The State of the System

The State of the System

The state of a system means the condition of the system which is described in terms of certain observable properties such as temperature, pressure ,volume of the system.

If any of these properties of the system changes ,the system is said to be in different state i.e. the state of system changes.

These properties of a system are called state variables.

The first and the last State of a system are called the initial state and the final state.

State function

A physical quantity is said to be state function if its value depends only upon the state of the system and does not depend upon the path by which this state has been attained.

or

A physical quantity is said to be a state function if the change in its value during the process depends upon initial state and final state of the system and does not depend upon the path or route by which this change has been brought about.

For ex: Physical quantities which are state function include pressure ,volume, temperature ,internal energy enthalpy, entropy ,free energy  etc.

Path function in Thermodynamics

thermodynamic property that depends on the path between the initial and final state is known as the path function. The path functions depend on the path taken or covered between two (initial and final) states. For example, work and heat. If different paths are chosen to reach from one point to another point, the work done will be different however you reach the same point in each case. So, work is not a state function as we cannot say that a system will have a specific amount of work at a specific state.

State Function

Path Function

It refers to a property whose value does not depend on the path followed to reach that value.

It is a property whose value depends on the path followed to reach that value.

They are also called point functions.

They are also called process functions.

It can be integrated using values of the initial and final state of a system.

They need multiple integrals and limits of integration for integrating the property of the system.

Value of state function remains the same regardless of the path or steps involved to reach that value.

Value of the path function will be different if a different path is taken to reach the final state.

Different paths give the same value

Different paths give different values

For example; entropy, mass, temperature, volume, etc.

Example; heat and mechanical work.

 

Thermodynamic Equilibrium

A system is said to be in thermodynamic equilibrium if its macroscopic properties like temperature, pressure do not change with time.

Thermodynamic process

A thermodynamic process is said to occur when the system changes from one state to another.

1)Isothermal process:

When a process is carried out in such a manner that the temperature remains constant throughout the process, it is called isothermal process. When such a process occur, heat can flow from the system to the surrounding and vice a Versa in order to keep the temperature of the system constant.

2)Adiabatic process:

When a process is carried out in such a manner that no heat can flow from the system to the surrounding or vice a versa i.e. the system is completely insulated from the surrounding, it is called adiabatic process.

3)Isochoric process

It is a process during which the volume of the system is kept constant.

4) Isobaric process

It is a process during which the pressure of the system is kept constant.

Reversible process

A process which is carried out infinitesimally slowly so that all changes occurring in the direct process can be exactly reversed and the system remains almost in a state of equilibrium with the surrounding at every stage of the process.