Fundamental Laws of Thermodynamics and Thermal Cycles

Understanding Thermodynamics

Thermodynamics is the science of converting heat into work or the reverse process of converting work into heat.

Thermodynamic Equilibrium

A system is said to be in thermodynamic equilibrium if there is no resultant force acting on the system and if the system temperature is the same as that of its surroundings.

The First Law of Thermodynamics

Act 1: This is a restatement of the principle of conservation of energy. A mathematical postulate of the 1st law is: in any thermodynamic process, the net heat absorbed by a system equals the sum of the thermal equivalent of work done by the system and the change in internal energy of the same.

Isochoric Process

An isochoric process is one in which the system volume remains constant (since no work is performed), thus: W = 0; ΔQ = ΔU.

Isothermal Process

The isothermal process is one in which the system temperature remains constant, indicating no change in the internal energy of the system: ΔU = 0; ΔQ = ΔW.

The Second Law of Thermodynamics

The 2nd Law states that in any process, some loss of energy takes place due to frictional forces or other forces of dissipation. A 100% efficient engine, or one that converts all heat input into useful work output, is not possible.

Heat Engines and Thermodynamic Cycles

• Motor efficiency: The relationship between work output and heat input.
• Refrigerator: A heat engine that works in reverse.
• Carnot cycle: An ideal reversible thermodynamic cycle between two sources of temperature and four processes in which performance is maximized.
• Carnot heat engine: An ideal machine, free of the problems typical for heat engines (such as heat loss through conduction, etc.). The Carnot engine has the maximum possible efficiency. The efficiency of a machine can be determined by comparison with the Carnot engine when operating between the same temperatures.
• Otto cycle: The ideal thermodynamic cycle applied in internal combustion engines.