Image for conceptualization of entropy

Conceptualization of entropy.

There are a number of ways to express the Second Law of Thermodynamics. One expression of the Second Law by Lord Kelvin is that it is impossible to engineer a transformation whose only result would be to convert heat from a source at constant temperature into work. Another expression of the Second Law is by Rudolf Clausius, that heat cannot of itself pass from a colder to a hotter body. What can unify these expressions expressions conceptually is to see that they are both ways of saying that the entropy of an isolated system will increase over time, approaching a maximum value at equilibrium. Entropy is a function which can only increase for a system and its surroundings. Entropy is time's arrow. A function which is always greater at a later time. While it is useful to think of entropy as 'disorder', make sure you learn to see this in an abstract statistical sense. Entropy is the number of possible microscopic configurations of a system. Think about that. As spontaneous change occurs, and a system approaches equilibrium, as entropy increases, differences in temperature or chemical potential smooth out. Because that direction of change is statistically more likely, it occurs spontaneously.

The Second Law appears in direct fashion with moderate frequency on the MCAT. For example, a physical sciences passage on a chemical reaction or thermodynamic process may pose a question as to whether a particular change corresponds to an increase or decrease of entropy. Bear in mind, though, that the direct importance for the test, with regard to this topic, is dwarfed by the significance of the Second Law of Thermodynamics for understanding the physical and biological sciences in a rich way.

This is an important conceptual bridge which in crossing you will not only seize an advantage over the competition, but also gain a foothold into a much deeper understanding of science. Try to make thinking about the 2nd law a walking around habit. Walk around with it and be patient. It takes a careful thought process to proceed step by step into a deeper understanding of this material, but it will be worth it both for the test and for your appreciation of science in general.

WikiPremed Resources

Engines & the 2nd Law Cards
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Thermodynamics Practice Items
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2nd Law of Thermodynamics Images
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Learning Goals


Be able to explain the entropy of a system as a concept of statistics and probability.

Understand how the entropy of a system may change through processes such as heat flow, expansion, and mixture.

Be prepared to express the Second Law of Thermodynamics in a variety of ways and conceptualize how the different ways of expressing the 2nd Law are related.

Describe thermodynamic cycles and understand how to follow them on a pressure - volume (P - V) diagram.

Know how to determine the work done in a thermodynamic cycle from a P - V diagram.

Be able to narrate the Carnot cycle and understand the significance of the Carnot efficiency.

Recall the formula for entropy change due to heat flows at constant temperature and be prepared to bring this understanding to the understanding of the Carnot cycle.

Understand the determinations underlying the efficiency of a heat engine.

Comprehend heat pumps and be prepared to compute coefficient of performance.

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Conceptual Vocabulary for 2nd Law of Thermodynamics

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 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 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.
In thermodynamics, entropy is often associated with the amount of order, disorder, and or chaos in a thermodynamic system.
The thermodynamic concept of entropy can be described qualitatively as a measure of energy dispersal at a specific temperature.
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.
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.
Advanced terms that may appear in context in MCAT passages
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.
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.