What is Thermodynamics?
Thermodynamics is the branch of Physics that deals with the relationship between heat, work, temperature, and energy.
It explains
- how energy changes from one form to another, and
- why certain processes occur spontaneously while others do not.
From running engines and power plants to refrigerators and even our own metabolism — thermodynamics governs how energy moves and transforms everywhere in the universe.
Key Terms and Concepts
System: The portion of the universe we study (e.g., gas in a cylinder).
Surroundings: Everything outside the system.
Boundary: The surface separating system and surroundings.
State Variables: Properties like pressure (P), volume (V), and temperature (T) that describe the system’s condition.
Equilibrium: When all state variables remain constant with no net change over time.
Types of Thermodynamic Systems
There are 3 types of Thermodynamic Systems:
- Open System
- Closed System
- Isolated System
Open System
An Open System can exchange both matter and energy (heat or work) with its surroundings.
Key Points:
- Matter can enter or leave.
- Energy can enter or leave.
Examples: Boiling water in an open pot, A cup of hot coffee, human body, or a running car engine.
2. Closed System
A Closed System can exchange energy but not matter with its surroundings.
Key Points:
No mass exchange.
Energy (like heat or work) can be transferred.
Examples: Pressure cooker, gas in a cylinder with a movable piston.
3. Isolated System
An Isolated System can exchange neither matter nor energy with its surroundings.
Key Points:
No transfer of matter.
No transfer of energy.
Example: A perfectly insulated thermos flask — ideally, it neither gains nor loses heat.
| System Type | Exchanges Matter | Exchanges Energy |
|---|---|---|
| Open System | ✅ | ✅ |
| Closed System | ❌ | ✅ |
| Isolated System | ❌ | ❌ |
Thermodynamic Processes
A thermodynamic process refers to the change that occurs in a system when it moves from one state to another due to variations in pressure, volume, temperature, or energy.
In other words, it’s the path or transformation through which a thermodynamic system passes as it exchanges heat (Q) or work (W) with its surroundings.
There are 4 types of Thermodynamic Processes:
- Isothermal
- Adiabatic
- Isobaric
- Isochoric (Isovolumetric)
| Type | Constant Quantity | Example |
|---|---|---|
| Isothermal Process | Temperature (T) | Expansion of gas at constant temperature |
| Adiabatic Process | No heat exchange (Q = 0) | Rapid compression of air in a piston |
| Isobaric Process | Pressure (P) | Heating water in an open vessel |
| Isochoric (Isovolumetric) Process | Volume (V) | Heating gas in a sealed container |
The Laws of Thermodynamics
Thermodynamics is based on four fundamental laws — the Zeroth, First, Second, and Third Laws. Each law describes a key principle of heat and energy.
Zeroth Law of Thermodynamics
If two systems are each in thermal equilibrium with a third system, they are in equilibrium with each other.
It defines temperature as a measurable and comparable property.
First Law of Thermodynamics
Energy cannot be created or destroyed; it can only change form.
Mathematically: ΔU = Q – W
Where:
- ΔU = change in internal energy
- Q = heat added to the system
- W = work done by the system
Example: When a gas expands, part of the heat supplied is used to do work, and the rest increases its internal energy.
Second Law of Thermodynamics
Heat cannot spontaneously flow from a colder body to a hotter body.
This law introduces entropy (S) — a measure of disorder. It explains why natural processes (like ice melting) have a preferred direction.
Third Law of Thermodynamics
As temperature approaches absolute zero (0 K), the entropy of a perfect crystal approaches zero.
This sets a limit — absolute zero is theoretically unattainable.
Understanding Entropy
Entropy (S) is a thermodynamic quantity that measures the degree of disorder or randomness in a system.
It also represents how energy is distributed or spread out within a system at a given temperature.
In natural processes, entropy tends to increase, which is why spontaneous processes are often irreversible — for example, ice melting or perfume spreading in air.
The change in entropy (ΔS) during a reversible process is given by:
where
ΔS = change in entropy (in joules per kelvin, J/K)
Qrev = heat absorbed or released reversibly (in joules)
T = absolute temperature at which the process occurs (in kelvin, K)
SI Unit: J K−1 (Joules per Kelvin)
Carnot Cycle
The Carnot cycle is an idealized thermodynamic cycle that represents the most efficient possible engine operating between two temperatures.
It shows how heat energy (Q) can be partially converted into work (W) in a perfectly reversible process.
Carnot Cycle was proposed by Nicolas Léonard Sadi Carnot (1796–1832) – a French physicist and engineer often called the “Father of Thermodynamics.” He was the first to study how heat can be converted into mechanical work.
The Four Stages of the Carnot Cycle
| Stage | Process | Description |
|---|---|---|
| 1 → 2 | Isothermal Expansion | Gas expands at constant temperature T1, absorbing heat Q1 from the hot reservoir. |
| 2 → 3 | Adiabatic Expansion | Gas continues to expand without heat exchange; temperature drops to T2. |
| 3 → 4 | Isothermal Compression | Gas is compressed at constant temperature T2, releasing heat Q2 to the cold reservoir. |
| 4 → 1 | Adiabatic Compression | Gas is compressed without heat exchange; temperature rises back to T1. |
Key Equations in Thermodynamics
| Concept | Equation | Description |
|---|---|---|
| First Law | ΔU = Q – W | Conservation of energy |
| Work (Isothermal) | W = nRT ln(V₂/V₁) | Work done by an ideal gas |
| Heat at Constant Pressure | Q = nCₚΔT | Heat absorbed at constant pressure |
| Heat at Constant Volume | Q = nCᵥΔT | Heat absorbed at constant volume |
| Relation between Cₚ and Cᵥ | Cₚ – Cᵥ = R | R = gas constant |
| Carnot Efficiency | η = 1 – T₂/T₁ | Ideal heat engine efficiency |
Applications of Thermodynamics
Thermodynamic principles are used in many fields:
Engines and Turbines: Convert heat energy into mechanical work.
Refrigeration and Air Conditioning: Use work to transfer heat from cold to hot regions.
Power Generation: Steam turbines, nuclear reactors, and solar plants all depend on heat-work conversion cycles.
Biological Systems: Human metabolism and body temperature regulation follow thermodynamic principles.
Glossary of Key Terms
Recap of the Key Terms in Thermodynamics
- Heat (Q): Energy transferred due to temperature difference
- Work (W): Energy transferred when a force moves an object
- Internal Energy (U): Total energy stored in a system
- Entropy (S): Measure of randomness or disorder
- Reversible Process: Ideal process that can be reversed perfectly
- Irreversible Process: Real process involving friction, heat loss, etc.
Quiz
Recap the concepts you have learnt. Try to answer the questions. You can find the answer to any question by clicking on the icon.
What does the First Law of Thermodynamics state?
The total energy of a system and its surroundings remains constant; energy can change forms but cannot be created or destroyed.
What is an adiabatic process?
A process where no heat enters or leaves the system (Q = 0).
What is the Carnot engine used for?
It’s a theoretical model that gives the maximum possible efficiency for converting heat into work.
What happens to entropy as a system approaches absolute zero?
It approaches zero, as per the Third Law of Thermodynamics.