What Is Thermodynamics? Research Areas, Funding, and Career Paths

Thermodynamics is the branch of physics concerned with heat, work, temperature, and energy, and with the laws that govern how these quantities move and transform in physical systems — from a single gas-filled cylinder to a star. It asks how energy is conserved as it converts between forms (mechanical, thermal, chemical, electrical), why some processes run forward but never spontaneously reverse, and how the microscopic behavior of enormous numbers of particles produces the macroscopic properties — temperature, pressure, volume — that can actually be measured in a lab. Thermodynamics is one of the oldest quantitatively precise branches of physics, and also one of the most broadly applied: its four laws hold for gases, liquids, solids, chemical reactions, biological cells, and (in a more exotic form) black holes alike, which is why the field sits underneath large parts of chemistry, mechanical and chemical engineering, materials science, and even cosmology, rather than staying confined to physics departments.

What thermodynamics actually studies

At its core, thermodynamics studies four things:

• Energy and its conservation — how energy moves into, out of, and within a system as heat and work, and the accounting principle (the first law of thermodynamics) that says total energy is conserved even as it changes form.
• Entropy and the direction of processes — why heat flows from hot to cold and not the reverse, why a dropped egg doesn’t spontaneously reassemble, and more generally why many everyday processes are irreversible; the second law of thermodynamics, built around the concept of entropy, is the formal statement of this asymmetry and is often called the only physical law with a built-in arrow of time.
• Equilibrium and state — the properties (temperature, pressure, volume, internal energy) that fully describe a system once it has settled into a stable state, and the mathematical relationships (equations of state, such as the ideal gas law) connecting those properties.
• The link between microscopic and macroscopic behavior — how the statistical behavior of huge numbers of atoms or molecules, none individually “hot” or “cold,” gives rise to macroscopic quantities like temperature and pressure; this connection is the subject matter of statistical mechanics, thermodynamics’ close sibling field, discussed further below.

Thermodynamics is traditionally organized around four laws. The zeroth law establishes that temperature is a well-defined, transitive property (if two systems are each in thermal equilibrium with a third, they’re in equilibrium with each other) — logically prior to the others but named after them because it was formalized later. The first law is energy conservation: the change in a system’s internal energy equals the heat added to it minus the work it does. The second law states that total entropy in an isolated system never decreases, which is why heat engines can never be perfectly efficient and why some energy is always “lost” as unusable heat in any real process. The third law states that a system’s entropy approaches a well-defined minimum as its temperature approaches absolute zero. Together these four laws are among the most experimentally robust statements in all of physics — no confirmed violation has ever been observed — and they apply regardless of what the system is actually made of, which is part of why thermodynamics generalizes so far beyond its origins in 19th-century steam-engine physics.

Classical thermodynamics vs. statistical mechanics

Thermodynamics can be approached two ways, and understanding the difference is central to the field. Classical (or “phenomenological”) thermodynamics works entirely with macroscopic, measurable quantities — temperature, pressure, volume, entropy — and the mathematical relationships between them, without needing any assumption about what matter is made of at the atomic scale; this is the form Sadi Carnot, Rudolf Clausius, and William Thomson (Lord Kelvin) developed in the 19th century to understand steam engines, and it remains the practical, engineering-facing form of the field. Statistical mechanics, developed later chiefly by Ludwig Boltzmann and J. Willard Gibbs, derives the same macroscopic laws from the statistical behavior of a system’s underlying particles, explaining, for example, that entropy is fundamentally a measure of the number of microscopic arrangements consistent with a given macroscopic state. The two are complementary rather than competing: classical thermodynamics gives the general, substance-independent laws, and statistical mechanics explains why those laws hold and lets researchers predict a specific material’s thermodynamic properties from its molecular structure.

Major sub-disciplines within thermodynamics

• Statistical mechanics — the microscopic, probability-based foundation of thermodynamics described above; increasingly its own specialty, with deep connections to condensed matter physics and, more recently, quantum information theory.
• Chemical thermodynamics — the application of thermodynamic principles to chemical reactions and phase changes: whether a reaction proceeds spontaneously, how far it goes before reaching equilibrium, and how much useful work it can do, generally analyzed through Gibbs free energy. This is a core part of physical chemistry.
• Non-equilibrium thermodynamics — extends the field beyond systems at or near equilibrium to systems with ongoing flows of heat, mass, or energy (a living cell, a weather system, a chemical reaction still in progress); a genuinely more mathematically demanding area, and an active modern research frontier.
• Engineering (applied) thermodynamics — the design analysis of heat engines, refrigeration and air-conditioning cycles, power plants, and propulsion systems; a foundational subject in mechanical and chemical engineering curricula, concerned with real-world efficiency limits set by the second law.
• Thermochemistry — the specific study of heat absorbed or released in chemical reactions and phase changes, measured experimentally through calorimetry (see Research methods below).
• Condensed matter and materials thermodynamics — thermodynamic behavior in solids and liquids, including phase diagrams, phase transitions (melting, boiling, and more exotic transitions like superconductivity), and the thermodynamic modeling used to design new materials.
• Biological thermodynamics (bioenergetics) — how living cells and organisms obey the same energy-conservation and entropy laws while maintaining highly ordered, far-from-equilibrium states, central to understanding metabolism and cellular energy transfer.
• Quantum thermodynamics — a newer field extending thermodynamic concepts like work, heat, and entropy to quantum-mechanical systems, motivated partly by the thermodynamic limits of quantum computing devices.
• Black hole and cosmological thermodynamics — a striking extension of the field’s laws to gravitational physics, following the discovery by Jacob Bekenstein and Stephen Hawking in the 1970s that black holes have a well-defined entropy (proportional to their event-horizon area) and temperature, and obey direct analogues of the ordinary laws of thermodynamics.

