Thermodynamics

Thermodynamics is the study of energy, work, and heat and the relationship between them. It deals with the macroscopic behaviour of matter and energy, explaining phenomena such as heat engines (Such as refrigerators, and air-conditioners) and the behaviour of gases.

In this chapter, we will cover the thermal properties of matter, Heat flow and energy stored by matter.

Thermodynamic Systems 

• System: A system refers to a specific portion of matter chosen for study.
• Surroundings: Everything outside the system is considered the surroundings.
• Boundary: The boundary separates the system from its surroundings.

Types of Thermodynamic Systems:

1. Isolated System: No exchange of energy or matter with the surroundings (e.g., a thermos).
2. Closed System: Exchange of energy but not matter with the surroundings (e.g., a sealed container).
3. Open System: Exchange of both energy and matter with the surroundings (e.g., a boiling pot).

The laws of Thermodynamics are the laws that govern the energy interactions in thermodynamic systems.

0th Law of Thermodynamics

According to the Zeroth Law of Thermodynamics, if two systems are in thermal equilibrium with a third system, they must also be in thermal equilibrium with each other.

• Concept of Thermal Equilibrium: Thermal equilibrium occurs when two or more objects, initially at different temperatures, are placed in contact and no heat flows between them. This happens when both objects reach the same temperature, meaning they are in a state of balance. In this state, the systems’ temperatures remain constant over time, and there is no net transfer of heat. Thermal equilibrium is crucial for defining temperature and for understanding heat transfer processes.
• Temperature: the degree or intensity of heat present in a substance or object, especially as expressed according to a comparative scale and shown by a thermometer or perceived by touch.
• Heat flows by virtue of Temperature difference i.e. Heat will flow when there is a temperature difference or in other words till the thermal equilibrium is reached.

1st Law of Thermodynamics 

The First law of thermodynamics is also known as the Law of Energy Conservation. This law states that energy cannot be created or destroyed, only transformed from one form to another. 

How can we supply heat to a system?

There are only two ways:

1. Heat (Q): Energy transferred due to a temperature difference. Flows from hot to cold, measured in Joules (J).
2. Work done on the system: We remember from the chapter on mechanics that work done is 

The change in internal energy (ΔU) is equal to the heat added to the system (Q) plus the work done on the system (W).

ΔU=Q + W

This law hints at the types of energies stored by matter in a thermodynamic system. Heat is stored as temperature.

• Internal Energy (U): Total energy inside a system, including molecular kinetic and potential energy. Depends on the system’s temperature and state.
• Work: In thermodynamics, work is defined as the energy transferred when an object is moved by a force. For an expanding gas, work done is given by:

W = P.ΔV

where P is pressure and ΔV is the change in volume.

Thermodynamic Processes 

A thermodynamic process involves the transfer of energy between a system and its surroundings. Some important processes are:

• Isothermal Process: Temperature remains constant (ΔT=0). For Example: Gas expansion/compression at constant temperature.
• Adiabatic Process: No heat exchange with the surroundings (Q=0). For Example Gas expansion in an insulated container.
• Isochoric Process: Volume remains constant (ΔV=0). For Example: Heating gas in a rigid container (no work done).
• Isobaric Process: Pressure remains constant (ΔP=0). For Example: Heating gas in a piston at constant pressure.

2nd Law of Thermodynamics: 

This law introduces the concept of entropy. It states that the total entropy of an isolated system always increases over time. Entropy is a measure of disorder or randomness.

Entropy

Entropy is a measure of the disorder or randomness in a system. The second law of thermodynamics implies that in any natural process, the total entropy of the system and surroundings always increases.

Mathematically, the change in entropy (ΔS) for a reversible process is given by:  Where Qrev​ is the heat exchanged in a reversible process and T is the temperature.

The 2nd law explains why natural processes are irreversible.

• Entropy Increase: Natural processes tend to move towards greater disorder.
• Heat cannot spontaneously flow from a colder to a hotter body, as every thermodynamic system tends to increase the randomness in the system.
• Inefficiency of all processes: Not all energy in a system can be converted into useful work. Some energy is always lost as waste heat, reducing efficiency (e.g., in heat engines).
• Direction of Time: It establishes the direction of natural processes. For example, heat flows from hot to cold bodies, and chemical reactions proceed towards equilibrium.
• Irreversibility: Many processes, like the mixing of gases or the diffusion of liquids, are irreversible. Once they occur, they cannot naturally reverse.
• Carnot’s Theorem: The efficiency of a heat engine depends only on the temperatures of the heat source and the heat sink.

