Thermodynamics Chapter: Second Law of Thermodynamics

Second Law of Thermodynamics

• Describes the directionality of processes that can be described by the first law of thermodynamics.
• Stems from the fact that some processes are irreversible, meaning they only occur in one direction.

Irreversible Processes

• Examples: aging, a glass bottle shattering on the ground, and heat transfer from a hot to a cold object.
• Work is the transfer of energy by a force acting on an object as it is displaced.
• Energy is the capacity to do work.

Entropy

• A measure of the disorder or disorganization of a system.
• The second law of thermodynamics states that the total entropy of a system either remains constant or increases.
• The entropy cannot decrease during a process of some system.
• Entropy increases during heat transfer from a hot to a cold object.

Consequences of Entropy

• As entropy increases, a certain amount of energy becomes permanently unavailable to do work.
• The amount of usable energy in a system over time must decrease.

Carnot Cycle

• Named after 19th-century French engineer Nicolas Sadi Carnot.
• The greatest possible efficiency of a heat engine occurs in one that only uses reversible processes (entropy = 0).
• A Carnot engine operates at the highest possible efficiency at any heat engine operating between two given temperatures.

Clausius' Theorem

• Heat transfer must occur from a higher-temperature body to a lower-temperature body.
• An obvious consequence of entropy and the second law of thermodynamics.

Kelvin-Planck Statement

• It is impossible to construct a device that operates on a cycle and produces the net equivalent of work to the heat transferred to the device.
• Heat cannot be converted completely into useful work.

Derivation of the Second Law of Thermodynamics

• Carnot determined the efficiency of the Carnot engine to be: η = 1 - (Tc / Th).
• The change in entropy (S) can be defined as the ratio of heat over temperature, for reversible processes.
• For real, irreversible processes, the total change in entropy can be calculated as ΔS = Qc / Tc - Qh / Th.

Applications of the Second Law of Thermodynamics

• Heat flow can be leveraged in applications such as refrigerators and heat engines to either cool or heat a space.
• Examples: refrigerators, heat engines, and burning of fuel to drive an engine.

Examples of the Second Law of Thermodynamics

• Burning of fuel to drive an engine, such as the gasoline that fuels an automobile.
• Aging is another spontaneous, irreversible process that cannot be undone and moves in one direction.

Learn about the Second Law of Thermodynamics, which describes the directionality of irreversible processes, such as aging and heat transfer. Understand how it builds upon the First Law of Thermodynamics.