Heat and the Second Law of Thermodynamics

Heat

A form of energy transfer between two objects or systems due to a temperature difference.

Thermal Equilibrium

The state reached when heat flows from an object at a higher temperature to one at a lower temperature until both are at the same temperature.

Units of Heat

The SI unit of heat is the joule (J), though it is often measured in calories (1 cal = 4.184 J).

Temperature

Measures the average kinetic energy of the particles in a substance (how hot or cold something is).

Conduction

The transfer of heat through a solid or between solids in direct contact, where heat moves from hot areas to cooler areas by vibrating particles transferring energy to neighboring particles.

Convection

The transfer of heat through a fluid (liquid or gas) by the movement of the fluid itself, where warm fluid rises while cooler fluid sinks, creating a circulation pattern.

Radiation

The transfer of heat through electromagnetic waves (e.g., infrared radiation) without the need for a medium.

Second Law of Thermodynamics

States that the total entropy (disorder) of an isolated system will always increase over time, and energy spontaneously disperses from areas of higher concentration to lower concentration.

Entropy

A measure of the disorder or randomness of a system.

Heat Engine

A device that takes in heat and converts part of it into useful work (e.g., in a car engine, steam turbine).

Efficiency of Heat Engines

The ratio of work done to the heat absorbed, calculated as η = W/Q_in = 1 - Q_out/Q_in.

Refrigerator

A device that moves heat from a cooler area to a warmer one, requiring work (energy input) to operate.

Heat Pump

A device similar to a refrigerator that moves heat from a cooler area to a warmer one, requiring work (energy input).

Coefficient of Performance (COP)

A measure of the efficiency of a heat pump or refrigerator, calculated as COP = Q_cold/W.

Irreversibility of Processes

The implication of the second law that natural processes are irreversible, such as heat always flowing from hot to cold.

Entropy Increase

The tendency of systems to evolve towards higher entropy or disorder, such as when a hot object cools down.

Work Done (W)

The useful energy output from a heat engine.

Heat Absorbed (Q_in)

The total heat energy taken in by a heat engine.

Heat Expelled (Q_out)

The total heat energy released by a heat engine.

Natural Processes

Processes that tend to increase the disorder or randomness of the universe, as stated by the second law of thermodynamics.