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.