Intro To Thermodynamics - Overview, Structure, Properties & Uses

Thermodynamics Introduction

Edited By Vishal kumar | Updated on Jul 02, 2025 08:10 PM IST

Thermodynamics is the branch of physics that deals with the study of heat, energy, and the transformation between them. It explores how energy moves and changes form, such as converting from heat to work. The fundamental laws of thermodynamics govern everything from the engines in our cars to the functioning of the universe.

This Story also Contains

Thermodynamics
What is the Thermodynamic System?
What are the Surroundings?
Types of Systems
Solved Examples Based on Introduction to Thermodynamics
Summary

For example, when you boil water, energy is transferred in the form of heat, causing a change in the water's state, which is a basic thermodynamic process. In real life, thermodynamics can be seen in refrigerators keeping food cold or in power plants generating electricity. By understanding thermodynamics, we can optimize systems for energy efficiency, improving everything from home heating to industrial processes.

Thermodynamics

Thermodynamics is a branch of science that focuses on the study of energy, particularly heat, and its transformation into different forms, such as mechanical work. It involves understanding how energy is transferred between systems and their surroundings and the principles that govern these processes.

What is the Thermodynamic System?

The collection of an extremely large number of atoms or molecules which are confined within certain boundaries (either fixed or moveable) such that it has a certain value of pressure, volume and temperature is called a thermodynamic system.

What are the Surroundings?

Anything outside the thermodynamic system to which energy or matter is exchanged is called its surroundings

Example: Suppose there is a Piston-cylinder arrangement which contains any gas within it, then the gas enclosed in a cylinder fitted with a piston forms the thermodynamic system but the atmospheric air which is outside the cylinder, and the movable piston are surroundings.

Types of Systems

Thermodynamic systems are classified into three major categories

(i) Open system: It exchanges both energy and matter with the surroundings.

(ii) Closed system: It exchanges only energy (not matter) with the surroundings.

(iii) Isolated system: It exchanges neither energy nor matter with the surroundings.

Solved Examples Based on Introduction to Thermodynamics

Example 1: Heat given to a body which raises its temperature by 1⁰ C is called its

1) water equivalent

2) thermal capacity

3) specific heat

4) temperature gradient.

Solution:

Thermodynamics

It deals with concepts of heat temperature and the interconversion of heat into another form of energy.

wherein

It does not take into account the atomic & molecular constitution of matter

Thermal capacity is defined as the amount of heat required to raise the temperature of a body by 1^oC.

Hence, the answer is the option (2).

Example 2: Any glass tube with an open tap is an example of

1) closed system

2) open system

3) isolated system

4) none of these

Solution:

Open System

When a Thermodynamics system can exchange both masses as well as heat with its surroundings is called an open system.

e.g. Any vessel with an open top will exchange mass and energy with its surroundings.

Hence, the answer is the option 2.

Example 3: When a thermodynamics system can exchange heat but not mass, then the system is

1) Closed system

2) Open system

3) Variable system

4) isolated system

Solution:

Closed System -When a Thermodynamics system can exchange heat but not mass with surroundings.

A conducting vessel with top top-closed will exchange only heat (energy) with the surroundings

Hence, the answer is the option 1.

Summary

Thermodynamics is the study of heat, energy, and their transformation into different forms. It defines the concept of thermodynamic systems, which are collections of particles confined by boundaries, and their interaction with surroundings. Systems can be open, closed, or isolated, depending on their exchange of energy and matter. Understanding these principles helps optimize processes like heating, cooling, and energy conversion in everyday applications.

Frequently Asked Questions (FAQs)

1. What is thermodynamics and why is it important?

Thermodynamics is the branch of physics that deals with heat, temperature, energy, and their relationships with work and matter. It's important because it explains how energy behaves in systems, from everyday occurrences like cooking to complex processes in engines and power plants. Thermodynamics helps us understand and predict how energy transfers and transforms, which is crucial for developing efficient technologies and understanding natural phenomena.

2. What's the difference between heat and temperature?

Heat is the transfer of thermal energy between objects, while temperature is a measure of the average kinetic energy of particles in an object. Heat flows from higher to lower temperature objects, but temperature is what we feel and measure with a thermometer. For example, a cup of coffee has less heat content than a swimming pool, but a higher temperature.

