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Thermodynamics studies energy transformations involving heat and work. A thermodynamic system is a chosen region of matter; everything else is the surroundings. Systems are classified as open (exchange matter and energy), closed (energy only), or isolated (neither). The state of an ideal gas is fully described by any two of P, V, T — the third follows from $PV = nRT$.

State variables (P, V, T, U, S) depend only on the current equilibrium state, not on the history of how the system reached it. Process variables (Q, W) depend on the path. This distinction is fundamental: internal energy change $\Delta U$ between two states is unique, but heat and work can vary depending on the process.

Thermodynamics is built on four laws: the Zeroth Law (defines temperature), the First Law (energy conservation), the Second Law (direction of processes and entropy), and the Third Law (absolute zero is unattainable). For JEE, the First and Second Laws dominate, together accounting for nearly all numerical and conceptual problems. The subject connects to heat engines, refrigerators, and the efficiency limits imposed by nature — making it both theoretically elegant and practically important.