Module 113. Heat and Thermodynamics

Heat and thermodynamics form a fundamental branch of physical science that deals with the study of energy, its transformation, and its relation to matter. The subject encompasses the principles governing temperature, heat flow, and the behaviour of systems in equilibrium. Thermodynamics provides the theoretical framework for understanding engines, refrigerators, and even biological and chemical processes. Its concepts are central to physics, chemistry, engineering, and environmental sciences, influencing technologies that sustain modern civilisation.

Fundamental Concepts of Heat and Temperature

Heat is a form of energy transferred between bodies or systems due to a difference in temperature. It always flows spontaneously from a body at a higher temperature to one at a lower temperature until equilibrium is reached. The SI unit of heat is the joule (J), though the calorie (cal) is also commonly used in certain contexts, where 1 cal = 4.186 J.
Temperature measures the degree of hotness or coldness of a body and indicates the direction of heat transfer. It is a measure of the average kinetic energy of the particles within a substance. Common temperature scales include:

• Celsius (°C): Based on the freezing and boiling points of water (0°C and 100°C).
• Fahrenheit (°F): Commonly used in the United States.
• Kelvin (K): The absolute temperature scale used in scientific calculations, where 0 K represents absolute zero — the point at which molecular motion ceases.

Thermometers and sensors based on thermal expansion or electrical resistance are used to measure temperature accurately in various applications.

The Concept of Heat Transfer

Heat can be transferred through three primary mechanisms:

1. Conduction: Transfer of heat through direct molecular contact, without movement of matter. Metals are good conductors, while wood and air are poor conductors. Example: heating one end of a metal rod causes the other end to become hot.
2. Convection: Transfer of heat through fluid (liquid or gas) motion. It occurs due to density differences created by temperature variations. Example: boiling water, where hot water rises and cold water sinks, forming convection currents.
3. Radiation: Transfer of heat in the form of electromagnetic waves without requiring a medium. The Sun’s heat reaching the Earth is an example of radiation.

The rate of heat transfer depends on the temperature difference, material properties, and surface area of the objects involved.

Thermodynamics: Definition and Scope

Thermodynamics is the branch of physics concerned with the relationships between heat, work, and energy. It describes how energy is transformed from one form to another and predicts the direction and feasibility of such transformations. Thermodynamics applies to all natural processes, from the expansion of gases to the metabolism of living organisms.
A thermodynamic system refers to a specific quantity of matter or a region in space chosen for study, separated from its surroundings by boundaries. Systems are classified as:

• Open system: Exchanges both energy and matter with surroundings (e.g. boiling water in an open pot).
• Closed system: Exchanges energy but not matter (e.g. a sealed steam engine).
• Isolated system: Exchanges neither energy nor matter (e.g. an insulated thermos flask).

The state of a system is defined by properties such as pressure (P), volume (V), and temperature (T).

The Laws of Thermodynamics

The principles governing all thermodynamic processes are summarised in the four fundamental laws of thermodynamics, which form the foundation of energy science.

1. Zeroth Law of Thermodynamics: States that if two bodies are each in thermal equilibrium with a third body, they are also in thermal equilibrium with each other. This law defines temperature as a measurable and comparable property and establishes the basis for thermometry.
2. First Law of Thermodynamics (Law of Energy Conservation): Energy can neither be created nor destroyed; it can only be converted from one form to another. The change in internal energy (ΔU) of a system is equal to the heat supplied (Q) minus the work done (W) by the system.

   ΔU=Q−WΔU = Q – WΔU=Q−W
   This law explains how energy transfers occur in processes such as heating, expansion, and compression. It forms the basis for the operation of engines and refrigeration systems.

3. Second Law of Thermodynamics: This law introduces the concept of entropy (S) — a measure of disorder or randomness in a system. It states that in any spontaneous process, the total entropy of a system and its surroundings always increases.Another form states that heat cannot spontaneously flow from a colder body to a hotter body.This law defines the direction of natural processes and the efficiency limits of heat engines.
4. Third Law of Thermodynamics: As the temperature of a pure crystalline substance approaches absolute zero, its entropy approaches zero. It implies that absolute zero (0 K) cannot be reached by any finite number of processes.

Thermodynamic Processes and Work

The behaviour of a thermodynamic system is described through various processes depending on how pressure, volume, and temperature change. Common processes include:

• Isothermal Process: Temperature remains constant (ΔT = 0). Example: expansion of gas in a thermostat bath.
• Adiabatic Process: No heat exchange with surroundings (Q = 0). Example: rapid compression in a gas cylinder.
• Isobaric Process: Pressure remains constant. Example: heating of water at atmospheric pressure.
• Isochoric Process: Volume remains constant. Example: heating a gas in a sealed container.

The work done (W) by or on a system during expansion or compression is represented as the area under the P–V curve on a pressure-volume graph.

Heat Engines, Refrigerators, and Efficiency

A heat engine is a device that converts thermal energy into mechanical work by exploiting temperature differences. It operates in a cyclic process, absorbing heat (Q₁) from a high-temperature source, converting part of it into work (W), and rejecting the remainder (Q₂) to a low-temperature sink.
Efficiency (η) of a heat engine is given by:
η=WQ1=1−Q2Q1η = \frac{W}{Q₁} = 1 – \frac{Q₂}{Q₁}η=Q1​W​=1−Q1​Q2​​
No real engine can achieve 100% efficiency because some energy is always lost as waste heat.
A refrigerator operates on the reverse principle of a heat engine. It extracts heat from a low-temperature region and expels it to a high-temperature region by consuming external work (usually electrical energy). The performance of a refrigerator is measured by the coefficient of performance (COP).

Entropy and the Direction of Natural Processes

Entropy is a key concept in thermodynamics that measures the degree of randomness or disorder within a system. The Second Law states that natural processes move toward a state of greater entropy, meaning energy tends to disperse spontaneously.
Examples include:

• Melting of ice into water.
• Diffusion of gases in an open space.
• Equalisation of temperature between hot and cold bodies.

Entropy helps explain why certain processes are irreversible and why perpetual motion machines of the second kind are impossible.

Applications of Thermodynamics in Daily Life and Industry

Thermodynamic principles are applied extensively in engineering, chemistry, and biological systems:

• Engines and Power Plants: Steam turbines, internal combustion engines, and nuclear reactors convert heat into work.
• Refrigeration and Air Conditioning: Maintain temperature control using thermodynamic cycles.
• Chemical Engineering: Determining reaction feasibility, heat of reaction, and equilibrium.
• Metallurgy: Extraction and refining of metals through heat treatment and phase changes.
• Biological Systems: Metabolic processes in living organisms obey thermodynamic principles, converting chemical energy into mechanical and thermal energy.

In modern science, thermodynamics also underpins statistical mechanics, quantum thermodynamics, and energy conservation strategies in sustainable technologies.

The Significance of Thermodynamics in Modern Science

Thermodynamics extends beyond mechanical systems, offering insights into natural phenomena such as weather patterns, phase transitions, and even cosmic processes. It governs the functioning of stars, the stability of ecosystems, and the efficiency of human-made machines.