lecture 4 Flashcards
entropy always
increases with time
work can easily be transformed into
heat but heat cannot easily be transformed into work
what makes a process irreversible
heat is generated
entropy =
change in heat/temperature
why does generating heat make a process irreversible
heat to work is difficult work to heat is easy
it generates entropy
entropy is a property of a
substance
more reversable a process is
the lower the entropy increase will be
entropy is the arrow of time
entropy always increases no matter what you do if you decreases entropy in one place you are increasing it by a greater amount somewhere else
friction prevents
an reversible process exsisting
entropy decreases
when a system is cooled
entropy increasse
when heat is added to a system
change in entropy =
irreversibility change + change in heat/temp
There is an entropy increase associated
with heat transfer, and friction, but not with work.
for adiabatic process entropy increase =
0
for a reversable process
ds = dq/T
substituting change in dq = Tds into first law
Tds = du +pdv ds = du/T + dpv/T
change in enthalpy =
Tds - vdp
entropy for solids and liquids
ds = du/T + pdv/T dv = 0 and du =cvdT ds = cv dT/T = cv lnT2/T1
entropy for ideal gas
in the LBoT
why for the given change in temperature for gas the entropy change greater than a solid
as extra component R =lnv2/v1
see LBoT for gas
isentropic process is one where
entropy does not change
draw the temperature entropy diagram for steam
see powerpoint
draw the temperature entropy diagram for an ideal gas
see powerpoint
if isentropic process describe the arrow of a pressure increase
straight arrow upwards
if non isentropic process describe the arrow of a pressure increase
arrow will move to the right as entropy always increases
compression
pressure increase (compressor)
expansion
pressure decreases (turbine)
isentropic process is
the most efficient
efficiency =
what you want/what you pay for
efficiency should always be
less than one
