Transcript Chapter 15

Chapter 15
The Laws of Thermodynamics
Thermodynamics
The study of the processes in which
energy is transferred as heat and as
work
heat--transfer of energy due to a
temperature difference
work--transfer of energy not due to a
temperature difference

Thermodynamics
System--any object or set of objects that
we wish to consider
closed system--a system for which no
mass enters or leaves (but energy can)
open system--mass as well as energy
can be exchanged with environment
isolated closed system--neither mass or
energy passes across system boundary
15.1 The First Law of
Thermodynamics


based on the law of conservation of
energy
the change in the internal energy of a
closed system, ( U), will be equal to
heat added to system minus work done
by the system

U=Q-W
15.1 The First Law of
Thermodynamics




U=Q-W
where:
 U, Internal energy (the total of all
energy of all the molecules of a system)
 U = positive when internal energy of
system increases
 U = negative when internal energy of
system decreases
15.1 The First Law of
Thermodynamics




U=Q-W
where:
Q = heat
heat added to the system is positive
heat lost from the system is negative
15.1 The First Law of
Thermodynamics




U=Q-W
where:
W = work
work done by the system is positive
work done to the system is negative
15.1 The First Law of
Thermodynamics





For isolated systems:
 U = 0 so
0 = Q - W and
Q=W=0
so no work is done by an isolated
system
15.1 The First Law of
Thermodynamics



The amount of internal energy is a property of
a system; heat and work is not
a system doesn’t have a given amount of
heat or work; rather they are added or
removed form the system by the change of
state of the system
so heat and work are part of a
thermodynamic process that can change a
system from one state to another not a
characteristic of the system such as P, V, T, n,
or U.
15-2 The 1st Law of
Thermodynamics Applied to
Some Simple Systems
Simples Processes:
 Isothermic Process
 Adiabatic Process
 Isobaric Process
 Isochoric (iosvolumetric) Process
Isothermic Process


Idealized process carried out at
constant temperature
for ideal gases; PV=nRT so PV=
constant
Isothermic Process



Example: ideal gas in a cylinder fitted
with a movable piston
gas is in contact with a heat reservoir
(body of mass so large that there is no
significant T to it when heat is
exchange with it)
assume that compression or expansion
is done slowly enough that all gas stays
at equilibrium at same constant T
Isothermic Process

If heat (Q = +) is
added to the system &
T=constant then the
gas must expand and
do work on the
surroundings (W = +)
Work
done by
system
Heat
added
to
system
Isothermic Process



Since T is constant
there is no U
(U=3/2nRT--Ch-14)
so
U = Q - W or Q = W
so in an isothermic
process work done by
the system is equal to
the heat added to the
system
Work
done by
system
Heat
added
to
system
Adiabatic Process



Process in which no heat is allowed to
flow into or out of the system
Q=0
can occur if process happens so quickly
that heat has no time to flow into or out
of system
Adiabatic Process




Since Q = 0 then
U = - W
U decreases (U = -)
if gas expands and
does work by the
system
(W = +)
temperature also
decreases as well Temperature
decreases
(U=3/2nRT)
Work
by the
system
decrease
in
internal
energy
Adiabatic Process


If gas contracts work
is done on the system
so U and T increases
diesel engines
compress fuel mixture
by a factor of 15+ so it
ignites without spark
plugs
Temperature
increases
Work
on the
system
increase
in
internal
energy
Adiabatic Process






Example:
hold a rubber band loosely with hands
gauge temperature by holding to lips
stretch band suddenly
quickly touch to lips
temperature increases, WHY?
Isobaric Process

Pressure is kept constant
since P = constant, P = F/A so F = PA
W = F x d so W = PAd
since V = Ad

when a piston expands: W



= PV
Isobaric Process




V = Vf - Vi
so expansion of gas
causes work done by
system (+W)
compression of gas
caused by work done
on system (-W)
and this work is
W = PV
Work
by the
system
Gas
expands
Isochoric Process





