# Thermodynamics - University of South Alabama

Mechanics

Oct 27, 2013 (4 years and 8 months ago)

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1
(Ch 18)
Thermodynamics
(Ch
.
18)
PH 104 W/ DR. G
LEC 20

General relationships among heat, work, temperature

Examples:
1.
Heating kettle of water on stove:
2.
Heating air in balloon: (i) gas T increase, (ii)
Notes:
1. Heating can increase temperature:
Thermodynamics:
= increases total
2. Heating can increase internal energy and do work.
2a. Slow
heating (pressure & T constant): can do just work
FIRST LAW: Int.energy increase + work =

(Energy output = energy input: energy is
Thermodynamics
First Law of Thermodynamics

Can’t get more energy than the energy put in.

Other examples: Adiabatic = no heat transfer:
1
Compressing gas:(i) piston works on gas,(ii) gas T increases
FIRST LAW: Int.energy increase + work =

(Energy output = energy input: energy is
FIRST LAW: Int.energy increase + work = heat added

(Energy output = energy input: energy is conserved.)
1
.
Compressing

gas:

(i)

piston

works

on

gas,

(ii)

gas

T

increases
2.
Expanding gas: (i) gas does work, (ii) gas T decreases.
Thermodynamics
First Law of Thermodynamics

Can’t get more energy than the energy put in.

1
Compressing gas:(
i
) piston works on gas,(ii)
FIRST LAW: Int.energy increase + work = heat added

(Energy output = energy input: energy is conserved.)
FIRST LAW: Int.energy increase + work = heat added

(Energy output = energy input: energy is conserved.)
1
.
Compressing

gas:

(
i
)

piston

works

on

gas,

(ii)

2.
Expanding gas: (i) gas does work, (ii)
1: work done to gas (in) = increase in
2: work done by gas (out) = decrease in
Note: int. energy decrease – (work by gas)
= int. energy decrease + (work to gas) = heat, still (=0)
Thermodynamics
First Law of Thermodynamics

Adiabatic process in weather: air masses (parcels)
1.
Compressing gas: (i) pressure increases, (ii)
2.
Ex
p
andin
g

g
as:
(
i
)

p
ressure decreases
(
ii
)

FIRST LAW: Int.energy increase + work =

(Energy output = energy input: energy is conserved.)
p g g (
)
p
( )
1: work done to gas (in) = increase in
2: work done by gas (out) = decrease in
-- if air mass has moisture, can condense: clouds, rain
NEXT: Where the heat input comes from (output goes to)
Thermodynamics
Heat flow: always from hot to cold.

Thermal equilibration: heat flows until

Consider: continuous heat flow:

Hot “reservoir” to cold “reservoir”(sink): T
hot
, T
cold
constant

葉   拾 ｷ ｵ 
= SECOND LAW : “one way” arrow for energy flow

Putting

system

in

this

flow:

source

for

2
Thermodynamics
Heat flow: always from hot to cold.

HEAT ENGINE: cyclical process, produces work
from heat

Steam engine: Hot steam, Turbine, Cold condenser

Others: Fuel combustion, nuclear plant, etc.

弄

 不  ﹥
= SECOND LAW : “one way” arrow for energy flow

HURRICANE
:

a

gigantic

heat

engine

Hot = warm ocean water, Cold = upper troposphere

Work = wind!
Thermodynamics

Heat engine efficiency =

NEVER 100 % : heat input – work output = nonzero
heat output
S ffi i (h i
h )/(h i )
SECOND LAW : “one way” arrow for energy flow

Heat flow: from hot to cold only

Heat engine: can use only part of this heat for work
S
o e
ffi
c
i
ency =
(h
eat
i
nput

h
eat output
)/(h
eat
i
nput
)
=
Most ideal process:
ch
T
C heat to
T
H fromheat
=
h
ch
h
c
T
T-T
T
T
1
H fromheat
C heat to
-1 efficiency Ideal =−==
NOTE:
T
c
= 0 is NOT
possible.
Thermodynamics
SECOND LAW : “one way” arrow for energy flow

Heat flow: from hot to cold only

Heat engine: can use only part of this heat for work
h
ch
T
T-T
efficiency Ideal =
H fro
m
heat C heat to
ch
T
C heat to
T
H fromheat
=
Most ideal process:
=
h
T
=
c
T
Decrease in Increase in
ENTROPY = amount of DISORDER
Overall change in entropy = an INCREASE
Reservoirs-engine system: process INCREASES disorder.
Or: Energy, concentrated/usable(for work) becomes unusable.
All natural processes are heat engines!...
Thermodynamics
SECOND LAW : “one way” arrow for energy flow

Heat flow: from hot to cold only

Heat engine: can use only part of this heat for work

Ove
rall
,

o
r
de
r t
e
n
ds
t
ow
ar
ds

d
i
so
r
de
r
.
h
ch
T
T-T
efficiency Ideal =
Ove,o de e ds ow ds
d so de.
• Establishes a direction for any process.
• The reverse is “highly improbable”.
• Local ordering: possible… at cost of
• Life processes? Building organized structures! …
• “disorder tax”: fuel broken down, exhaust released, etc.
Ultimate end: no usable energy left in universe:
Laws of Thermodynamics:
FIRST LAW:
You can only obtain as much energy as you put in.
SECOND LAW:
All processes degrade energy into a less usable form.