Introduction to Thermodynamics
Fundamental concepts, laws, and key terminology
1. What Is Thermodynamics?
- Definition: The branch of physics that studies the relationships between heat, work, energy, and the properties of matter.
- Scope: Applies to all systems—from microscopic particles to engines, stars, and the atmosphere.
- Goal: Predict how energy is transferred and transformed, and determine the direction of natural processes.
2. Basic Terminology
| Term |
Symbol |
Description |
| System |
– |
Portion of the universe under study (e.g., a gas in a cylinder). |
| Surroundings |
– |
Everything external to the system. |
| Boundary |
– |
Interface separating system and surroundings (real or imaginary). |
| Closed System |
– |
No mass exchange, but energy (heat/work) can cross the boundary. |
| Open System |
– |
Both mass and energy can cross the boundary. |
| Isolated System |
– |
Neither mass nor energy cross the boundary. |
| State Variable |
– |
Property that defines the system’s condition (e.g., pressure P, volume V, temperature T). |
| Process |
– |
Transformation from one state to another (e.g., isothermal, adiabatic). |
| Cycle |
– |
Sequence of processes that returns a system to its initial state. |
| Equilibrium |
– |
No net macroscopic changes; all state variables are uniform and constant. |
| Heat (Q) |
– |
Energy transfer due to temperature difference. |
| Work (W) |
– |
Energy transfer due to forces acting through a distance (e.g., expansion work). |
| Internal Energy (U) |
– |
Total microscopic kinetic + potential energy of a system. |
| Enthalpy (H) |
H = U + PV |
Useful for processes at constant pressure. |
| Entropy (S) |
– |
Measure of disorder or the number of microscopic configurations. |
3. The Four Laws of Thermodynamics
0th Law – Thermal Equilibrium
- Statement: If system A is in thermal equilibrium with system B, and B with system C, then A is in thermal equilibrium with C.
- Consequence: Defines temperature as an empirical property; allows the use of thermometers.
1st Law – Conservation of Energy
- Statement: The change in internal energy of a closed system equals the net heat added to the system minus the net work done by the system.
- Mathematical Form:
[
\Delta U = Q - W
]
- Implications: Energy cannot be created or destroyed, only transferred or converted.
2nd Law – Directionality & Entropy
- Multiple formulations (choose the most convenient):
- Clausius: Heat cannot spontaneously flow from a colder body to a hotter one.
- Kelvin‑Planck: No cyclic device can convert all absorbed heat into work; some heat must be rejected to a colder reservoir.
- Entropy Form: For any isolated system, the total entropy never decreases; it increases for irreversible processes and remains constant for reversible ones.
- Mathematical expression for a reversible process:
[
dS = \frac{\delta Q_{\text{rev}}}{T}
]
3rd Law – Absolute Zero
- Statement: As the temperature of