### 2nd Law of Thermodynamics

Each list begins with basic conceptual vocabulary you need to know for MCAT questions and proceeds to advanced terms that might appear in context in MCAT passages. The terms are links to Wikipedia articles.

**Second law of thermodynamics**

The second law of thermodynamics is an expression of the universal law of increasing entropy, stating that the entropy of an isolated system which is not in equilibrium will tend to increase over time, approaching a maximum value at equilibrium.

**Carnot cycle**

The Carnot cycle represents the most efficient cycle possible for converting a given amount of thermal energy into work or, conversely, for using a given amount of work for refrigeration purposes.

**Thermodynamic cycle**

A thermodynamic cycle is a series of thermodynamic processes which returns a system to its initial state.

**Entropy**

Entropy is a measure of the unavailability of a system's energy to do work.

**Thermodynamic equilibrium**

A thermodynamic system is said to be in thermodynamic equilibrium when it's state is characterized by the minimum of a thermodynamic potential, such as the Helmholtz free energy.

**Heat engine**

A heat engine is a physical or theoretical device that converts thermal energy to mechanical output.

**Reversible process**

A reversible process, or reversible cycle if the process is cyclic, is a process that can be reversed by means of infinitesimal changes in some property of the system without loss or dissipation of energy.

**Refrigeration**

Refrigeration is the process of removing heat from an enclosed space, or from a substance, and rejecting it elsewhere in order to lower the temperature of the enclosed space or substance and then maintain that lower temperature.

**Heat pump**

A heat pump is a machine or device that moves heat from one location to another via work.

**Coefficient of performance**

The coefficient of performance of a heat pump is the ratio of the output heat to the supplied work

**Thermal efficiency**

The thermal efficiency is a dimensionless performance measure of a thermal device such as an internal combustion engine, a boiler, or a furnace.

**Carnot's theorem**

Carnot's theorem sets a limit on the maximum amount of efficiency any possible engine can obtain based on the difference between the hot and cold reservoir temperatures.

**Third law of thermodynamics**

The third law of thermodynamics is an axiom of nature regarding entropy and the impossibility of reaching absolute zero of temperature.

**Entropy**

In thermodynamics, entropy is often associated with the amount of order, disorder, and or chaos in a thermodynamic system.

**Entropy**

The thermodynamic concept of entropy can be described qualitatively as a measure of energy dispersal at a specific temperature.

**Dissipation**

Dissipation embodies the concept of a dynamical system where important mechanical modes, such as waves or oscillations, lose energy over time, typically due to the action of friction or turbulence.

**Isentropic process**

In thermodynamics, an isentropic process is one during which the entropy of the system remains constant.

**Probability distribution**

A probability distribution is a probability measure defined over a state space instead of the sample space.

**Statistical thermodynamics**

Statistical thermodynamics is the study of the microscopic behaviors of thermodynamic systems using probability theory.

**Rudolf Clausius**

Rudolf Clausius (1822 - 1888), was a German physicist and mathematician considered one of the central founders of the science of thermodynamics. His restatement of Carnot's principle put the theory of heat on sounder basis.

**Nicolas Léonard Sadi Carnot**

Nicolas Carnot (1796 - 1832) was a French physicist and military engineer who gave the first successful theoretical account of heat engines, thereby laying the foundations of the second law of thermodynamics.

**William Thomson, 1st Baron Kelvin**

William Thomson, 1st Baron Kelvin, (1824 - 1907) was a British mathematical physicist and engineer who did important work in the mathematical analysis of electricity and thermodynamics. He is widely known for developing the scale of absolute temperature measurement.

**Quasistatic process**

A quasistatic process is a thermodynamic process that happens infinitely slowly, which in practice, can be approximated by performing the process very slowly.

**Waste heat**

Waste heat refers to heat produced by machines and technical processes for which no useful application is found.

**Microstate**

In statistical mechanics, a microstate describes a specific detailed microscopic configuration of a system, that the system visits in the course of its thermal fluctuations.

**Maxwell's demon**

Maxwell's demon was an 1867 thought experiment by the Scottish physicist James Clerk Maxwell, meant to raise questions about the possibility of violating the second law of thermodynamics.

**Recuperator**

A recuperator is a special purpose counter-flow heat exchanger used to recover waste heat from exhaust gases.

**Thermal expansion valve**

A thermal expansion valve is a component in an air conditioning system that controls the rate at which liquid refrigerant can flow into an evaporator.

**Steam turbine**

A steam turbine is a mechanical device that extracts thermal energy from pressurized steam, and converts it into useful mechanical work. It has almost completely replaced the reciprocating piston steam engine.

**Geometrical frustration**

Geometrical frustration is a phenomenon in condensed matter physics in which the geometrical properties of the atomic lattice forbid the existence of a unique ground state, resulting in a nonzero residual entropy.

**Zero-point energy**

The zero-point energy is the lowest possible energy that a quantum mechanical physical system may possess and is the energy of the ground state of the system.

**Magnetic refrigeration**

Magnetic refrigeration is a cooling technology based on the magnetocaloric effect which can be used to attain extremely low temperatures (well below 1 kelvin).

**Stirling engine**

In the family of heat engines, Stirling engine defines a closed-cycle regenerative hot air engine, though the term is often used incorrectly to refer generically to a much wider range of hot air engine types.

**Watt steam engine**

The Watt steam engine was the first type of steam engine to make use of steam at a pressure above atmospheric.

**Statistical ensemble**

A statistical ensemble is an idealization consisting of a large number of mental copies of a system, considered all at once, each of which represents a possible state that the real system might be in.

**Ergodic hypothesis**

The ergodic hypothesis says that all accessible microstates are equally probable over a long period of time.

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