Heat Pumps

Heat Pump is a device that uses work to transfer heat from a colder body to a hotter one. Heat pumps are used in air conditioning systems and heating.  For example, a Refrigerator.

Mechanism of Air Conditioner/Refridgerator

A Refrigerator is a device that uses work to transfer heat from a cold body to a hot body, maintaining a lower temperature in the cold reservoir. It involves the following steps:

• Compression: The compressor compresses the refrigerant gas, increasing its pressure which increases its temperature. This hot, high-pressure gas passes through heat-sink coils, which use fans to release heat into the environment. This condenses the gas into a cooler liquid (Condensation).
• Expansion Valve: The high-pressure liquid refrigerant moves through an expansion valve, causing it to expand, and cooling it further.
• Absorption of heat from the room: The cold refrigerant is then passed through a coil through which the air inside the room is passed using a fan. This allows it to absorb heat from the air inside the room or fridge, heating the gas (or evaporating it if the refrigerant is a liquid).

The cycle repeats, continuously removing heat and cooling the space.

Coefficient of Performance (COP): A measure of the efficiency of refrigerators and heat pumps, defined as the ratio of the heat removed (or added) to the work input.

Heat Engines 

A heat engine is a device that converts heat energy into work. A Carnot Engine is a theoretical heat engine that operates between two heat reservoirs. 

Efficiency: The efficiency of a heat engine is the ratio of work output to the heat input. Real engines are less efficient due to losses like friction, heat dissipation, etc.

It is given by the relation:

η = 1 − TcTₕ

Where TC is the temperature of the cold reservoir and TH is the temperature of the hot reservoir (in Kelvin).

3rd Law of Thermodynamics: 

As the temperature of a system approaches absolute zero, the entropy of the system approaches a minimum (or constant) value.

The third law of thermodynamics states that the entropy of a closed system at thermodynamic equilibrium approaches a constant value when its temperature approaches absolute zero. 

Reversible and irreversible processes

The reversibility of a process is governed by the Entropy of the system. All natural systems tend to increase their randomness, i.e. their entropy. 

Concept of Absolute Zero

Absolute zero is the lowest possible temperature, at which there is no heat or motion. It is the temperature at which the particles in a substance stop moving entirely. It is the temperature at which a thermodynamic system has the lowest energy. 

 Such a situation occurs at -273.15° Celsius. We have defined a new scale of temperature that starts at this point. It is known as the Kelvin Scale. 

At this point, certain interesting behaviour occurs: 

1. The entropy of a system is zero, i.e. there is no randomness. No lower degree of randomness is possible.
2. Satendranath Bose showed that at this point the substance would behave more like a wave rather than a matter. We shall study the wave nature of matter in Quantum Physics.

Thermodynamic State Variables

State variables are properties that describe the state of a thermodynamic system. Common state variables include Temperature (T), Pressure (P), Volume (V), Internal Energy (U), and Entropy (S). These variables define the state of a system at any given time, and changes in these variables determine the changes in the system’s state.

Heat Capacity

Different substances have different heat capacities. A substance with a high specific heat can absorb more heat without a significant temperature change.

Specific Heat Capacity (c): 

The amount of heat required to raise the temperature of 1 kg of a substance by 1°C (or 1 K).

where: c = specific heat capacity, Q = heat added, m = mass of the substance, ΔT = change in temperature.

For Gases, it is known as the specific heat for gas. However, one must note that the specific heat for a gas is noted at a constant pressure, i.e. it is a measure of heat for raising temperature by 1°C, keeping all other thermodynamic parameters constant.

Latent Heat

We can keep supplying heat to a matter to increase its temperature, but only to a certain limit. Once enough heat is provided, the matter might change its state at some point. The solid might turn into vapour and vapour might turn into a liquid. 

At a macroscopic level, this change of state happens at a constant temperature. It means that once you start heating water, its temperature will keep on increasing, but once it is at its boiling point, the temperature will not rise further. 

Latent Heat of Vaporisation

Latent heat of vaporization is the amount of heat required to change 1 kg of a liquid into a gas without changing its temperature.

Latent heat of Condensation

The latent heat of condensation is the heat released when a gas condenses into a liquid. It is the amount of heat required to change 1 kg of a solid into a liquid without changing its temperature.

State of Matter

States of Matter describe the distinct forms that different phases of matter can take, and these states are influenced by temperature and pressure. These are solid, liquid, and gas.

