CCh

ap. 1

4: H

eat

•To study tem

perature and temperature scales.

•To describe therm

al expansion and its applications.

•To explore and solve problem

s involving heat, phase changes and calorim

etry. •

To study heat transfer. •

To describe solar energy and see how technology can lead to resource conservation.

1

HHow

to S

tudy C

hap. 1

4

1)H

eat transfer, equilibrium,

temperature: P.14-5, 27,

53, 56, 62, 71, 72, 77 2)

Thermal expansion: P.14-

15, 16, 73 3)

Phase change, calorimetry:

P.14-32, 44, 49

2

1 1 2 3

3 E

xam

s an

d 2

More

3

1)D

EC 3 (Tue): PLAN

for Exam 4 – Chap. 12, 14 and 15

1)P.1 … 12

2)P.2 … 14

3)P.3 … 15

4)P.4 … Q

uick quizzes from 12, 14 and/or 15

2)D

EC 8 (Tue): Comm

on Makeup Test at 7 pm

3)

DEC 11 (Fri): Final Exam

– Comprehensive (all

chapters) – Please start reviewing Eaxm1~Exam

3 m

aterials now!

EEn

ergy an

d H

eat (in

Ch

ap. 7

) �

Energy is conserved. �

Kinetic Energy describes motion

and relates to the mass of the

object and it’s velocity squared. �

Energy on earth originates from

the sun. �

Energy on earth is stored therm

ally and chemically.

�Chem

ical energy is released by m

etabolism.

�Energy is stored as potential energy in object (through elastic deform

ation or in height and m

ass)

4

�Energy can be dissipated by heat and noise (m

otion transferred at the m

olecular level). This is referred to as dissipation.

[What’s N

ew Here?] M

echanical energy and heat are equivalent. Equivalent on what?

MMech

an

ical E

qu

ivalen

ce of H

eat

�Done

by Jam

es Joule

in the

1800’s.

�Potential

energy stored

in a

raised mass was used to pull a

cord wound on a rod mounted to

a paddle in a water bath.

�The

measured

temperature

change of the water proved the equivalence

of mechanical

energy and heat.

�The

unit for

potential energy,

kinetic energy, and heat is the Joule in honor of his work.

5

Tem

pera

ture

�Tem

perature is

an attem

pt to

measure the “hotness” or “coldness”

on a scale you devise.

�A

device to

do this

is called

a therm

ometer

and is

usually calibrated

by the

melting

and freezing points of a substance. This is m

ost often water with corrections for atm

ospheric pressure well known.

�The

thermom

eter is

often a

container filled

with a

substance that will expand or contract as heat flows in its surroundings.

66

Tem

pera

ture S

cales

�Based on the boiling and freezing points of water, two system

s developed to measure

temperature.

�In the U

nited States, the Fahrenheit scale was developed with boiling at 212

oF and freezing at 32

oF; In many other countries,

the Celsius (also called Centigrade) scale was developed with water freezing at 0

oC and boiling at 100

oC.

�Conversion: T

f =9/5Tc +32

o

77

CCalo

ries an

d J

ou

les �

The food calorie is properly noted as a kilo-calorie in SI units.

�A calorie is 4.184J. A

mount of heat required to raise

the temperature of 1 gram

of water from 14.5 C to

15.5 C. �

So, the Big Mac you’re about to eat will cost your diet

about three and a half million joules.

8

Work and Energy

Work

=H

eat =

Tra

nsfer o

f En

ergy

A tractor is doing w

ork on (or “energy transfer” to) a sled of firew

ood as it exerts the force over a displacement.

Engine

Energy Transfer

9

Chap. 16

We form

ulate this part.

W = F cos � x (D

istance)

blank page

110

H is H

eat T

ran

sfer (Hea

t Cu

rrent)

e.g., P.14-53, 14-56, 14-62 11

k = Thermal Conductivity

UUn

dersta

ndin

g Pro

blem

1

12

UUn

dersta

ndin

g Pro

blem

2

13

T

blank page

114

m

c

V

c

Hea

t Capacity (I)

�Substances have an ability to “hold heat” that goes to the atom

ic level.

