Second Law of Thermodynamics

• Heat generally cannot flow spontaneously from a material at lower temperature to a material at higher temperature.

• The entropy of an isolated macroscopic system never decreases, or (equivalently) that perpetual motion machines are impossible.

Second Law of Thermodynamics

• Identifies the direction of a process. (e.g.: Heat can only spontaneously transfer from a hot object to a cold object, not vice versa)

• Used to determine the “Quality” of energy. (e.g.: A high-temperature energy source has a higher quality since it is easier to extract energy from it to deliver useable work.)

• Used to exclude the possibility of constructing 100% efficient heat engine and perpetual-motion machines

• Used to introduce concepts of reversible processes and irreversibilities.

• Determines the theoretical performance limits of engineering systems. (e.g.: A Carnot engine is theoretically the most efficient heat engine; its performance can be used as a standard for other practical engines)

Second Law (cont)• A process can not happen unless it satisfies both the first and second laws of thermodynamics. The first law characterizes the “quantity” of energy. The second law defines the “quality”.

• Define a “Heat Engine”: A device that converts heat into work while operating in a cycle.

Heat engine

QH

QL

TH

TL

Wnet

Q-Wnet=U (since U=0 for a cycle)Wnet=QH-QL

Thermal efficiency (Carnot efficiency), th is defined as:th=Wnet/QH=(QH-QL)/QH

=1-(QL/QH)

Kevin-Planck Statement• The Kelvin-Planck Statement is another expression of the second law of thermodynamics. It states that:

It is impossible for any device that operates on a cycle to receive heat from a single reservoir and produce net work.

Heat engine

QH

TH

Wnet

• A heat engine has to reject some energy into a lower temperature sink in order to complete the cycle.

• TH>TL in order to operate the engine. Therefore, the higher the temperature, TH, the higher the quality of the energy source and more work is produced.

Reversible Processes and Irreversibilities

• A reversible process is one that can be executed in the reverse direction with no net change in the system or the surroundings. • At the end of a forwards and backwards reversible process, both system and the surroundings are returned to their initial states.• No real processes are reversible. • However, reversible processes are theoretically the most efficient processes. • All real processes are irreversible due to irreversibilities. Hence, real processes are less efficient than reversible processes.Common Sources of Irreversibility:

• Friction • Sudden Expansion and compression• Heat Transfer between bodies with a finite temperature difference.

Entropy• Systems will tend to progress towards a

state of entropy (lower levels of order).

Summary

• While the first law of thermodynamics is considered “written in stone”, the second law is about the probability of an occurrence.

• Sometimes order does appear from disorder. However, unless work is added to the system, the likelihood of that happening is small.