Thermodynamics notes

Thermal equilibrium is the condition in which two bodies in contact stop exchanging net heat because they have the same temperature. Temperature is the physical quantity that tells the direction of heat flow: heat flows naturally from higher temperature to lower temperature. The Zeroth Law states that if two systems are separately in thermal equilibrium with a third system, they are in thermal equilibrium with each other. This law makes temperature measurement possible because a thermometer can act as the third system. For NEET, remember that thermal equilibrium does not mean equal heat content or equal internal energy; it means equal temperature and no net heat flow.

A thermodynamic system is the part of the universe chosen for study, while everything outside it is the surroundings. Systems may be open, closed or isolated depending on exchange of mass and energy. The macroscopic condition of a system is described by state variables such as pressure, volume, temperature and internal energy. A state function depends only on the state, not on the path followed. For an ideal gas, pressure, volume and temperature are connected by the equation of state PV = nRT. NEET questions often test whether a quantity is a state variable or a path variable, and whether the system is open, closed or isolated.

Heat, work and internal energy are central quantities in thermodynamics. Internal energy is the total microscopic energy of molecules due to random motion and interactions. Heat is energy transferred between system and surroundings because of temperature difference. Work is energy transferred when a macroscopic force acts through displacement, such as gas pushing a piston. In thermodynamics, work done by a gas during expansion is positive in the NCERT convention commonly used for physics: W = ∫P dV. Heat and work are path dependent, but change in internal energy is path independent. For an ideal gas, internal energy depends only on temperature.

The First Law of Thermodynamics is the law of conservation of energy applied to thermal systems. In the NCERT physics sign convention, heat supplied to a system is used partly to increase internal energy and partly to do external work: Q = ΔU + W. If heat is supplied but no work is done, internal energy increases. If a gas expands without heat input, its internal energy decreases. The law does not tell whether a process is naturally possible; that is the role of the second law. NEET problems usually involve identifying signs of Q, W and ΔU for different processes and applying the equation carefully.

A thermodynamic process is a change of a system from one equilibrium state to another. The most important NEET processes are isothermal, adiabatic, isobaric, isochoric and cyclic. In an isothermal process, temperature remains constant and for an ideal gas ΔU = 0. In an adiabatic process, no heat is exchanged and temperature usually changes. In an isobaric process, pressure is constant; in an isochoric process, volume is constant and work is zero. A cyclic process returns to its initial state, so net change in internal energy is zero. P-V graphs are essential because the area under the curve gives work.

Heat capacity measures how much heat is required to raise the temperature of a body. Specific heat capacity is heat required per unit mass per unit temperature rise, while molar heat capacity is heat required per mole per unit temperature rise. For gases, heat required depends strongly on the process. At constant volume, supplied heat only increases internal energy, so Cv is used. At constant pressure, heat also does work of expansion, so Cp is greater than Cv. For an ideal gas, Mayer’s relation is Cp - Cv = R. The ratio γ = Cp/Cv is crucial in adiabatic processes and heat engine questions.

The First Law permits many energy-conserving processes, but the Second Law decides which processes occur naturally. Heat naturally flows from hot to cold, not cold to hot without external work. No heat engine can convert all absorbed heat into work in a cyclic process. Entropy is a measure of energy dispersal or disorder, and the entropy of an isolated system never decreases. Reversible processes are ideal and can be exactly retraced; real processes are irreversible due to friction, turbulence, finite temperature differences and dissipation. For NEET, focus on Kelvin-Planck and Clausius statements, entropy change, and why 100% efficient heat engines are impossible.

A heat engine is a cyclic device that absorbs heat from a hot reservoir, converts part of it into work, and rejects the remaining heat to a cold reservoir. Its efficiency is the fraction of absorbed heat converted into work. A refrigerator works in the reverse direction: it removes heat from a cold body and rejects it to a hotter surroundings by using external work. A heat pump is similar but its useful output is heat delivered to the hot region. NEET questions commonly ask for efficiency, coefficient of performance, energy balance, and why an engine cannot have 100% efficiency due to the second law.

The Carnot engine is an ideal reversible heat engine operating between two reservoirs at temperatures TH and TC. Its cycle has four reversible steps: isothermal expansion at TH, adiabatic expansion, isothermal compression at TC and adiabatic compression. It has the maximum possible efficiency for any engine working between the same two temperatures. Carnot efficiency depends only on reservoir temperatures and is given by η = 1 - TC/TH, where temperatures must be in kelvin. No real engine can exceed Carnot efficiency because real engines have irreversibilities such as friction and finite temperature heat transfer. NEET often asks cycle sequence, efficiency and comparison of engines.

Calorimetry deals with measurement of heat exchanged between bodies. When hot and cold bodies are mixed in an insulated container, heat lost by the hot body equals heat gained by the cold body, assuming no loss to surroundings. Temperature change without phase change is calculated using Q = mcΔT. During a phase change, temperature remains constant while heat changes the internal molecular arrangement; this heat is called latent heat and is calculated using Q = mL. Although calorimetry is often introduced in thermal properties, it supports thermodynamics by clarifying heat transfer, thermal equilibrium and energy conservation. NEET questions often combine calorimetry with phase change.

Thermodynamics NEET problems become easy when you first identify the process, sign convention and known state variables. Always draw or inspect the P-V graph: area under the curve gives work, and enclosed area gives net work in a cycle. Next use the first law Q = ΔU + W and the ideal-gas fact that ΔU depends only on temperature. For engines and refrigerators, apply energy balance before efficiency or COP. For Carnot questions, convert all temperatures to kelvin. Conceptual traps usually involve confusing heat with internal energy, using Celsius, assuming all processes are reversible, or forgetting that second law limits efficiency.