3. Can energy be created or destroyed?

No, energy cannot be created or destroyed, only converted from one form to another. This is the essence of the First Law of Thermodynamics, also known as the law of conservation of energy. In any process, the total energy of an isolated system remains constant, though it may change forms (e.g., from kinetic to potential energy).

4. What is entropy and why does it always increase?

Entropy is a measure of the disorder or randomness in a system. It always increases in natural processes due to the Second Law of Thermodynamics. This law states that the total entropy of an isolated system always increases over time. For example, when an ice cube melts, the ordered structure of ice becomes the more disordered liquid water, increasing entropy.

5. How does a refrigerator work?

A refrigerator works by moving heat from inside the fridge to the outside, making the interior colder. It uses a refrigerant that evaporates at low temperatures, absorbing heat from inside the fridge. The refrigerant is then compressed, raising its temperature above the surrounding air, allowing it to release heat to the environment. This cycle repeats, continuously removing heat from inside the fridge.

6. How does a thermos (vacuum flask) keep drinks hot or cold?

A thermos uses several methods to minimize heat transfer:

7. How does sweating cool the body?

Sweating cools the body through evaporative cooling. When sweat evaporates from the skin, it requires energy in the form of heat. This heat is taken from the body, lowering its temperature. The process is endothermic, meaning it absorbs heat from its surroundings (in this case, your body), resulting in a cooling effect.

8. Why does pressure increase with temperature in a closed container?

In a closed container, as temperature increases, the gas particles move faster, colliding with the container walls more frequently and with greater force. This increased collision rate and force results in higher pressure. This relationship is described by Gay-Lussac's law, which states that for a fixed volume and amount of gas, pressure is directly proportional to temperature.

9. How does a heat pump work?

A heat pump works by moving heat from a cold space to a warmer one, against the natural direction of heat flow. It uses the same principle as a refrigerator but in reverse. By compressing and expanding a refrigerant, it can extract heat from outside air (even when it's cold) and move it inside to warm a building. In summer, it can be reversed to cool the building.

10. How does atmospheric pressure change with altitude?

Atmospheric pressure decreases with increasing altitude. This is because as you go higher, there's less air above you, resulting in fewer air molecules exerting pressure. The relationship is exponential, not linear, with pressure decreasing more rapidly near the Earth's surface and more slowly at higher altitudes. This principle is used in altimeters to measure altitude based on pressure changes.

11. What is the difference between an exothermic and endothermic reaction?

An exothermic reaction releases heat to its surroundings, while an endothermic reaction absorbs heat from its surroundings. In an exothermic reaction, the temperature of the surroundings increases (e.g., burning fuel). In an endothermic reaction, the temperature of the surroundings decreases (e.g., melting ice).

12. How does the expansion of gases cause cooling?

When a gas expands, it does work against its surroundings, which requires energy. In an adiabatic expansion (no heat exchange with surroundings), this energy comes from the gas's internal energy, lowering its temperature. This principle is used in refrigeration cycles and explains why compressed air cans feel cold when sprayed.

13. What is the difference between reversible and irreversible processes?

A reversible process is one that can be reversed without leaving any trace on the surroundings, while an irreversible process cannot be undone without changing the surroundings. Reversible processes are idealized and don't occur in reality, but they're useful for theoretical analysis. All real processes are irreversible due to factors like friction, heat loss, and increase in entropy. Understanding this difference is crucial for analyzing efficiency and energy losses in thermodynamic systems.

14. What is the Joule-Thomson effect?

The Joule-Thomson effect describes the temperature change of a gas when it expands through a valve or porous plug without exchanging heat with its surroundings. For most gases at room temperature, expansion causes cooling (e.g., the cold spray from an aerosol can). However, at high temperatures, some gases may warm upon expansion. This effect is crucial in processes like gas liquefaction and refrigeration.

15. What is the difference between adiabatic and isothermal processes?

In an adiabatic process, no heat is exchanged between the system and its surroundings. Temperature changes as work is done on or by the system. In an isothermal process, temperature remains constant as heat is exchanged with the surroundings. For example, rapid compression of a gas is nearly adiabatic, while slow compression allowing heat exchange is more isothermal.

