# Ch 5: Heat, Temperature and Thermodynamics

Mechanics

Oct 27, 2013 (4 years and 6 months ago)

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Ch 5: Heat, Temperature and
Thermodynamics

Chapter 5

Heat, Temperature and Thermodynamics

The Three Basic Phases of Matter

Microscopic behavior:

Macroscopic result:

Atoms are tightly packed

Material is non
-
compressible

fixed positions

Material does not flow

Solid

Chapter 5

Heat, Temperature and Thermodynamics

The Three Basic Phases of Matter

Microscopic behavior:

Macroscopic result:

Atoms are tightly packed

Material is non
-
compressible

Atoms free to roam around

Material does flow

Liquid

Chapter 5

Heat, Temperature and Thermodynamics

The Three Basic Phases of Matter

Microscopic behavior:

Macroscopic result:

Atoms are widely separated

Material is compressible

Atoms free to roam around

Material does flow

Gas

Chapter 5

Heat, Temperature and Thermodynamics

The Three Basic Phases of Matter

Sequence of increasing molecule motion (and energy)

Solid

Liquid

Gas

Internal Energy:

The sum total of all the microscopic and
random kinetic and potential energies of all
the atoms in an object.

Chapter 5

Heat, Temperature and Thermodynamics

The amount of internal energy depends on …

has more internal
energy than

… the amount of material

Internal Energy:

The sum total of all the microscopic and
random kinetic and potential energies of all
the atoms in an object.

Chapter 5

Heat, Temperature and Thermodynamics

The amount of internal energy depends on …

has more internal
energy than

… the phase of the material

Internal Energy:

The sum total of all the microscopic and
random kinetic and potential energies of all
the atoms in an object.

Chapter 5

Heat, Temperature and Thermodynamics

The amount of internal energy depends on …

has more internal
energy than

… the temperature

Temperature:

Temperature is a measure of average
atom/molecule speeds.

Chapter 5

Heat, Temperature and Thermodynamics

For Example:

At 25
o
F, air molecules have an average speed ~1080 mi/hr

At 100
o
F, air molecules have an average speed ~1160 mi/hr

Temperature:

Temperature is a measure of average
atom/molecule speeds.

Chapter 5

Heat, Temperature and Thermodynamics

Temperature Measurement:

Average molecule speeds are difficult to measure directly.

Temperature is almost always measured
indirectly (by one of it’s side effects).

One such side effect is thermal
expansion. Most materials expand when
hotter and contract when cooler.

Cool Iron bar

Temperature:

Temperature is a measure of average
atom/molecule speeds.

Chapter 5

Heat, Temperature and Thermodynamics

Temperature Measurement:

Average molecule speeds are difficult to measure directly.

Temperature is almost always measured
indirectly (by one of it’s side effects).

One such side effect is thermal
expansion. Most materials expand when
hotter and contract when cooler.

Hot Iron bar

Chapter 5

Heat, Temperature and Thermodynamics

Thermal expansion is a very small effect.

For example: Steel expands 0.006% for every 10
o
F

From 20
o
F to 100
o
F:

A 1.0 m steel bar expands 0.5 mm

From 20
o
F to 100
o
F:

A 500 ft steel bridge expands 3 in.

Chapter 5

Heat, Temperature and Thermodynamics

Thermal expansion is a very small effect.

The effect is greatly exaggerated in a ‘liquid in glass’
thermometer.

Chapter 5

Heat, Temperature and Thermodynamics

Thermal expansion is a very small effect.

Temperature Scales

100
o
C

212
o
F

0
o
C

32
o
F

F = C + 32

C = (F

32)

9

5

9

5

Chapter 5

Heat, Temperature and Thermodynamics

Thermal expansion is a very small effect.

Bi
-
metallic Strip or Spring

Upon
heating
:
metal 2 expands
more than metal 1

Chapter 5

Heat, Temperature and Thermodynamics

Thermal expansion is a very small effect.

Bi
-
metallic Strip or Spring

Upon
cooling
:
metal 2 contracts
more than metal 1

Chapter 5

Heat, Temperature and Thermodynamics

How Cold Can It Get?

Boiling pt for water

100
o
C

212
o
F

0
o
C

32
o
F

Air molecules average
1260 mi/hr.

Freezing pt for water

-
89
o
C

-
129
o
F

Coldest naturally
occurring on earth
[Vostok Antarctica,
July 1983]

Air molecules average
880 mi/hr.

Chapter 5

Heat, Temperature and Thermodynamics

How Cold Can It Get?

Boiling pt for water

100
o
C

212
o
F

0
o
C

32
o
F

Freezing pt for water

-
89
o
C

-
129
o
F

Coldest on earth

Air starts to liquefy

-
196
o
C

-
321
o
F

Air starts to freeze
into a solid

-
210
o
C

-
346
o
F

Molecule motion
essentially stop

-
273
o
C

-
460
o
F

Chapter 5

Heat, Temperature and Thermodynamics

How Cold Can It Get?

Boiling pt for water

100
o
C

212
o
F

0
o
C

32
o
F

Freezing pt for water

-
89
o
C

-
129
o
F

Coldest on earth

Air starts to liquefy

-
196
o
C

-
321
o
F

Air starts to freeze

-
210
o
C

-
346
o
F

Molecule motion
essentially stop

-
273
o
C

-
460
o
F

This is the coldest possible
temperature. Referred to as
‘absolute zero’

0 K

Kelvin scale:

K = C + 273

63 K

77 K

273 K

Chapter 5

Heat, Temperature and Thermodynamics

Thermodynamics

Other forms of energy (kinetic, chemical,
potential, etc.) can be converted 100% into heat.

Heat plays a special role in the conversion of one form of
energy into another.

There is no process whose sole effect is to
convert heat 100% into other forms of energy.

2
nd

Law of Thermodynamics

Chapter 5

Heat, Temperature and Thermodynamics

2
nd

Law of Thermodynamics

Related to/caused by the difficulty in organizing a vast
number of random motions.

This is the fundamental reason that physical processes
look ‘odd’ when viewed backward in time.

[All other basic law’s of physics don’t ‘care’ which way
time runs]

Chapter 5

Heat, Temperature and Thermodynamics

Thermodynamics

Any machine or process that does convert heat into other forms
of energy is referred to as a ‘heat engine’.

Any heat engine has a maximum upper limit to it’s efficiency
given by:

Max. Eff. = x 100%

( T
hot

T
cold
)

T
hot

T
hot

& T
cold
must be in K

Chapter 5

Heat, Temperature and Thermodynamics

Thermodynamics

Any heat engine has a maximum upper limit to it’s efficiency
given by:

Max. Eff. = x 100%

( T
hot

T
cold
)

T
hot

Note that the efficiency is always less than 100%.

Note that the efficiency = 0%, if T
hot

= T
cold
.

Real efficiencies are usually much lower.

Chapter 5

Heat, Temperature and Thermodynamics

Example:

Find Max. Eff. of a Steam Generator

Max. Eff. = x 100%

( 870 K

300 K

)

870 K

T
hot

= 600
o
C

T
cold
= 27
o
C

Max. Eff. = x 100%

( T
hot

T
cold
)

T
hot

Max. Eff. = x 100%

570 K

870 K

= 0.65 x 100%

= 65%

[
870

K ]

[
300

K ]