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Thermodynamic Terms : System and Surroundings, Types of System


  • Energy can be transformed from one form to another. For example, energy change during combustion of wood, conversion of liquid water into vapour, conversion of potential energy of water stored in a reservoir at top of a dam to kinetic energy when it falls down the reservoir.
  • Thermodynamics is the branch of science that deals with energy changes that take place during physical and chemical transformations.
  • However, it is silent on the rates involving these transformations.

The System and the Surroundings

  • System − Part of the universe in which observations are made

  • Surroundings − Part of universe excluding system

  • Universe = System + Surroundings

Types of the System

  • Open system − There is exchange of energy and matter between system and surroundings. (shown in figure)

Example − Presence of reactants in an open beaker

  • Closed system − There is no exchange of matter, but exchange of energy is possible between the system and the surroundings. (shown in figure)

Example − Presence of reactants in closed vessel made of conducting material

  • Isolated system − There is no exchange of energy or matter between the system and the surroundings. (shown in figure)

Example − Presence of reactants in a thermos flask or any other closed insulated vessel

Extensive and Intensive Properties

  • Extensive property: Value depends on the quantity or size of matter in the system

Examples − mass, volume, internal energy, heat capacity, etc.

  • Intensive property: Value does not depend on the quantity or size of matter in the system

Examples − temperature, density, pressure, etc.

The State of System

  • State of a thermodynamic system can be described by properties such as its pressure (p), temperature (T), volume (V), composition of the system, etc.

  • Variables such as p, V, T are called state variables or state functions.

  • The values of state functions or state variables depend only on the state of the system and not on how it is reached.

  • To define the state of a system, it is not necessary to define all the properties of the system.

Thermodynamic equilibrium:

In an isolated system, when there is no change in the macroscopic property of the system like entropy, internal energy etc. with time, It is said to be in thermodynamic equilibrium. The state of the system which is in thermodynamic equilibrium is determined by intensive properties such as temperature, pressure, volume etc. Whenever, the system is in thermodynamic equilibrium, it tends to remain in this state infinitely and will not change spontaneously.  

The operation by which a system changes from one state to another state is called a process.

Thermodynamic process:

A thermodynamic process is a passage of a system from an initial state to a final state of thermodynamic equilibrium. The initial and final states are the defining elements for the process. Whenever, a system changes from one state to another, it is accompanied by change in energy but in case of open systems, there may be change of matter as well.

Various types of thermodynamic processes are isothermal process, adiabatic process, isochoric process, isobaric process, reversible process and irreversible process.

1. Isothermal Process:
A process is said to be isothermal, if the temperature of the system remains constant during each stage of the process. When such a process occurs, heat transfer may take place from system to the surroundings in order to keep the temperature of the system constant. An isothermal proceā€¦

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