16. How does a steam engine convert thermal energy to mechanical work?

A steam engine uses the expansion of steam to drive a piston or turbine. Water is heated in a boiler to produce high-pressure steam. This steam expands in a cylinder, pushing a piston, which converts the steam's thermal energy into mechanical work. The expanded steam is then condensed back to water and returned to the boiler, completing the cycle. This process demonstrates the conversion of heat to work, a key concept in thermodynamics.

17. How does the First Law of Thermodynamics relate to energy conservation?

The First Law of Thermodynamics is essentially the law of energy conservation applied to thermodynamic systems. It states that the change in internal energy of a system equals the heat added to the system minus the work done by the system. This means energy can't be created or destroyed, only converted between different forms (e.g., heat to work, or vice versa). It's crucial for understanding energy transfers in all physical processes.

18. How do phase changes relate to energy and temperature?

Phase changes involve energy transfer without temperature change. During a phase change (e.g., melting, boiling), energy is used to break or form intermolecular bonds rather than increase kinetic energy. This is why water's temperature remains at 100°C while boiling, despite continued heat addition. The energy absorbed or released during a phase change is called latent heat.

19. What is the difference between state functions and path functions?

State functions are properties that depend only on the current state of the system, not on how it got there. Examples include internal energy, entropy, and pressure. Path functions, on the other hand, depend on the path taken between states. Examples include heat and work. Understanding this distinction is crucial in thermodynamic calculations. For instance, the change in a state function can be calculated just by knowing the initial and final states, while path functions require knowledge of the entire process.

20. How does the concept of chemical potential relate to phase equilibrium?

Chemical potential is a measure of how the Gibbs free energy of a system changes with the addition or removal of a component. In phase equilibrium, the chemical potential of a substance must be the same in all coexisting phases. This concept is crucial for understanding phase transitions, osmosis, and chemical equilibrium. For example, it explains why water evaporates from a surface until the chemical potential of water vapor in the air equals that of liquid water, reaching equilibrium.

21. What is thermal equilibrium?

Thermal equilibrium is the state where two or more objects in contact with each other have reached the same temperature. At this point, there is no net flow of thermal energy between the objects. For instance, when you put an ice cube in a glass of water, thermal equilibrium is reached when the ice has melted and the water temperature is uniform throughout.

22. What is the ideal gas law and when does it apply?

The ideal gas law is PV = nRT, where P is pressure, V is volume, n is the number of moles, R is the gas constant, and T is temperature. It describes the behavior of an ideal gas, which is a theoretical gas composed of point particles with no intermolecular forces. Real gases behave most like ideal gases at high temperatures and low pressures, when the gas particles are far apart and interactions are minimal.

23. What is the difference between heat capacity and specific heat?

Heat capacity is the amount of heat required to raise the temperature of an object by one degree, while specific heat is the heat capacity per unit mass. Heat capacity depends on the size and material of the object, while specific heat is a property of the material itself. For example, a large pot of water has a higher heat capacity than a small cup, but both have the same specific heat.

24. What is the Mpemba effect?

The Mpemba effect is the counterintuitive phenomenon where, under certain conditions, hot water can freeze faster than cold water. While not fully understood, possible explanations include evaporation, dissolved gases, and convection currents. It's named after Erasto Mpemba, a Tanzanian student who observed this effect while making ice cream.

25. What is the relationship between pressure and volume in a gas?

For a fixed amount of gas at constant temperature, pressure and volume are inversely proportional. This relationship is described by Boyle's Law: PV = constant. As volume decreases, pressure increases, and vice versa. This explains why you can compress a gas into a smaller volume, increasing its pressure, or why a balloon expands when you decrease external pressure.

26. What is the significance of absolute zero temperature?

Absolute zero (-273.15°C or 0 K) is the lowest possible temperature, where particles have the minimum possible energy. It's significant because:

27. What is the Maxwell-Boltzmann distribution and why is it important?

The Maxwell-Boltzmann distribution describes the statistical distribution of molecular speeds in a gas at thermal equilibrium. It shows that gas molecules have a range of speeds, with most molecules moving at speeds close to the average, and fewer moving much faster or slower. This distribution is crucial for understanding gas behavior, reaction rates, and many other phenomena in statistical thermodynamics.

