Short Answer

The First Law of Thermodynamics states that energy cannot be created or destroyed; it can only be changed from one form to another. This means the total energy of a system and its surroundings always remains constant.

In simple terms, if heat is supplied to a system, it either increases the internal energy of the system or does work on the surroundings. This law explains how energy flows in machines, engines, and natural processes.

Detailed Explanation :

First Law of Thermodynamics

The First Law of Thermodynamics is a fundamental principle that governs all energy-related processes in nature and technology. It states that energy cannot be created from nothing and cannot disappear on its own. Instead, it only transforms from one form to another. This law helps explain how heat, work, and internal energy are related inside a physical system.

This law is also known as the Law of Conservation of Energy for thermodynamic systems. It forms the basis of all engines, refrigerators, heating systems, and even biological processes like eating and breathing. It ensures that whenever energy seems to be “lost,” it has actually been converted into another form.

Statement of the First Law of Thermodynamics

The First Law states:

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

Mathematically,

Where:

• Q = heat supplied to the system
• ΔU = change in internal energy
• W = work done by the system

This equation shows how the energy entering a system is distributed.

Meaning of Each Term

1. Heat (Q)

Heat is the energy transferred into the system due to temperature difference. When heat enters, the system gains energy.

2. Internal Energy (ΔU)

Internal energy is the total kinetic and potential energy of the molecules inside a substance.
When heat is supplied, particles move faster, increasing internal energy.

3. Work (W)

Work is energy used by the system to expand or move.
If the system expands, it does work on the surroundings.

Example:
When steam expands in an engine, it pushes the piston and does work.

Explaining the First Law with Simple Examples

1. Heating a Gas in a Cylinder

When heat is supplied to a gas:

• Part of the heat increases the internal energy (temperature rises).
• Part of the heat is used to push the piston (work is done).

Thus,

2. Human Body

Food provides energy (similar to heat input).
This energy:

• Increases body energy
• Helps us do work like walking or lifting objects

Energy is not created but transformed.

3. Boiling Water

When heat is supplied:

• Internal energy increases
• This energy helps water molecules change from liquid to gas (doing work in expanding)

4. Car Engine

Fuel contains chemical energy.
When burned:

• Part becomes mechanical work (movement of car)
• Part becomes internal energy (heat of engine)

Energy is conserved but changes forms.

Implications of the First Law

1. Energy Conservation

Energy never disappears.
If something heats up, cools down, or moves, its energy just changes form.

2. Machines Cannot Produce Energy from Nothing

No engine can run without fuel.
No machine can produce more energy than it consumes.
This proves perpetual motion machines are impossible.

3. Heat Converts Into Work

Heat energy can be changed into mechanical work but only partially.
All heat cannot be converted into work.

4. Work Converts Into Heat

When you rub your hands, the work you do converts into heat.

Applications of the First Law

1. Heat Engines

Steam engines, petrol engines, and diesel engines use the First Law to convert heat into work.

2. Refrigerators and Air Conditioners

These devices use work to move heat from cold regions to hot regions.

3. Power Plants

Thermal power plants convert heat energy into electrical energy using this law.

4. Chemistry and Biology

Reactions involving energy changes (exothermic or endothermic) follow the First Law.

5. Everyday Activities

Walking, eating, cooking, and even breathing involve energy transfer and transformation.

Why It Is Called the First Law

This law is placed first because:

• It is basic and universal
• It applies to all systems—living or non-living
• It forms the foundation for other laws of thermodynamics

Conclusion

The First Law of Thermodynamics states that energy cannot be created or destroyed but only transformed from one form to another. It explains the relationship between heat supplied, internal energy, and work done by a system. This law is essential for understanding engines, refrigerators, power systems, and all natural processes involving energy. It ensures that the total energy of the universe remains constant.