Physics

# Thermodynamics and Laws of Thermodynamics

Thermodynamics is the branch of physics that deals with heat and temperature and their relationship to energy and work. Broadly speaking, thermodynamics deals with the transfer of energy from one system to another and from one form to another.

The behavior of the above quantities is determined by the four laws of thermodynamics. The four laws of thermodynamics describe all the minute changes that occur as a body’s energy system changes, in addition to explaining the ability of an energy system to do some work around it. It was only in the 19th century that studies on thermodynamics expanded; To optimize the performance of steam engines. There are eight founding schools of thermodynamics.

Before we begin our discussion of the four laws of thermodynamics, we will familiarize ourselves with some terms.

Thermodynamic System: An assembly of a very large number of particles with a fixed value of pressure (P), volume (V), and temperature (T).

Surroundings: Everything outside the system can have a direct effect on the system.

Reversible Process: An ideal process in which the system is in equilibrium at every point.

Cyclical in Process: When a system returns to its starting point.

Extensive State Variables: Variables that indicate the size of the system. These variables are internal energy (U), volume (V), and total mass (M).

Intensive state variables: variables that do not indicate the size of the system; For example, pressure (P), temperature (T), and density (ρ).

Entropy: Degree of disorder in the system. It is maximum for gas and minimum for solid.

## Law (I): The Zeroth Law of Thermodynamics

Zeroth’s law states that if A is in thermal equilibrium with B and B is in thermal equilibrium with C, then A and C are also in thermal equilibrium with each other.

• Zeroth’s law helps us determine the temperature of different bodies. Bodies in thermal equilibrium with each other will have the same temperature.
• When two bodies of different temperatures are placed in contact, heat flows from the body with the higher temperature to the body with the lower temperature until their temperatures are equal.
• The stage at which body temperature is equal is called thermal equilibrium. Thus, the direction of heat flow is controlled by temperature.

## Law (II): First Law of Thermodynamics

The first law states that the heat (Q) supplied to a system is used partly in increasing the internal energy (∆U) of the system and partly in expanding the gas (∆W); For example, Q=∆U+W

Sign conventions are very important when following the first law of thermodynamics:

• This law is also known as the law of conservation of energy.
• Heat flow describes the transfer of energy.
• The internal energy of a system is related to temperature as follows, Etotal = KEsystem + PEsystem + Usystem.
• The transfer of energy to or from a system is called work.
• It can be rightly said that a machine cannot work forever unless a suitable amount of energy is given as input.

### Thermodynamic process:

• Isothermal: constant temperature
• Isochoric: constant volume
• Isobaric: constant pressure

Limitations of the first law of thermodynamics:

• It does not indicate the direction of heat change.
• It gives no idea about the amount of heat change.
• It does not give any information about the heat source.

## Law (III): Second Law of Thermodynamics

The second law of thermodynamics states that there cannot be a process in which the entropy of an isolated system decreases.

• According to Clausius, no process is possible that results in the transfer of heat from a colder object to a hotter object.
• In the terms of entropy, it either stays the same or increases but never decreases. However, a decrease in entropy can occur in non-isolated systems, if they increase the entropy of the environment by the same amount.
• The entropy of a system remains constant for any process that goes from an initial state to a final state; Regardless of whether a process is reversible or irreversible.
• According to the second law, the heat transferred (∆Q) is the product of the temperature (of the system and the heat source or destination) with the increase (ds) of the entropy (S) of the system; ∆Q=TdS.

## Law (IV): Third Law of Thermodynamics

The third law states that the entropy of the system becomes minimum when the temperature of the system approaches absolute zero (−273.15°C, 0 K).

• At zero K the value of entropy is zero. Nevertheless, in some cases, there is some amount of residual energy in the system.
• When the temperature of the system is zero, the system will have minimum thermal energy. This statement is true if the perfect crystal has only one state and that too with minimum energy.