State the First Law of Thermodynamics.

Short Answer

The First Law of Thermodynamics states that energy can neither be created nor destroyed, but it can be changed from one form to another. In simple words, the total energy of a system and its surroundings always remains constant.

The law also explains the relationship between heat, internal energy, and work. It states that when heat is supplied to a system, some part of it increases the internal energy of the system, and the remaining part is used to do work. This forms the basis of energy conservation in thermodynamics.

Detailed Explanation :

First Law of Thermodynamics

The First Law of Thermodynamics is one of the most fundamental principles in physics. It expresses the law of conservation of energy in the context of thermodynamic systems. According to the First Law, energy does not appear or disappear; instead, it simply changes forms. Heat energy can become mechanical work, mechanical work can become internal energy, and so on. However, the total energy of the universe remains unchanged.

This law helps explain how heat and work interact with the internal energy of a system. It provides a mathematical and conceptual framework for understanding all thermodynamic processes such as heating, cooling, expansion, compression, and phase changes.

Statement of the First Law of Thermodynamics

The First Law of Thermodynamics can be stated as:

“The heat supplied to a system is equal to the sum of the increase in the internal energy of the system and the work done by the system.”

This can be written mathematically as:

ΔU = Q − W

Where:

  • ΔU = Change in internal energy
  • Q = Heat supplied to the system
  • W = Work done by the system

This formula explains how heat added to a system is distributed.

Explanation of Terms in the First Law

  1. Heat (Q)

Heat is the energy transferred to the system because of temperature difference.

  • If heat is added → Q is positive
  • If heat is removed → Q is negative
  1. Work (W)

Work is the energy transferred due to movement of the system’s boundary.

  • If work is done by the system → W is positive
  • If work is done on the system → W is negative
  1. Internal Energy (U)

Internal energy is the total energy stored in the system due to molecular motion and molecular interactions.

  • If internal energy increases → ΔU is positive
  • If internal energy decreases → ΔU is negative

Significance of the First Law

The First Law helps us understand how energy behaves in thermodynamic systems. It explains:

  • How heat affects temperature
  • How energy changes during expansion or compression
  • How engines convert heat into work
  • How refrigerators move heat from cold to hot regions

The law ensures that in any thermodynamic process, the total energy remains constant.

Application in Different Thermodynamic Processes

The First Law applies differently in different processes:

  1. Isochoric Process (Constant Volume)

Volume does not change, so no work is done (W = 0).
Thus,
ΔU = Q
Heat supplied only increases internal energy.

  1. Isobaric Process (Constant Pressure)

Work is done as volume changes.
Heat supplied increases both internal energy and work output.

  1. Isothermal Process (Constant Temperature)

Internal energy remains constant (ΔU = 0).
Thus,
Q = W
All heat supplied is converted into work.

  1. Adiabatic Process (No Heat Transfer)

Heat exchange is zero (Q = 0).
Thus,
ΔU = −W
Internal energy decreases when work is done by the system.

  1. Cyclic Process

System returns to initial state, so internal energy change is zero (ΔU = 0).
Thus,
Q = W
Heat absorbed equals work done.

Examples of the First Law in Real Life

  1. Car Engines

Fuel burns and releases heat.
Part of this heat increases internal energy, while the rest moves the piston to produce work.

  1. Pressure Cooker

Steam builds up pressure. Heat increases internal energy of water and does work by lifting the whistle.

  1. Human Body

Food provides heat energy. Some increases internal energy, and some is used to do physical work.

  1. Electric Appliances

Electric energy converts to heat and work, obeying the conservation of energy.

  1. Air Conditioner

Work is done on the system to transfer heat from inside to outside.

Importance of the First Law

The First Law is extremely important because it:

  • Forms the foundation of all energy systems
  • Helps design engines, turbines, compressors, and refrigerators
  • Explains energy flow in natural and artificial processes
  • Ensures that energy calculations remain consistent
  • Helps understand the efficiency of machines

It basically tells us that energy never disappears; it only changes forms.

Conclusion

The First Law of Thermodynamics states that energy cannot be created or destroyed but can be transformed from one form to another. It explains how heat supplied to a system is used to increase internal energy and perform work. This law is a pillar of thermodynamics, helping us understand energy flow in engines, refrigerators, nature, and everyday life. It is a powerful expression of the universal conservation of energy.