Thermodynamics (Mech)

Laws of Thermodynamics:

Zeroth Law: if two systems are in thermal equilibrium with a third, they are with each other. Basis of temperature.

First Law: Energy is conserved. dQ = dU + dW. Heat added = increase in internal energy + work done by system.

Second Law: No heat engine can be 100% efficient. Heat flows naturally from hot to cold. Entropy of universe increases.

Third Law: Entropy approaches zero as temperature approaches absolute zero.

Thermodynamic Cycles (memorize):

• Carnot cycle: maximum efficiency = 1 − T_cold/T_hot. Reversible isothermal + adiabatic processes.
• Otto cycle: petrol engines. Compression + combustion + expansion + exhaust.
• Diesel cycle: diesel engines. Compression > Otto, no spark.
• Rankine cycle: steam power plants. Boiler → turbine → condenser → pump.
• Refrigeration cycle: reverse of heat engine. Refrigerant absorbs heat from cold reservoir, releases to hot.

IC Engine basics:

• 4-stroke: intake, compression, power, exhaust.
• 2-stroke: combined intake+compression and power+exhaust.
• Diesel: compression ignition (auto-ignition).
• Petrol: spark ignition.

Key formulas:

• Efficiency η = W_net / Q_in.
• COP_refrigerator = Q_cold / W_in.
• Specific heat: Q = mcΔT.

RRB JE focus: cycle efficiency, IC engine differences, simple gas law problems.