Laws of Thermodynamics: Meaning, Terms, Laws & Examples

Laws of Thermodynamics

Laws of Thermodynamics: The laws of thermodynamics are a group of physical quantities, such as temperature, energy, work, heat and entropy, that characterize thermodynamic systems in thermodynamic equilibrium. The laws also use various parameters for thermodynamic processes in order to establish relationships between them. Thermodynamics is the branch of physics that deals with the concepts relating to heat and temperature and the interconversion of heat and other forms of energy. Thermodynamics is part of macroscopic science. It only deals with bulk systems and does not go into detail about the molecular constitution of matter. In this article, we will discuss the laws of thermodynamics in detail.

Terms Related to Laws of Thermodynamics

There are some terms which will be frequently used in stating the laws of thermodynamics. So it is necessary for you to be familiar with the term. So some of the terms relating to laws of thermodynamics are-

Thermal Equilibrium: The state of the system is said to be in thermal equilibrium when the state of the system in the macroscopic variables such as temperature, heat, work, and energy does not change with time.

Thermodynamic System: A thermodynamic system is a system in which a body of matter is confined within the walls that separates the system from the surrounding. Thermodynamic systems can be three types:

• Isolated thermodynamics system: In an isolated system, no exchange of energy or matter takes place between the system and the surroundings. The Insulated Vessel is an example of an isolated system.
• Closed thermodynamics System: In a closed system, no exchange of matter takes place, but the exchange of energy is possible between the system and the surroundings. Copper or Steel is an example of a closed system.
• Open thermodynamics System: In an open system, the transfer of both mass and energy takes place between the system and surroundings.

Surroundings: It includes everything other than the system. The universe together comprises both systems and surroundings.

Thermodynamic process: A process in which the thermodynamic state of the system is changed by changing the heat, pressure and internal energy. Four types of thermodynamic processes are-

• Adiabatic Process – A process in which no heat transfer happens in or out of the system.
• Isochoric Process – A process in which no change in volume occurs and the volume in the system remains the same.
• Isobaric Process – A process in which no change in pressure occurs and the pressure remains the same in the system.
• Isothermal Process – A process in which no change in temperature occurs and the volume in the system remains the same.

Four Laws of Thermodynamics

Thermodynamics is the branch of physics that deals with the relations between heat, work, temperature, and energy. The laws of thermodynamics describe how these physical variables behave under different circumstances. There are basically four types of laws of thermodynamics-

1. Zeroth law of thermodynamics
2. The first law of thermodynamics
3. The second law of thermodynamics
4. Third law of thermodynamics

Zeroth Law of Thermodynamics

Zeroth's Law of Thermodynamics, states that “When two systems are in thermal equilibrium with each other and also separately in thermal equilibrium with the third system, then the three are said to be in equilibrium with each other.”

For Example: Suppose two systems A and B, are separated by an adiabatic wall, while each system is in contact with a third system C with the help of conducting wall. The states of the systems (i.e., their physical variables) will change until both A and B attains thermal equilibrium with C. After this is achieved, imagine that the adiabatic wall between A and B is replaced by a conducting wall and C is insulated from A and B by an adiabatic wall. It can be analysed that the states of A and B change no further. Hence they are in equilibrium with each other.

First Law of Thermodynamics

The first law of thermodynamics (commonly called the law of conservation of energy) states that “ Energy can neither be created nor be destroyed it can be converted from one form to the other”.

The internal energy(U) of a system can be changed by two ways of energy transfer: Heat and Work. 

Suppose, ∆Q is the Heat supplied to the system by the surroundings

∆W is the work done

∆U is the internal energy change

So, the general principle according to the conservation of energy is, ∆Q = ∆U + ∆W

Second Law of Thermodynamics

The second law of thermodynamics states that "Any spontaneous process occurring will always lead to an escalation of entropy, that measures the randomness of the system in the universe. The concept of entropy is related to the second law of thermodynamics. It can be represented as-

ΔSuniv > 0

Where,

 ΔSuniv is the entropy change

Third Law of Thermodynamics

The third law of thermodynamics states that the entropy of a system reaches a constant value when its temperature reaches absolute zero(in Kelvin).