How thermodynamics relates to neighboring disciplines

Thermodynamics is formally a sub-discipline of physics — CASRAI’s companion guide on what physics is covers the field as a whole, including thermodynamics’ place alongside classical mechanics, electromagnetism, and quantum mechanics as one of physics’ foundational branches — but in practice its laws are load-bearing well outside physics departments:

• Chemistry — chemical thermodynamics and thermochemistry are core subfields of physical chemistry, governing why reactions happen, how far they proceed, and how much energy they release or absorb; readers researching that broader neighboring field can see CASRAI’s guide on organic chemistry for how reaction thermodynamics shows up in synthetic chemistry specifically.
• Mechanical and chemical engineering — applied thermodynamics is a required foundation for designing engines, turbines, refrigeration systems, and industrial chemical processes, and for understanding the hard efficiency limits the second law imposes on all of them.
• Materials science — phase diagrams and phase-transition thermodynamics are central tools for predicting and designing a material’s structure and properties; see CASRAI’s companion guide on materials science.
• Quantum physics — statistical mechanics and quantum thermodynamics both draw directly on quantum-mechanical descriptions of matter, and the thermodynamic behavior of quantum computing hardware is an active shared research area; see CASRAI’s companion guide on quantum physics.
• Earth and planetary science — thermodynamic principles govern the Earth’s internal heat flow, atmospheric and ocean circulation, and planetary formation and evolution, connecting directly to geology-adjacent research.
• Biology — bioenergetics applies thermodynamic accounting to metabolism, cellular respiration, and the energy flows that keep living systems far from equilibrium.

Thermodynamics is one of the disciplines mapped in CASRAI’s overview guide to the branches of science, which shows how it relates to the full landscape of scientific disciplines rather than just its closest neighbors.

Who funds thermodynamics research

Thermodynamics research spans basic physics, chemistry, and applied engineering, so its funding is spread across several federal directorates and program offices rather than concentrated in one place. In the United States, the core federal funders are:

• The National Science Foundation (NSF), primarily through its Directorate for Mathematical and Physical Sciences (MPS) — the Division of Physics and Division of Chemistry both fund fundamental thermodynamic and statistical-mechanics research, and the Division of Materials Research funds thermodynamics-of-materials work — and through its Directorate for Engineering, whose Division of Chemical, Bioengineering, Environmental, and Transport Systems (CBET) funds applied thermal-transport and thermodynamics research with direct engineering relevance.
• The Department of Energy (DOE), through its Office of Science: the Office of Basic Energy Sciences (BES) is the largest single funder of fundamental thermodynamics-adjacent research in the US, supporting both condensed-matter/materials thermodynamics and chemical-sciences programs, since understanding and controlling energy conversion at the molecular and materials level is directly central to DOE’s energy mission. See CASRAI’s guide on writing a DOE Office of Science grant proposal narrative and its comparison of ARPA-E and DOE Office of Science funding mechanisms for how DOE’s energy-focused funding differs from more open-ended basic-science support.
• NASA, for thermodynamics as it applies to planetary science, astrophysics, and spacecraft thermal systems.
• The National Institutes of Health (NIH), mainly through the National Institute of General Medical Sciences (NIGMS), for biological and biophysical thermodynamics research relevant to protein folding, cellular energetics, and molecular biophysics — a narrower but real funding channel compared with the physics- and chemistry-side agencies above.
• Defense agencies, including the Air Force Office of Scientific Research (AFOSR) and the Office of Naval Research (ONR), fund thermal-management and energy-conversion research with dual-use relevance, similar to their broader role in physics funding — see CASRAI’s guide to the DOD research funding landscape.

Outside the US, national funders play the equivalent role — the UK’s Science and Technology Facilities Council (STFC) and the Engineering and Physical Sciences Research Council fund UK physical-sciences and engineering thermodynamics research respectively. Unlike some other physics subfields, thermodynamics does not have a small set of large, field-specific private foundations; where private funding exists, it tends to come from foundations supporting physics, chemistry, or energy research more broadly, such as the Simons Foundation’s support for theoretical physics and mathematics. Always confirm current solicitation details, eligibility, and program names directly with the funding agency before relying on them for a specific proposal — program structures and priorities change from year to year.

