First law of thermodynamics state that "heat energy of any system converts in work done +internal energy of the system."
Hence based on conservation of energy.

First Law of Thermodynamics: Conservation of Energy

The first law of thermodynamics, also known as the law of energy conservation, is a fundamental principle in thermodynamics. It states that energy cannot be created or destroyed in an isolated system; it can only be transferred or transformed from one form to another.

Explanation:

1. Conservation of Energy:
- The first law of thermodynamics is based on the principle of conservation of energy. This principle, also known as the law of energy conservation, states that the total energy of an isolated system remains constant over time.
- In other words, energy can neither be created nor destroyed, but it can be converted from one form to another or transferred between different objects within the system.
- This principle is derived from our understanding of the fundamental nature of energy and its interaction with matter.

2. Energy Transfer and Transformation:
- The first law of thermodynamics applies to all forms of energy, including mechanical energy, thermal energy, chemical energy, and electromagnetic energy.
- It provides a framework to understand how energy is transferred or transformed within a system, and how it affects the system's internal energy.
- For example, when work is done on a system, such as a gas being compressed, energy is transferred to the system and its internal energy increases.
- Similarly, when work is done by a system, such as a gas expanding and pushing a piston, energy is transferred out of the system and its internal energy decreases.

3. Mathematical Formulation:
- The first law of thermodynamics can be mathematically expressed as ΔU = Q - W, where ΔU represents the change in internal energy of the system, Q represents the heat transferred to the system, and W represents the work done by the system.
- This equation shows that any increase in internal energy of the system must be balanced by the sum of heat transfer into the system and work done by the system.
- It provides a quantitative understanding of how energy is conserved within a system and how it is related to heat and work.

Conclusion:
The first law of thermodynamics, based on the principle of conservation of energy, is a fundamental principle in thermodynamics. It states that energy cannot be created or destroyed in an isolated system, but it can be transferred or transformed from one form to another. This law provides the basis for understanding energy transfer and transformation within a system and is essential in the study of thermodynamics and related fields.