http://www.ck12.org Chapter 14. Thermodynamics
(or more properly, the change in entropy) is also a matter ofprobabilitywhich we discuss below. Entropy can be
expressed also as theinformationa system has (or loses). The link between entropy and “information theory” is
quite complicated and beyond the scope of this book.
Entropy as Probability
There is a statistical explanation of entropy which says that the most disorder state is the most probable one. Suppose
you flip two coins at once. Let’s list the possible outcomes.
We’ll useT=tailsandH=heads
One possibility for two tails:T T
One possibility for two heads:HH
Two possibilities for one head and one tail:T HorHT
There are a total of four possible outcomes with a 25% chance of getting two tails and a 25% chance of getting two
heads. But there is a 50% chance of getting one head and one tail.
The more organized states (those above with a 25% probability) have a lower probability of occurring.
The most probable outcomes in the natural world are those with the greatest disorder. A tea cup falling and shattering
is a disordered state and much more likely to occur than is a shattered tea cup reconstructing itself into a more ordered
state. Similarly, stars (highly organized objects) transfer energy to the near-absolute-zero vacuum of interstellar
space until all that remains are burned out cinders and a slightly warmer interstellar vacuum. One cosmological
possibility is that the highest probable state of universe is the so-called “heat death” of the universe: A universe with
one low (uniform) temperature.
- The ideal gas law statesPV=nRTwherePis pressure,Vis volume,nis the number of moles of substance,Ris
the universal gas constant andTis temperature. - Thermodynamics is the study of processes in which energy is transferred by heat and work.
- In an isothermal process, temperature is constant
- In an isochoric process, volume is constant
- In an isobaric process, pressure is constant
- In an adiabatic process, no heat flows into or out of the system
- The first law of thermodynamics: The change in the internal energy of a closed system is equal to the heat into
(or out of) a system plus the work done on the system (or by the system).
∆U=Q+W,∆Uis the change in internal energy of the system,Qis the heat, andWthe work.
- The second law of thermodynamics describes the direction in which physical phenomena can occur.
Statements describing the second law:
a. Heat can flow spontaneously from hot to cold but never from cold to hot.
b. No heat machine is 100% efficient.
The most general statement of the second law:
For all natural processes, the total entropy of a system increases.
- The efficiency of an actual heat engine can be expressed as
e=
(
1 −QQLH
)
× 100
The high temperature thermal energy isQHand the low temperature thermal energy isQL.