Some Basic Terms

Some Basic Terms

System

A system is defined as any specified portion of matter under study which is separated from the rest of the universe with a bounding surface. A system may consist of one or more substances.

Surroundings

The rest of the universe which might be in a position to exchange energy and matter with the system is called the surroundings.

Types of system

(i) Isolated system

A system which can exchange neither energy nor matter with its surrounding is called an isolated system.

(ii) Open system

A system which can exchange matter as well as energy with its surroundings is said to be an open system.

(iii) Closed system

A system which can exchange energy but not matter with its surroundings is called a closed system.

Macroscopic properties

The properties associated with a macroscopic system (i.e. consisting of large number of particles) are called macroscopic properties. These properties are pressure, volume, temperature, composition, density etc.

Extensive and Intensive properties

An extensive property of a system is that which depends upon the amount of the substance present in the system like mass, volume and energy.

An intensive property of a system is that which is independent of the amount of the substance present in the system like temperature, pressure, density, concentration, viscosity, surface tension, refractive index etc.

State of a system

When macroscopic properties of a system have definite values, the system is said to be in definite state. Whenever there is a change in any one of the macroscopic properties, the system is said to change into a different state. Thus, the state of a system is fixed by its macroscopic properties.

State variables

Since the state of a system changed with change in any of the macroscopic properties, these properties are called state variables or the thermodynamics parameters which depends only upon the initial and final states of the system and independent of the manner as to how the change is brought are called state functions. Some common state functions are internal energy, enthalpy, entropy, free energy, pressure, temperature, volume etc.

Thermodynamic equilibrium

A system in which the macroscopic properties do not undergo any change with time is said to be in thermodynamic equilibrium.

Thermodynamic process and their types

The operation by which a system changes form one state to another is called a process. Whenever a system changes from one state to another it is accompanied by change in energy. In case of open systems, there may be change of matter as well.

The following types of process are known

Isothermal process

A process is said to be isothermal if the temperature of the system remains constant during each stage of the process.

Adiabatic process

A process is said to be adiabatic if the heat enters or leaves the system during any step of the process.

Isobaric process

A process is said to be isobaric if the pressure of the system remains constant during each step of the process.