Isovolumetric
one in which volume
doesn’t change
since W = PV and
there is no V process
does no work on
surrounding
all heat added
changes internal
Internal
energy
energy
increases
U = Q
No
movement
of piston
Heat
added
PV Diagram
Graph of the relationship between
pressure and volume at various
temperatures
isotherms--lines (curves) with same
temperature
 area under the curve (PV) gives you
work done as conditions change for the
various processes

PV Diagram
isothermic
adiabatic
isobaric
isochoric
volume
15-3 Human Metabolism and the
First Law
Metabolism--the many energy transforming
processes that occur within an organism
 U = Q - W
 our bodies does work on environment and
loses heat so to maintain our internal energy
we must increase U by eating
metabolic rate--rate at which U is transformed
inside our body (kcal/h or watts)
15-4 The Second Law of
Thermodynamics




The Second Law of Thermodynamics
it as many equivalent statements
Clausius statement--heat flows naturally
from a hot object to a cold object, but not
spontaneously (by itself without input of some
kind of work) from cold to hot
the study of heat engines (any device that
changes thermal energy into mechanical
energy) will help develop a more general form
15-5 Heat Engines


Any device that changes thermal energy
into mechanical work
the basic idea behind a heat engine is
that mechanical energy can be obtained
from thermal energy only when heat is
allowed to flow from a high temperature
to a cold temperature
15-5 Heat Engines


Heat input (QH) at a
High Temp TH
high temperature (TH)
is partly transformed
QH
into work (W) and
partly exhausted as
engine
heat (QL) at a lower
W
temperature (TL)
TH and TL are called the
QL
operating
temperatures of the
engine
Low Temp TL
15-5 Heat Engines
We will examine heat engines that run
in a repeating cycle; continuously
 ****Note*** New Sign Convention: QH,
QL, and W are always positive now
Two types of practical engines:
 Steam engine
 Internal combustion engine

15-5 Heat Engines
Steam engine
 Steps (OH):
 high pressure steam is produced by
combustion or nuclear fission
 steam turns turbine or expands piston
(work)
 low pressure steam is condensed and
re-vaporized
15-5 Heat Engines
Internal combustion engine
steps (four-stroke engine)(OH):
 intake stroke--fuel mixture enters cylinder
 compression stroke--mixture is compressed
by piston
 power stroke--fuel mixture is ignited and
expanding gases force piston down (work)
 exhaust stroke-exhaust gas pushed from
cylinder
15-5 Heat Engines





Why is T needed to drive a heat
engine?
Efficiency of heat engine e = W/QH
QH = W + QL (energy conserved)
so W = QH - QL
e = W/QH = (QH - QL)/QH = 1- QL/QH
15-5 Heat Engines
Carnot Engine
 ideal engine
 each process of heat addition and exhaust,
expansion or compression would be
reversible (each step done slow enough to
achieve equilibrium before next step occurs)
real engines--process acts quickly, irreversible
(due to friction and turbulence)
15-5 Heat Engines
Carnot cycle
four step cycle (OH)
 gas is expanded isothermally with addition of
heat (T constant)
 gas expands adiabatically (no Q added but
T)
 gas compressed isothermally
 gas expands adiabatically until its at original
state
15-5 Heat Engines
Carnot efficiency (ideal)
 eideal = (TH - TL)/TH = 1- TL/TH
 theoretical limit to efficiency
 60-80% efficiency is excellent
15-5 Heat Engines
Second Law of Thermodynamics
Kelvin-Planck statement
 no device is possible whose sole effect
is to transform a given amount of heat
completely into work
 only possible if TL = 0K
Third Law of Thermodynamics
 absolute zero is unattainable
15-6 Refrigerators, Air
Conditioners, and Heat Pumps


