A simple, complete guide to all four laws of thermodynamics — thermal equilibrium, conservation of energy, entropy, and absolute zero — with definitions, diagrams, real-life examples, and why perpetual motion machines can never work.
Last updated: September 22, 2026
Thermodynamics is the branch of physics that deals with heat, work, temperature, and energy, and how they transform from one form to another within a system. The word comes from the Greek therme (heat) and dynamis (power). It explains everything from how engines and refrigerators work to why ice melts and why heat always flows from hot to cold.
The laws of thermodynamics are among the most fundamental and universally applicable laws in all of science — they govern chemical reactions, biological processes, engines, weather systems, and even the ultimate fate of the universe.
Before diving into the laws themselves, it helps to understand a few core terms used throughout thermodynamics.
Energy is the capacity to do work. It exists in many forms of energy, including:
Entropy (S) is a measure of the disorder or randomness of a system. The more spread out and disordered the energy and particles in a system, the higher its entropy. Entropy is central to the Second Law of Thermodynamics.
Enthalpy (H) is a measure of the total heat content of a system at constant pressure, defined as H = U + PV (where U is internal energy, P is pressure, and V is volume). It is widely used in chemistry to describe the heat absorbed or released during a chemical reaction.
There are four laws of thermodynamics, numbered 0, 1, 2, and 3 (not 1–4) because the Zeroth Law was formulated after the First and Second Laws, but was considered so fundamental that it needed to come "before" them logically — hence the unusual numbering starting at zero.
Figure 1: Overview of all four laws of thermodynamics — zeroth, first, second, and third.
"If two systems are each in thermal equilibrium with a third system, then they are also in thermal equilibrium with each other."
This law establishes the very basis for defining and measuring temperature. It's why a thermometer works: the thermometer reaches thermal equilibrium with the object being measured, and its reading tells you the temperature reliably.
Example: If a cup of tea (A) and a thermometer (C) reach the same temperature, and that thermometer later reads the same value on a second cup of tea (B), then cups A and B are at the same temperature — even though A and B never touched.
"Energy cannot be created or destroyed — only converted from one form to another." This is also known as the Law of Conservation of Energy.
ΔU = Q − W
Where ΔU = change in internal energy, Q = heat added to the system, W = work done by the system.
The first law of thermodynamics tells us that the total energy of an isolated system remains constant. Energy can move between forms — heat can become work, chemical energy can become kinetic energy — but the total amount never changes.
Example: In a car engine, chemical energy from fuel converts into heat and then into mechanical work that moves the car, but the total energy input (fuel) equals the total energy output (motion + heat lost).
"The total entropy of an isolated system always increases over time; heat flows spontaneously from a hotter object to a colder one, never the reverse."
The second law explains why natural processes are irreversible — why a broken egg doesn't spontaneously reassemble, why ice cubes melt in warm water but water doesn't spontaneously freeze part of itself, and why no heat engine can ever be 100% efficient (some energy is always lost as unusable heat, increasing entropy).
Example: A hot cup of coffee left on a table always cools down to room temperature, transferring heat to the cooler surroundings — it never spontaneously gets hotter by absorbing heat from the cooler room.
"As the temperature of a perfect crystalline system approaches absolute zero (0 Kelvin, or −273.15°C), its entropy approaches a minimum constant value (zero, for a perfect crystal)."
The third law also implies a very important consequence: absolute zero can never actually be reached through any finite number of physical processes — it can only be approached infinitely closely. This is why scientists can get extremely close to absolute zero in laboratories (within billionths of a degree) but never truly reach it.
