Thermodynamics

Thermodynamic Terms : System and Surroundings, Types of System

Thermodynamics

• 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 process implies that the product of the volume and the pressure is constant for an ideal gas. i.e.

PV = Constant

For example: Isothermal processes can occur in any kind of system, including highly-structured machines and even in living cells. Various parts of the cycles of some heat engines are carried out isothermally and may be approximated by a Carnot cycle.

2. Adiabatic Process:
A process is said to be adiabatic, if no heat can flow from system to the surroundings or vice-versa. An adiabatic process is also known as isocaloric process which is a thermodynamic process, in which no heat is transferred to or from the working fluid. This system is completely insulated from the surroundings.

3. Isobaric Process:
A process is said to be isobaric, if the pressure of the system remains constant during each stage of the process. The heat transferred to the system does work but also changes the internal energy of the system.

4. Isochoric (or iso-volumetric) Process:
A process is said to be Isochoric (or iso-volumetric), if the volume of the system remains constant during each step of the process. In any Isochoric process, the work done by the system is always zero. For any two dimensional system, the heat energy transferred to that system is absorbed by it as its internal energy. The other name of this process is isometric process.
For example: When we heat up any empty container, the air inside gains internal energy which can be felt due to increase in pressure and temperature.

Difference between Reversible process and Irreversible process:
 

Reversible process	Irreversible process
a. The process is carried out infinitesimally slowly, i.e. all changes occuring in the direct process can be exactly reversed and the difference between driving force and the opposing force is very small.	a.The process is carried out infinitesimally rapidly, i.e. the successive steps of the direct process cannot be retraced and the difference between driving force and the opposing force is very large.
b.The system remains in the state of equilibrium with the surroundings.	b. After the completion of a process, equilibrium may exist.
c. In this process, maximum work is obtained.	c. In this process, minimum work is obtained.
d. It is an imaginary process and cannot be achieved practically.	d. These processes occur in nature.
e. Infinite time is required for the completion of a process.	e. Finite time is required for the completion of a process.

Cyclic process:

When a system returns to its original state after completing a series of changes, then it is known as cyclic process. In a cyclic process, the initial and the final state is same. As the internal energy U of the system depends only on the state of the system. So, in a cyclic process, the net change of internal energy will be equal to zero i.e. ∆U = 0. Hence

From the first law:

∆U = q + w

0 = q + w

Hence, q = – w

Examples of a Cyclic Process:

1. Expansion at constant temperature (T₁).

2. Removal of heat at constant volume (V₂).

3. Compression at constant temperature (T₂).

4. Addition of heat at constant volume (V₁).

If the process takes place at constant temperature, then the cycle is known as isothermal cycle.

If the process takes place reversibly, then cycle is known as reversible cycle.

The process is cyclic so, work is done. There is no change in internal energy after each cycle. Therefore, the net work done in each cycle equals to the heat added to the system. Now, we …