First Law of Thermodynamics — Prelims Strategy

Prelims Strategy

To excel in NEET questions on the First Law of Thermodynamics, a systematic approach is key. First, thoroughly understand the definitions of internal energy, heat, and work, and crucially, master the sign conventions: $Q$ is positive if absorbed, negative if released; $W$ is positive if done *by* the system, negative if done *on* the system.

Always write down the First Law as $\Delta U = Q - W$. Second, memorize the characteristics and specific formulas for work, heat, and internal energy change for each standard thermodynamic process: isochoric ($W=0$), isobaric ($W=P\Delta V$), isothermal ($\Delta U=0$ for ideal gas), and adiabatic ($Q=0$).

Practice drawing and interpreting P-V diagrams, as the area under the curve represents work done, and the area enclosed by a cycle represents net work. For ideal gases, remember that $\Delta U = nC_V\Delta T$ holds for *any* process, and use Mayer's relation $C_P - C_V = R$.

When solving numerical problems, convert all temperatures to Kelvin and ensure consistent units. For conceptual questions, focus on the implications of each process on temperature, pressure, volume, and internal energy.

Be wary of trap options that confuse heat with internal energy or misapply sign conventions.