Example: At absolute zero, a perfect crystal would have zero entropy because all its atoms would be perfectly ordered with no thermal motion — a state that is theoretically the most ordered possible arrangement of matter.
| Law | Short Statement | Key Concept |
|---|---|---|
| Zeroth Law | Systems in thermal equilibrium with a third system are in equilibrium with each other | Basis of temperature |
| First Law | Energy cannot be created or destroyed, only transformed | Conservation of energy (ΔU = Q − W) |
| Second Law | Entropy of an isolated system always increases | Entropy, irreversibility |
| Third Law | Entropy approaches a minimum as temperature approaches absolute zero | Absolute zero, entropy limit |
Closely related to thermodynamics are the ideal gas laws, which describe how pressure, volume, and temperature of a gas relate to one another — essentially specific applications of thermodynamic principles to gases.
| Gas Law | Relationship | Condition Held Constant |
|---|---|---|
| Boyle's Law | P × V = constant | Temperature |
| Charles's Law | V/T = constant | Pressure |
| Gay-Lussac's Law | P/T = constant | Volume |
| Ideal Gas Law | PV = nRT | Combines all three |
These gas laws are frequently used alongside the First Law of Thermodynamics to analyze how gases do work and exchange heat during compression, expansion, and heating — foundational to understanding engines, refrigerators, and weather systems.
A perpetual motion machine is a hypothetical device that could run forever without any external energy input, or produce more energy than it consumes. Despite centuries of attempts by inventors, such a machine is fundamentally impossible according to the laws of thermodynamics.
This is why every real engine, motor, and machine ever built inevitably loses some energy as waste heat, and why no device has ever run forever without an external power source.
The laws of thermodynamics are studied in both physics and chemistry, though with a different emphasis:
| Context | Focus | Key Applications |
|---|---|---|
| Laws of thermodynamics in physics | Heat engines, entropy, energy transfer between physical systems | Engines, refrigerators, statistical mechanics |
| Laws of thermodynamics in chemistry | Energy changes in chemical reactions, spontaneity of reactions | Enthalpy of reaction, Gibbs free energy, reaction spontaneity |
In chemistry, the laws are used to determine whether a chemical reaction will occur spontaneously (via Gibbs free energy, ΔG = ΔH − TΔS, which directly combines enthalpy, temperature, and entropy) and how much heat a reaction releases or absorbs.
Laws of thermodynamics form a core chapter in the Class 11 Physics and Chemistry curriculum. In physics, the focus is on internal energy, heat engines, and the Carnot cycle; in chemistry, the focus shifts toward enthalpy of reactions, Hess's Law, and spontaneity of processes using Gibbs free energy. Both subjects build from the same four foundational laws covered in this guide.
| Zeroth Law | Defines temperature via thermal equilibrium |
| First Law | ΔU = Q − W (energy conservation) |
| Second Law | Entropy of the universe always increases |
| Third Law | Entropy → minimum as T → absolute zero |
Thermodynamics is the branch of physics dealing with heat, work, temperature, and energy, and how they transform from one form to another within a system.
There are four laws of thermodynamics: the Zeroth Law (thermal equilibrium), First Law (conservation of energy), Second Law (entropy always increases), and Third Law (entropy approaches a minimum near absolute zero).
The Zeroth Law states that if two systems are each in thermal equilibrium with a third system, they are also in thermal equilibrium with each other. It forms the basis of temperature measurement.
The First Law, or Law of Conservation of Energy, states that energy cannot be created or destroyed, only converted between forms. It is expressed as ΔU = Q − W.
The Second Law states that the total entropy of an isolated system always increases over time, and heat naturally flows from hot to cold, never spontaneously the reverse. It explains why no engine can be 100% efficient.
The Third Law states that as temperature approaches absolute zero, the entropy of a perfect crystalline system approaches a minimum constant value, and absolute zero itself can never be fully reached.
Entropy is a measure of the disorder or randomness in a system. According to the Second Law, entropy in an isolated system tends to increase over time.
A perpetual motion machine violates the laws of thermodynamics — producing energy from nothing violates the First Law, while achieving 100% efficiency with no energy loss violates the Second Law, since entropy must always increase.
No. Newton's laws of motion describe how forces cause objects to move (mechanics), while the laws of thermodynamics describe heat, energy, and entropy — two entirely separate branches of physics.
