The laws of thermodynamics describe the fundamental principles governing energy, heat, and work in physical systems. These laws form the basis of mechanical, thermal, and chemical engineering, and are essential for understanding engines, refrigeration systems, and energy transfer processes.
The three laws of thermodynamics as noted below are simple statements based on observations of natural processes. They are of fundamental importance to all physical and chemical systems and underpin the analysis of energy transfer and conversion.
When two objects are separately in thermodynamic equilibrium with a third object, they are also in equilibrium with each other.
Objects in thermodynamic equilibrium are at the same temperature. This principle forms the basis of temperature measurement using thermometers.
The first law expresses the principle of conservation of energy and states that heat and work are mutually convertible.
Heat input equals work output over a complete cycle:
Σ dQ = Σ dW
Over a complete cycle, the algebraic sum of heat supplied to a system equals the algebraic sum of work done by the system.
In a cyclic process, the system returns to its original state. The term (δQ − δW) represents the change in internal energy δE:
δQ = δE + δW
The total internal energy includes:
For most mechanical engineering applications, atomic and chemical energy are neglected:
E = U + P.E + K.E + S.E
Where U is internal energy. The first law is often written as:
δQ = δU + δW
This is known as the restricted energy equation.
Further examples:
Non-Flow Processes
Steady Flow Equation
The second law is based on experimental observation and defines the direction of heat transfer and efficiency limits.
Heat flows naturally from hot bodies to cold bodies and not in the reverse direction without external work.
In thermodynamic cycles:
The maximum theoretical efficiency is achieved by the Carnot Cycle.
Common interpretations include:
The Clausius inequality states that for any thermodynamic cycle:
This principle confirms the second law and is demonstrated using Carnot cycles.
Entropy is a thermodynamic property defined by:
Entropy measures the degree of disorder or energy distribution within a system.
Further details: Entropy Notes
The laws of thermodynamics define how energy is transferred and transformed within physical systems. They establish the limits of efficiency and the direction of natural processes.
| Law | Statement | Key Principle |
|---|---|---|
| Zeroth Law | If two systems are in equilibrium with a third system, they are in equilibrium with each other. | Defines temperature and thermal equilibrium |
| First Law | Energy cannot be created or destroyed, only transferred or converted. | Conservation of energy (δQ = δU + δW) |
| Second Law | Heat flows from hot to cold and not the reverse without external work. | Defines efficiency limits and entropy increase |
Together, these laws provide the foundation for analysing energy systems such as engines, turbines, refrigeration systems, and thermal processes in engineering.
The laws of thermodynamics are used in the design and analysis of: