First Law of Thermodynamics

The First Law of Thermodynamics, also known as the Law of Energy Conservation, states that energy can be neither created nor destroyed, only transferred or converted from one form to another.

mathematically the first law of thermodynamics is expressed as

Q = ∆U + W.

In other words whenever a heat is applied to a thermodynamic system there is a change in internal energy of the system and also a certain work is done by the system to the surroundings.

This above equation can also be written as:

∆U = Q-W.

where,

∆U = change in internal energy of the system.

Q = amount of heat transferred to the system.

W = work done by the system.

Important concepts in first law of thermodynamics.

Internal energy(U):

Internal Energy (U) in thermodynamics refers to the total energy contained within a system due to the microscopic motion (Kinetic energy)and interactions of its particles(Potential energy).

hence internal energy is the sum of potential energy and kinetic energy of gas molecules in thermodynamics system.

Internal energy U = Potential energy+ Kinetic energy.

Heat (Q):

Energy transferred to the system due to a temperature difference. Heat flows from a hotter body to a cooler one.

Work (W):

Represents energy transfer due to mechanical processes. Positive means work is done by the system; negative means work is done on the system.

These above three terms are used to describe the first law of thermodynamics.

Applications of the First Law of Thermodynamics:

The First Law of Thermodynamics has wide-ranging applications across various fields of science and engineering. Some of the key applications include:

1. Isothermal Processes (∆U = 0 ):

In an isothermal process, the temperature of the system remains constant, so the internal energy does not change.

Application: Gas expansion or compression: For ideal gases, the work done by the gas is equal to the heat added (Q = W).

2. Adiabatic Processes (Q=0):

In an adiabatic process, there is no heat exchange with the surroundings.

Application:Rapid expansion or compression of gases, such as in internal combustion engines or air compressors, where the change in internal energy equals the work done (∆U = -W ).

3. Isobaric Processes (p = constant ):

In an isobaric process, the pressure remains constant, and both heat and work contribute to changes in internal energy.

Application:Heating or cooling gases at constant pressure, such as in heat exchangers.

4. Isochoric Processes (W= 0):

In an isochoric process, the volume remains constant, so no work is done.

Application:Heating of gases in a rigid container, where all heat added changes the internal energy (∆U = Q).

5. Heat Engines:

The first law is used to calculate work output and efficiency of heat engines.Application:Steam turbines, gasoline and diesel engines, where heat is converted into mechanical work.

6. Refrigerators and Heat Pumps:

The first law governs the energy balance in refrigeration cycles, where work is done to transfer heat from a colder to a hotter region.

Application:Refrigerators, air conditioners, and heat pumps.

7. Phase Changes:

The first law is used to analyze energy changes during phase transitions (e.g., melting, boiling) where heat is added or removed without a temperature change.

Application: Latent heat calculations in processes like vaporization and condensation.

8. Biological Systems:

The first law applies to energy transformations in living organisms.

Application: Metabolic processes: Energy from food is converted into work and heat.

9. Thermal Power Plants:

The first law helps analyze the energy transfer in power plants where fuel combustion generates heat, which is used to produce work.

Application: Efficiency calculations of thermal plants (e.g., steam or gas turbines).

10. Rocket Propulsion:

The first law is used to calculate the energy changes in gases expelled from rockets to produce thrust.

Application: Energy balance in propulsion systems.

These examples highlight how the First Law of Thermodynamics serves as a foundation for understanding and designing systems that involve energy transformations.