First Law of Thermodynamics free note for Diploma / BTech.

1. What is First law of Thermodynamics ?

The First Law of Thermodynamics is the law of conservation of energy applied to thermodynamic systems. It states that energy can neither be created nor destroyed; it can only be transferred or converted from one form to another.

Whenever heat is supplied to a system, the energy is either:

Stored as internal energy, or
Used by the system to perform work on its surroundings.

This law forms the basis for analyzing engines, refrigerators, turbines, compressors, boilers, heat exchangers, and other thermal systems.

2. Statement of the First Law of Thermodynamics

The heat supplied to a system is equal to the increase in its internal energy plus the work done by the system.

Mathematical Expression

For a closed system, $Q = \Delta U + W$

where:

Q = Heat supplied to the system (J or kJ)
ΔU = Change in internal energy (J or kJ)
W = Work done by the system (J or kJ)

This equation is often rearranged as: $\Delta U = Q – W$

Explanation

When heat is added to a system:

Part of the energy increases the molecular energy (internal energy).
The remaining energy may be used to expand the system and perform work.

For example, heating air inside a piston-cylinder arrangement causes the gas to expand, pushing the piston upward. The supplied heat is therefore divided between increasing the gas’s internal energy and doing expansion work.

Sign Convention

Quantity	Positive When
Heat (Q)	Added to the system
Work (W)	Done by the system
Internal Energy (ΔU)	Increases

Thus,

Heat added → Positive
Heat rejected → Negative
Work done by the system → Positive
Work done on the system → Negative

3. Special Cases

1. Constant Volume Process

Since there is no boundary movement, $W = 0$

Therefore, $Q = \Delta U$

All supplied heat increases the internal energy.

2. Adiabatic Process

No heat transfer occurs. $Q = 0$

Hence, $\Delta U = -W$

The work done comes entirely from the system’s internal energy.

3. Cyclic Process

At the end of a complete cycle, $\Delta U = 0$

Therefore, $Q = W$

The net heat supplied equals the net work done during the cycle.

4. Isolated System

Neither heat nor work crosses the system boundary. $Q = 0,\qquad W = 0$

Thus, $\Delta U = 0$

The total energy remains constant.

4. Internal Energy

Internal energy is the total microscopic energy contained within a substance due to:

Molecular motion (kinetic energy)
Molecular interactions (potential energy)

It is represented by U and is a state property, meaning it depends only on the current state of the system.

5. Limitations of the First Law

The first law:

Does not indicate the direction of heat flow.
Does not explain why heat naturally flows from hot to cold.
Does not account for energy quality or irreversibility.
Cannot determine whether a process is feasible.

These aspects are addressed by the Second Law of Thermodynamics.

6. Applications

The first law is applied in the analysis and design of:

Steam power plants
Internal combustion engines
Gas turbines
Compressors
Pumps
Refrigerators
Air conditioners
Boilers
Heat exchangers
Industrial heating systems

7. Numerical Example

Problem:

A closed system receives 800 kJ of heat and performs 300 kJ of work. Determine the change in internal energy.

Solution:

Given:

Heat supplied, $Q = 800$ Q=800 kJ
Work done, $W = 300$ W=300 kJ

Using the first law, $\Delta U = Q – W$ $\Delta U = 800 – 300 = 500 \text{ kJ}$

Answer: The internal energy increases by 500 kJ.

8. Advantages of the First Law

Based on the principle of energy conservation.
Helps analyze energy interactions in thermal systems.
Provides the foundation for thermodynamic cycle analysis.
Widely used in engineering design and performance calculations.

9. Disadvantages

Does not distinguish between useful and unusable energy.
Cannot predict process direction.
Does not explain entropy generation.
Does not establish the maximum possible efficiency of heat engines.

10. Key Points

Energy cannot be created or destroyed.
Heat supplied is converted into internal energy and work.
Internal energy is a state property.
The first law is an expression of energy conservation.
It applies to both closed and open thermodynamic systems (with appropriate energy balance equations).

Frequently Asked Questions (FAQs)

1. What is the First Law of Thermodynamics?
It states that energy cannot be created or destroyed; it can only be transferred or transformed. The heat supplied to a system equals the increase in internal energy plus the work done by the system.

2. What is the mathematical form of the first law?
For a closed system: $Q = \Delta U + W$

or equivalently, $\Delta U = Q – W$

3. What is internal energy?
Internal energy is the microscopic energy stored within a system due to the motion and interactions of its molecules.

4. What happens during an adiabatic process?
No heat is transferred ( $Q = 0$ Q=0). Any work done by or on the system changes its internal energy.

5. What happens in a cyclic process?
The system returns to its initial state, so the change in internal energy is zero, and the net heat supplied equals the net work done.

6. Why is the First Law important in engineering?
It provides the fundamental energy balance used to analyze engines, turbines, compressors, boilers, refrigeration systems, and power plants.

7. Can the First Law determine whether a process is possible?
No. It only accounts for energy conservation. The feasibility and direction of a process are determined by the Second Law of Thermodynamics.

8. What are the SI units of heat, work, and internal energy?
The SI unit for all three is the joule (J). In engineering, kilojoules (kJ) are commonly used.