Solid

In a solid, particles are tightly packed and vibrate around fixed positions. 

• This arrangement gives solids a definite shape and volume. 
• The intermolecular forces in solids are strong, holding the particles in place. 
• Solids have low kinetic energy, which is why they maintain a rigid structure and do not flow.

Liquid

The particles are still close together but can move past one another, allowing liquids to flow. 

• Liquids have a definite volume but no definite shape, meaning they take the shape of their container.
• The intermolecular forces in liquids are weaker than those in solids, giving them the ability to change shape while retaining volume. 
• The kinetic energy of liquid particles is moderate compared to solids, allowing them to move freely.
• Further, they exhibit the property of Surface Tension.

Gas

Gases can flow like Liquids, but exhibit certain distinguishing behaviour:

• Gases have neither a definite shape nor volume. 
• The particles are far apart and move quickly, filling the space available. 
• Gases have weak intermolecular forces, and the kinetic energy of the particles is high, which results in rapid motion and expansion. As a result, gases will expand to fill any container they are in.

The difference between gases and solids or liquids is that for solids or liquids, the change in volume due to a change of external pressure is rather small. In other words solids and liquids have much lower compressibility as compared to gases.

The biggest difference between gases and liquids is that gases cannot form a surface, and do not have surface tension. Whereas, liquids have a surface. We shall study surface tension in the chapter related to fluids.

Change in state of matter

Boiling Point

The temperature at which a liquid turns into a gas at a pressure of 1 atm. For water, it’s 100°C at standard atmospheric pressure (1 atm).

Melting point

The temperature at which a solid turns into a liquid. It might also be known as the freezing point. For ice, it is 0°C at standard pressure (1 atm).

Sublimation Point

The temperature at which a solid directly transitions into a gas without passing through the liquid phase. Dry ice (solid CO₂) sublimates at -78.5°C. 

Phase Diagram

A graph between the temperature T and the Pressure P of the substance is called a phase diagram or P – T diagram. The following figure shows the phase diagram of water and CO2. Such a phase diagram divides the P – T plane into a solid region, a vapour region and a liquid region. The regions are separated by curves such as sublimation curve (BO), fusion curve (AO) and vaporisation curve (CO). The points on the sublimation curve represent states in which solid and vapour phases coexist. The point on the sublimation curve BO represents states in which the solid and vapour phases co-exist. Points on the fusion curve AO represent states in which solid and liquid phases coexist. Points on the vapourisation curve CO represent states in which the liquid and vapour phases coexist. The temperature and pressure at which the fusion curve, the vaporisation curve and the sublimation curve meet and all the three phases of a substance coexist are called the triple point of the substance. For example, the triple point of water is represented by the temperature 273.16 K and pressure 6.11×10–3 Pa.

Triple Point

The temperature of a substance remains constant during its change of state (phase change).

Exotic states of matter

Plasma

Plasma is an ionized gas consisting of free electrons and positively charged ions. It occurs at extremely high temperatures, such as in stars or lightning. Plasma is highly energetic, with particles moving at very high speeds, and is affected by electric and magnetic fields. It conducts electricity and can be found in places like the sun or plasma TVs.

Bose-Einstein Condensates (BEC):

Bose-Einstein Condensates (BEC) form at near absolute zero, where particles act as a single quantum entity. Similarly, Fermionic Condensates exist under extreme conditions, showcasing unique properties not found in the other common states.

Plasma

Plasma is the fourth state of matter, after solid, liquid, and gas. It is a hot, charged state of matter made up of ions, electrons, and neutral atoms.

Bose-Einstein Condensate

A Bose-Einstein condensate (BEC) is a state of matter where a large number of particles are cooled to near absolute zero and merge into a single quantum object.

What is Thermodynamics in simple words?

Thermodynamics is the branch of physics that deals with heat, temperature, energy, and their transformation from one form to another.

How many laws of Thermodynamics are there?

There are four laws of thermodynamics — Zeroth Law, First Law, Second Law, and Third Law — each explaining different energy principles.

Why is Thermodynamics important for UPSC?

Thermodynamics helps in understanding energy management, environment-friendly technologies, engines, refrigerators, and power plants, often asked in UPSC exams.

What are the key terms in Thermodynamics?

Important terms include system, surroundings, heat, work, internal energy, entropy, enthalpy, and thermodynamic equilibrium.

Where is Thermodynamics applied in daily life?

Thermodynamics is used in air conditioners, refrigerators, engines, power plants, renewable energy systems, and understanding climate change.