Research methods, tools, and equipment

Thermodynamics research combines direct measurement with modeling and simulation:

• Calorimetry — the direct measurement of heat absorbed or released by a physical or chemical process; techniques range from bomb calorimetry (measuring the heat of combustion) to differential scanning calorimetry (measuring heat flow as a sample is heated or cooled, used to characterize phase transitions and reaction thermodynamics).
• Pressure-volume-temperature (PVT) measurement — apparatus for directly measuring how a substance’s pressure, volume, and temperature relate, used to determine and validate equations of state.
• Spectroscopy — used to infer thermodynamic properties (vibrational energy levels, phase behavior) indirectly from how a material interacts with light or other radiation.
• Cryogenics and precision temperature control — apparatus for holding samples at very low or very precisely controlled temperatures, essential for studying phenomena like superconductivity and for testing the third law’s low-temperature predictions.
• Computational thermodynamics — molecular dynamics and Monte Carlo simulations, and electronic-structure methods such as density functional theory, used to predict a material or molecule’s thermodynamic properties computationally rather than (or before) measuring them experimentally; increasingly central as computing power has grown, particularly for materials design.

Careers and training in thermodynamics

Thermodynamics is rarely a standalone degree program; researchers working primarily on thermodynamic problems typically hold a bachelor’s degree and PhD in physics, chemistry, chemical engineering, or mechanical engineering, with thermodynamics or statistical mechanics as a specialization within that broader field. The typical US graduate path runs a bachelor’s degree followed by a PhD of roughly 5-6 years, involving coursework, qualifying examinations, and original dissertation research, often followed by one or more postdoctoral positions before a permanent faculty, national-laboratory, or industry role. Industry demand is substantial in energy, aerospace, semiconductor, and materials sectors, where thermal management and energy-conversion efficiency are direct engineering concerns, which means thermodynamics specialists have somewhat more industry (versus purely academic) career pathways available than some other physics subfields.

Researchers in this area are typically members of the professional societies for their home discipline rather than a thermodynamics-specific society: the American Physical Society (APS) for the physics side (including its statistical and nonlinear physics community), the American Chemical Society (ACS) for chemical thermodynamics and thermochemistry, and the American Society of Mechanical Engineers (ASME) and American Institute of Chemical Engineers (AIChE) for applied and engineering thermodynamics.

Frequently asked questions

What is thermodynamics in simple terms?

Thermodynamics is the branch of physics that studies heat, work, temperature, and energy, and the laws governing how energy moves and transforms in physical systems — including why some processes happen spontaneously and others don’t.

What are the four laws of thermodynamics?

The zeroth law establishes that temperature is a well-defined, transitive property. The first law is conservation of energy: energy can change form but not be created or destroyed. The second law states that total entropy in an isolated system never decreases, which sets a fundamental limit on the efficiency of any energy-conversion process. The third law states that a system’s entropy approaches a well-defined minimum as temperature approaches absolute zero.

What is the difference between thermodynamics and statistical mechanics?

Classical thermodynamics describes systems using only macroscopic, measurable quantities like temperature, pressure, and entropy, without reference to atomic-scale structure. Statistical mechanics derives those same macroscopic laws from the statistical behavior of a system’s underlying particles, explaining, for example, why entropy increases in terms of the number of microscopic arrangements available to a system.

How does thermodynamics relate to physics and chemistry?

Thermodynamics is formally a sub-discipline of physics — see CASRAI’s guide on what physics is — but its principles are equally central to chemistry, where chemical thermodynamics determines whether and how far a reaction proceeds, and to mechanical and chemical engineering, where applied thermodynamics governs the design of engines, refrigeration systems, and industrial processes.

Who funds thermodynamics research?

In the US, the main federal funders are the National Science Foundation (through its Physics, Chemistry, and Materials Research divisions, and its engineering-focused CBET division), the Department of Energy’s Office of Science (particularly the Office of Basic Energy Sciences), NASA for space- and planetary-related work, and NIH’s National Institute of General Medical Sciences for biological and biophysical thermodynamics. Defense agencies such as AFOSR and ONR also fund thermal-management research with dual-use relevance.

What careers use thermodynamics?

Beyond academic and national-laboratory physics and chemistry research, thermodynamics specialists work in the energy, aerospace, semiconductor, and materials industries on problems like power generation, propulsion, refrigeration and thermal management, and materials and chemical process design.

Related CASRAI resources

Thermodynamics is one of many major scientific disciplines covered in CASRAI’s overview guide to the branches of science, which maps how it relates to neighboring fields across the natural and applied sciences. For the broader parent discipline, see CASRAI’s guide on what physics is; for closely related fields covered in this same series, see CASRAI’s guides on quantum physics, materials science, and organic chemistry. On the funding side, see CASRAI’s guides to writing a DOE Office of Science grant proposal narrative, the comparison of ARPA-E and DOE Office of Science funding, the DOD research funding landscape, and Simons Foundation funding for mathematics and the physical sciences.