Thermodynamics is the branch of physics that deals with the relationships between heat, work, temperature, and energy. The framework is governed by four fundamental laws, numbered from zero to three, which define physical quantities such as temperature, internal energy, entropy, and absolute zero.
The Zeroth Law of Thermodynamics: Concept of Temperature
The Zeroth Law establishes the basis for temperature measurement and validates the use of thermometers. It was formulated by Ralph H. Fowler in 1931, well after the First and Second laws had been established.
Statement of the Law
If two thermodynamic systems are each in thermal equilibrium with a third system, then they are in thermal equilibrium with each other.
Thermodynamic Mechanism
- When a body A is in thermal equilibrium with body C, their temperatures are equal (TA = TC).
- If body B is also in thermal equilibrium with body C, then TB = TC.
- Consequently, TA = TB, meaning no net heat flows between A and B if they are brought into direct contact.
Real-World Applications
- Thermometers: A thermometer acts as the third body (C). When placed in contact with a human body (A), it exchanges heat until it reaches thermal equilibrium. The expansion of the thermometric fluid (like mercury) reflects the temperature of both the thermometer and the human body.
The First Law of Thermodynamics: Conservation of Energy
The First Law is an extension of the law of conservation of energy to thermodynamic systems. It introduces the concept of internal energy (U).
Statement of the Law
The net heat energy (Δ Q) supplied to a thermodynamic system is equal to the sum of the increase in its internal energy (Δ U) and the external work done (Δ W) by the system on its surroundings.
Mathematical Formula
- Δ Q = Heat absorbed or released by the system.
- Δ U = Change in internal energy (a state function depending only on initial and final states).
- Δ W = Work done (P · Δ V, where P is pressure and V is volume).
IUPAC Sign Conventions for UPSC Prelims
| Thermodynamic Quantity | Positive Value (+) | Negative Value (−) |
| Heat (Δ Q) | Heat is supplied to the system | Heat is rejected by the system |
| Work (Δ W) | Work is done by the system (Expansion) | Work is done on the system (Compression) |
| Internal Energy (Δ U) | Internal energy increases (Temperature rises) | Internal energy decreases (Temperature drops) |
Thermodynamic Processes Derived from the First Law
- Isothermal Process (Δ T = 0): Temperature remains constant. Since internal energy of an ideal gas depends solely on temperature, Δ U = 0. Therefore, Δ Q = Δ W.
- Adiabatic Process (Δ Q = 0): No heat enters or leaves the system. Thus, Δ U = -Δ W. If a gas expands adiabatically, work is done by the system (Δ W > 0), causing internal energy to drop and the system to cool down.
- Isochoric Process (Δ V = 0): Volume remains constant, meaning work done Δ W = P · Δ V = 0. Therefore, Δ Q = Δ U. All absorbed heat goes entirely into raising internal energy.
- Isobaric Process (Δ P = 0): Pressure remains constant. Heat supplied changes both internal energy and performs work: Δ Q = Δ U + P(V2 – V1).
The Second Law of Thermodynamics: Direction of Heat Flow
While the First Law states that energy is conserved, it does not explain why heat flows spontaneously from hot to cold bodies and never the reverse. The Second Law introduces the concept of Entropy (S), a measure of molecular disorder or randomness.
Core Statements
1. Clausius Statement (Focus on Heat Transfer)
It is impossible to construct a device that operates in a cycle and produces no other effect than the transfer of heat from a cooler body to a hotter body.
- Implication: Spontaneous heat transfer occurs only from high temperature to low temperature. External work must be input to reverse this process (e.g., refrigerators, air conditioners).
2. Kelvin-Planck Statement (Focus on Heat Engines)
It is impossible to construct an engine operating in a cycle that absorbs heat from a single reservoir and converts it completely into work without rejecting any heat to a cooler reservoir.
- Implication: No heat engine can have a thermal efficiency (η) of 100%. Some energy is always dissipated into the environment as waste heat.
Mathematical Concept of Entropy
Real-World Applications
- Heat Engines: Automobile engines consume fuel at high temperatures, convert a fraction into mechanical work, and reject the remaining thermal exhaust into the atmosphere.
- Refrigerators: Electrical work is consumed to pump heat out of a cold interior and discard it into the warmer kitchen environment.
The Third Law of Thermodynamics: Absolute Zero
The Third Law explores the behavior of systems as their temperature approaches absolute zero (0 K or -273.15°C).
Statement of the Law
The entropy of a perfectly crystalline substance approaches zero as the absolute temperature approaches absolute zero (0 K).
Thermodynamic Mechanism
At absolute zero, all molecular motion and thermal vibrations cease entirely. In a perfect crystal, there is only one possible microscopic energy state available to the atoms, meaning the system possesses perfect order.
Mathematical Form
Essential Consequence
- Inaccessibility of Absolute Zero: It is physically impossible to reduce the temperature of any system to exactly absolute zero in a finite number of steps or operations. Modern cryogenics can achieve temperatures within billionths of a Kelvin, but absolute zero remains a theoretical limit.
Summary Matrix of the Thermodynamic Laws
| Law | Primary Focus | Definitive Parameter | Core Practical Relevance |
| Zeroth Law | Thermal Equilibrium | Temperature (T) | Formulates the working principle of thermometers. |
| First Law | Energy Conservation | Internal Energy (U) | Defines work-heat relationships in engines and compressors. |
| Second Law | Direction of Heat Flow | Entropy (S) | Establishes efficiency boundaries for power cycles and engines. |
| Third Law | Absolute Zero Limit | Minimum Entropy (S = 0) | Sets the baseline for low-temperature physics and chemical calculations. |