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JEE Main 2026 Preparation: Question Papers, Solutions, Mock Tests & Strategy Unacademy » JEE Study Material » Chemistry » The First Law of Thermodynamics

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The First Law of Thermodynamics

The first law of thermodynamics is centred around the idea that the total amount of energy always remains constant as it can neither be created nor destroyed.

Table of Content

Introduction

In Physics, Thermodynamics is the branch that deals with the relationship among work, heat, energy and temperature. Lord Kelvin was the first Scots-Irish physicist who defined thermodynamics as the subject of the relation between heat and forces. There are four laws of thermodynamics:

1) Zeroth law of thermodynamics

2) The first law of thermodynamics

3) The second law of thermodynamics

4) The third law of thermodynamics

The first law of thermodynamics

The first law of thermodynamics states that if heat is recognised as a form of energy, then the total energy of its surroundings remains constant. This implies that the total energy of the universe is constant. The first law of thermodynamics is said to be in action when energy flows across the boundary that separates a system from its surroundings. The first law of thermodynamics is also called the law of conservation of energy. The equation of the first law of thermodynamics is: ΔU = Q – W Here,

ΔU is the change in the internal energy of the system
Q is the algebraic sum of heat energy transferred between the system and the surrounding
W is the work done by the system with its surroundings
For an isolated system, the energy always remains constant

Application of first law of thermodynamics

Some Applications of the first law of thermodynamics are as follows:

Refrigerators: If the refrigerator door is open inside a closed room, the temperature of the room will increase: The temperature increases when the gas is compressed
Thermal power plants: The first law of thermodynamicsstates that energy can neither be created nor destroyed. This law is used in power plants to generate power through fossil fuels
In steam engines: The steam engine performs a cyclic function with a piston moving up and down once every cycle
Heat pumps: Heat pumps are air conditioners that run in reverse. These are used as engines that convert mechanical energy into heat energy
Radiant energy: It is the energy that is transferred by electromagnetic radiation. For example, light bulbs transform electrical energy into light energy
Human metabolism: Human metabolism process is also an example of the first law of thermodynamics. In this process, when the food is getting digested, chemical energy is getting converted into kinetic energy

Some applications of the first law of thermodynamics in different chemical processes:

Adiabatic process: This is when heat cannot leave or enter the system. The work done in this process increases the internal energy
Isothermal process: In this process, the system’s temperature remains constant in the system
Isochoric process: In this, the system’s volume does not change
Isobaric process: In this process, the system’s pressure remains constant

Limitations of the first law of thermodynamics

Some limitations of the first law of thermodynamics are:

It does not tell anything about the direction of the flow of the heat
It does not tell whether the process is spontaneous or not
If any system or reaction follows the first law of thermodynamics, the process would not be reversible
It does not specify whether the reaction is feasible or not
In general, it is not possible to convert the total energy into equivalent work

Conclusion

According to the first law of thermodynamics, energy can neither be created nor be destroyed. This implies that during an interaction between a system and its surroundings, the amount of energy gained by the system must be exactly equal to the amount of energy lost by the surroundings. Therefore, the first law of thermodynamics is called the law of conservation of energy. This law also states that the internal energy of any system has to be equal to the work being done by the system.