not matter with the surroundings is called a closed system.
Open System A system which can exchange matter as well as energy with the surroundings is called an open system.
Macroscopic Properties The properties of the system which depend on the bulk of the matter are called macroscopic properties. They are classified into two Intensive properties and extensive properties.
Intensive Properties Those properties of the system which are independent of the amount of substance present are called intensive properties. E.g. temperature, pressure, viscosity, surface tension, density, dielectric constant, specific heat capacity, refractive index, etc.
Extensive properties Are those properties that depend on the amount of substance present in the system. E.g. Volume, Mass, Internal energy, Enthalpy, Entropy, Gibbs Free Energy etc. State variables The measurable properties required to describe the state of the system are called state variables.
State function It is a property of a system whose value depends only upon the state of the system and is independent of the path or manner by which the state is reached.
Isothermal process A process in which the temperature of the system remains constant. Adiabatic process It is defined as a process in which no heat enters or leaves the system during any step of the process.
Isobaric process A process in which the pressure of the system remains constant.
Isochoric process A process in which the volume of the system remains constant.
Reversible process A process is said to be reversible if it is carried out infinitely slowly so that the driving force is only slightly greater than the opposing force.
Irreversible process A process is said to be irreversible if it is not carried out infinitely slowly so that the system does not get a chance to attain equilibrium. An irreversible equilibrium cannot be reversed without the help of an external agency and without changing the properties of the surroundings.
Cyclic process A process in which the system undergoes a series of changes and ultimately returns to its original state.

### Laws of Thermodynamics

#### First Law of thermodynamics:

The total energy in the universe is a constant.

Energy can neither be created nor destroyed. But it can be transferred from one system to another.
The mathematical representation of the first law is written as
$\Delta E\$ = q $\pm w$
Where $\Delta E\$ represents the change in energy or E $\ _{2}$ – E $\ _{1}$ where 2 and 1 represents the final and initial states of the system. Depending upon the conditions of measurement, that is whether the energy change was measured at constant volume or constant pressure.

‘q’ represents the heat energy that is supplied to the system and ‘w’ is the work done on the system which is positive or work done by the system which is negative. Work can be $\pm\$ depending on weather work is done by the system or work is done on the system.
Work w = P $\Delta V$ where $\Delta V$ is the change in volume V $\ _{2}$ – V $\ _{1}$