Operation of these
devices is the
reverse of a heat
engine
engine
by doing work heat
is taken from a low
temperature region
to a high
temperature region
High Temp TH
QH
W
QL
Low Temp TL
15-6 Refrigerators, Air
Conditioners, and Heat Pumps
Refrigerators and Air Conditioners
process
 inside (low T) expanding gas absorbs
heat; liquidgas
 condenser/pump; pumps warm gas
outside and compresses it so that it
loses heat;
gas liquid

15-6 Refrigerators, Air
Conditioners, and Heat Pumps
Refrigerators and Air Conditioners
coefficient of performance (CP)
 CP = QL/W = QL/(QH - QL)
 CPideal = TL/(TH- TL)

15-6 Refrigerators, Air
Conditioners, and Heat Pumps
Heat Pump
 device that heats a house by taking
heat from QL (low T) outside house and
delivering heat QH (high T) to warm
inside of house
 works like refrigerator and A/C, can be
reversed to cool in summer
CP = QH/W

15-7 Entropy and the Second
Law of Thermodynamics
Entropy (S)
 is a state function of a system
 we deal with a change in entropy (S) during
processes
 according to Clausius, the change in entropy
of a system when an amount of heat (Q) is
added to it by a reversible process at a
constant temperature (K) is: S =Q/T
15-7 Entropy and the Second
Law of Thermodynamics
Entropy (S)
 if the temperature change is not too great an
average value for temperature can be used, if
not that’s what they made calculus for
Second Law of Thermodynamics (again)
 the entropy of an isolated system never
decreases. It can only stay the same or
increase
15-7 Entropy and the Second
Law of Thermodynamics
Entropy (S)
Second Law of Thermodynamics (and again)
 the entropy of an isolated system never
decreases. It can only stay the same or
increase
 for real processes; S>0
 if system is not isolated:
S = Ss + Senv0
15-7 Entropy and the Second
Law of Thermodynamics
Entropy (S)
Second Law of Thermodynamics (and again)
 the total entropy of any system plus that of its
environment increases as a result of any
natural process
 only in ideal processes is S = 0
 entropy is not conserved it is always
increasing
15-8 Order to Disorder
Entropy--a measure of the disorder of a
system
Second Law of Thermodynamics (once
again)
 Natural processes tend to move toward
a state of greater disorder
 Examples: salt/pepper, broken cup,
mixing hot/cold
15-8 Order to Disorder




An increase in entropy corresponds to
an increase in disorder (randomness)
Information theory
built on the idea that the more orderly
an arrangement the more information is
needed to specific it or classify it
hot/cold liquidscool liquid
15-9 Unavailability of Energy;
Heat Death



In the process of heat conduction from a
hot body to a cold one entropy
increases and order goes to disorder
the separation of hot/cold objects can
serve as high/low temperature regions
for a heat engine and be used to obtain
useful work
when the two objects reach the same
temperature no work can be obtained
15-9 Unavailability of Energy;
Heat Death
Second Law of Thermodynamics
 in any natural process, some energy
becomes unavailable to do useful work
 in any process energy is never lost but it
becomes less useful to do work
 energy is degraded as it goes from
more ordered forms (mechanical) to
less ordered forms (internal to thermal)
15-9 Unavailability of Energy;
Heat Death




A natural outcome is that as time goes on, the
universe will approach a state of maximum
disorder
matter will become a uniform mixture at one
temperature, no work can then be done
all energy would be degraded to thermal
energy and all change would cease (Heat
Death)
this is based on the assumption that the
universe is finite
15-10 Evolution and Growth;
“Time’s Arrow”


Evolution of organisms and entropy
Why has entropy called time’s arrow?
15.12 Energy Resources:
Thermal Pollution
Know this:
 greenhouse effect
 thermal pollution
Processes for electricity generation
 Fossil-fuel steam plants
 Nuclear energy
 Geothermal energy
 Hydroelectric power plants
 Tidal energy
 Wind power
 Solar energy (solar cell/photovoltaic cell)