28. What is the significance of critical point in phase diagrams?

The critical point on a phase diagram represents the conditions (temperature and pressure) at which the distinction between liquid and gas phases disappears. Above this point, the substance exists as a supercritical fluid with properties of both liquids and gases. The critical point is significant because:

29. What is the significance of the Clausius-Clapeyron equation?

The Clausius-Clapeyron equation describes how the vapor pressure of a liquid changes with temperature. It's crucial for understanding phase transitions, particularly vaporization and sublimation. The equation shows that vapor pressure increases exponentially with temperature, explaining phenomena like:

30. Can we achieve 100% efficiency in heat engines?

No, it's impossible to achieve 100% efficiency in heat engines. This is a consequence of the Second Law of Thermodynamics. Some energy is always lost as waste heat in the process of converting thermal energy to mechanical work. The theoretical maximum efficiency is given by the Carnot efficiency, which depends on the temperature difference between the hot and cold reservoirs.

31. What is the zeroth law of thermodynamics?

The Zeroth Law of Thermodynamics states that if two systems are in thermal equilibrium with a third system, they are in thermal equilibrium with each other. This law establishes temperature as a fundamental and measurable property, allowing us to use thermometers and compare the temperatures of different objects.

32. What is the Third Law of Thermodynamics?

The Third Law of Thermodynamics states that as a system approaches absolute zero temperature, all processes cease and the entropy of the system approaches a minimum value. In other words, it's impossible to reach absolute zero temperature in a finite number of steps. This law helps explain why we can't achieve 100% efficiency in heat engines.

33. What is the difference between intensive and extensive properties in thermodynamics?

Intensive properties are independent of the amount of substance present, while extensive properties depend on the amount. For example:

34. How does a Carnot engine work, and why is it important?

A Carnot engine is a theoretical heat engine operating in a reversible cycle between two temperature reservoirs. It consists of two isothermal and two adiabatic processes. While not practically achievable, it represents the maximum possible efficiency for converting thermal energy to work. The Carnot efficiency serves as a benchmark for real heat engines and helps us understand the limitations imposed by the Second Law of Thermodynamics.

35. How does the concept of free energy relate to spontaneous processes?

Free energy, particularly Gibbs free energy, determines the spontaneity of a process at constant temperature and pressure. A spontaneous process has a negative change in Gibbs free energy (ΔG < 0). This concept combines the First and Second Laws of Thermodynamics, accounting for both energy and entropy changes. It's crucial in understanding chemical reactions, phase changes, and other natural processes, explaining why some reactions occur spontaneously while others require energy input.

36. How do heat engines differ from refrigerators in terms of energy flow?

Heat engines and refrigerators operate on similar principles but with opposite energy flows:

37. What is the role of entropy in the arrow of time?

Entropy plays a crucial role in determining the direction of time in physical processes. The Second Law of Thermodynamics states that the total entropy of an isolated system always increases over time. This increase in entropy provides a direction to time, explaining why we observe certain processes (like mixing of gases or heat flow from hot to cold) happening spontaneously in one direction but not in reverse. This concept is fundamental to our understanding of irreversibility in nature.

38. How does pressure affect the boiling point of liquids?

Pressure has a significant effect on the boiling point of liquids. As pressure increases, the boiling point rises, and vice versa. This is because boiling occurs when the vapor pressure of the liquid equals the atmospheric pressure. At higher pressures, more energy (and thus higher temperature) is needed for the vapor pressure to overcome the atmospheric pressure. This principle explains why water boils at lower temperatures at high altitudes and is used in pressure cookers to cook food faster.

39. How does the equipartition theorem relate to the specific heat of gases?

The equipartition theorem states that in thermal equilibrium, energy is equally distributed among all accessible degrees of freedom of a system. For an ideal gas, each degree of freedom contributes 1/2 kT to the average energy per molecule (where k is Boltzmann's constant and T is temperature). This theorem explains why the specific heat of diatomic gases is higher than that of monatomic gases – diatomic molecules have more degrees of freedom (translational, rotational, and sometimes vibrational) to store energy.