Thermodynamics is the branch of physics that studies energy, heat, work, temperature, and the transformations that occur between different forms of energy in physical systems. Its central purpose is to establish general relationships between the macroscopic properties of matter without requiring a complete description of the motion of every individual atom or molecule.
A thermodynamic system can be anything selected for analysis, such as a gas in a cylinder, an engine, a refrigerator, a chemical reaction, Earth's atmosphere, a biological organism, or an entire power station. The surroundings are everything outside the chosen system, and the boundary separates the system from its surroundings. Thermodynamics is concerned with quantities such as internal energy, enthalpy, entropy, pressure, volume, temperature, and heat capacity.
It provides laws that constrain what processes are possible and determine how energy is transferred and transformed. The subject is therefore much broader than the study of engines or heat. It provides a general framework for understanding equilibrium, energy conversion, phase changes, chemical processes, refrigeration, combustion, atmospheric processes, and many biological and technological systems.
The foundations of thermodynamics developed from attempts to understand heat engines and improve the efficiency of machines during the eighteenth and nineteenth centuries. Early work by engineers and physicists studying steam engines eventually led to the recognition that heat and work are related forms of energy transfer. Sadi Carnot analysed idealised heat engines and established principles concerning their maximum possible efficiency.
James Prescott Joule demonstrated the quantitative relationship between mechanical work and thermal effects, helping establish the equivalence of mechanical and thermal energy. Rudolf Clausius formulated important mathematical ideas concerning entropy, while William Thomson, later Lord Kelvin, contributed to the absolute temperature scale and the second law. The first law formalised energy conservation, the second law established the directionality and limitations of energy transformations, and the third law clarified behaviour near absolute zero. Statistical mechanics later provided a microscopic interpretation of thermodynamic quantities by relating macroscopic behaviour to atoms, molecules, and probability.
Thermodynamics uses several important concepts to describe systems. A state variable is a property determined by the current equilibrium state rather than by the particular process through which that state was reached. Examples include pressure, temperature, volume, internal energy, enthalpy, and entropy. State functions contrast with path dependent quantities such as heat and work, whose values depend on the process connecting two states.
A thermodynamic process may be isothermal when temperature is constant, isobaric when pressure is constant, isochoric when volume is constant, or adiabatic when no heat is transferred across the system boundary. Processes may also be reversible or irreversible. A reversible process is an idealisation in which the system remains arbitrarily close to equilibrium and can be reversed without net changes to the system and surroundings. Real processes involve finite gradients, friction, mixing, turbulence, heat transfer across finite temperature differences, or other sources of irreversibility. These distinctions allow thermodynamic calculations to describe both ideal limits and real physical behaviour.
The first law of thermodynamics is a statement of energy conservation applied to thermodynamic systems. In one common sign convention, the change in a system's internal energy equals heat supplied to the system minus work done by the system. Internal energy represents the microscopic energy associated with the system, including molecular translational, rotational, vibrational, electronic, and interaction energies where relevant. Heat and work are not substances stored inside an object. They are modes of energy transfer across a system boundary.
When energy enters a system as heat, its internal energy can increase, although the result depends on whether energy is simultaneously transferred out through work or other mechanisms. When a gas expands against an external pressure, it can perform work on its surroundings and consequently experience a reduction in internal energy unless sufficient energy enters through another pathway. The first law therefore provides an accounting framework that prevents energy from being created or destroyed during thermodynamic processes.
The second law of thermodynamics concerns the direction in which processes occur and introduces entropy as a fundamental thermodynamic quantity. Although energy is conserved, not every imaginable energy transformation is physically possible. Heat naturally flows from a hotter body to a colder body rather than spontaneously in the reverse direction. A heat engine cannot convert all heat supplied to it into useful work while operating cyclically with only a single thermal reservoir.
The second law can be expressed through the requirement that the total entropy of an isolated system does not decrease. Entropy is associated with the number and statistical weighting of microscopic configurations compatible with a macroscopic state, while thermodynamically it quantifies aspects of energy dispersal and irreversibility. Entropy does not simply mean disorder in every scientific context, because that informal description can be misleading. The second law is central to understanding why processes have preferred directions and why useful energy conversion always involves constraints.
The third law addresses the behaviour of matter as temperature approaches absolute zero. For a perfect crystalline substance under appropriate assumptions, the entropy approaches a limiting value as temperature approaches zero kelvin. The third law has several equivalent formulations and provides a foundation for defining absolute entropy and understanding low temperature physics. Reaching absolute zero exactly is not achievable through a finite sequence of ordinary thermodynamic operations, although temperatures extremely close to it can be produced using sophisticated techniques.
Quantum effects become increasingly important at very low temperatures, and classical approximations may cease to be adequate. The third law also explains why heat capacities and other thermodynamic properties behave in particular ways near very low temperatures. It is therefore not merely a statement about coldness but a fundamental constraint on the thermodynamic structure of matter and on the practical attainability of the lowest temperatures.
Thermodynamics can be formulated at several levels. Classical thermodynamics uses macroscopic variables and empirical laws without explicitly tracking individual particles. Statistical mechanics connects thermodynamics to microscopic states and probability distributions, explaining quantities such as temperature, entropy, and pressure in terms of molecular behaviour. Chemical thermodynamics applies the same principles to reactions, mixtures, phase equilibria, chemical potentials, and changes in composition. Engineering thermodynamics analyses engines, turbines, compressors, pumps, refrigeration systems, heat exchangers, and power cycles. Biological thermodynamics investigates energy transformations in living systems, while atmospheric and geophysical thermodynamics describe processes involving air, water, phase changes, radiation, and planetary environments. Despite these different applications, the same fundamental principles of energy conservation, equilibrium, entropy, and state relationships provide the underlying framework.
Thermodynamics has practical importance throughout modern technology and science. Internal combustion engines, steam turbines, gas turbines, refrigeration systems, heat pumps, industrial furnaces, chemical plants, power stations, and electronic cooling systems all depend on thermodynamic principles. Thermodynamics also determines the efficiency limits of energy conversion and helps engineers identify where energy is transferred, lost, or degraded in practical systems. It is closely related to fluid mechanics, electromagnetism, chemistry, materials science, statistical mechanics, and quantum physics.
A common misconception is that thermodynamics says energy is lost during inefficient processes. Energy itself is conserved, but energy can become less available for conversion into useful work, with entropy increasing as irreversible processes occur. Another misconception is that thermodynamics is exclusively about heat. In reality, it is a general theory of energy, equilibrium, and transformations that applies even to systems where mechanical, chemical, electrical, or other forms of energy dominate.