Q

= m c �T

[J] = [kg] [?] [K]

�c = specific heat capacity [J / (kg * K)]

�cwater = 4.19 x 10

3 J/(kg*K) vs. ccopper = 0.39 x 10

3 J/(kg*K)

[Q] W

hat is c? How

effectively the

substance can

hold heat. [Q

] Why Q

is proportional to m?

The vibration of each atom is a reason for

holding heat. “Many atom

s” means holding

more heat.

[Q] W

hy Q is proportional to �T?

It depends on how much the heat transfer

( �T) is made.

15

HHea

t Capacity (II)

�Substances have an ability to “hold heat” that goes to the atom

ic level.

Q

= m c �T

[J] = [kg] [?] [K]

16

�c = specific heat capacity [J / (kg * K)]

�cwater = 4.19 x 10

3 J/(kg*K) vs. ccopper = 0.39 x 10

3 J/(kg*K) �

What we see in life? �

One of the best reasons to spray water on a fire is that it

suffocates com

bustion. But,

another reason

is that

water has

a huge

heat capacity.

Stated

differently, it

has im

mense therm

al inertia. In plain terms, it’s good at cooling

things off because it’s good at holding heat. �

Taking a

copper frying

pan off

the stove

with your

bare hands

is an

awful idea

because metals

have sm

all heat

capacity. In plain terms, m

etals give heat away as fast as they can.

�Exam

ples 14.6 and 14.7

Wh

at is h

appen

ing a

t A�

B ?

117

Wh

at D

o Y

ou

See b

�c a

nd d

�e?

118

Ph

ase C

han

ges (I) �

The snowflake needs to absorb the heat of fusion to becom

e a liquid. �

Put ice in water. You have a refreshing drink but also solid water and liquid water in equilibrium

.

119

Q/m

= Lf = 3.34 x 10

5 J/kg

e.g., P.14-32

Ph

ase C

han

ges (II) �

Which is worse to touch for a burn? 100°C water (at d)

or 100°C steam (at e) ? -- The steam

, because it also contains the energy (heat of vaporization) that it took to becom

e a gas. This is 2.3 MILLIO

N joules per kg of

water.

Q/m

= Lv = 2.26 x 10

6 J/kg Q

/m= L

f = 3.34 x 105 J/kg

20

UUn

dersta

ndin

g Pro

blem

32

21

Q/m

= Lv = 2.26 x 10

6 J/kg

Phase Change

Heat capacity

15�

100 oC UUn

dersta

ndin

g Pro

blem

32

22

Un

dersta

ndin

g Pro

blem

56

23

Q/m

= Lv = 2.26 x 10

6 J/kg Q

/m= L

f = 3.34 x 105 J/kg

e.g., P.14-32

Un

dersta

ndin

g Pro

blem

56

224

UUn

dersta

ndin

g Pro

blem

25

blank page

226

227

Un

dersta

ndin

g Pro

blem

74

74

228

Un

dersta

ndin

g Pro

blem

74

74

Th

ermal E

xpan

sion

���: The expansion is proportional to the original length and the tem

perature change (for reasonable �T). (Table 14.1)

29

��: Volum

e expansion (Table 14.2; Example 14.4)

Stress o

n a

Spacer

�Consider a alum

inum spacer

(L0 =10 cm

) at 17,2 oC. �

Thermal Expansion

�

Stress

�Therm

al Stress

�

Example: Road Expansion and

Contraction

330

Example 14.5 Pa

10

700

(alminum

)

K 10

42

(aluminum

)11

15

��

��

��

.Y

.�

UUn

dersta

ndin

g Pro

blem

At �78 oC

A

t � oC

Length (L) � diam

eter (d)

31

Thermal Stress

Thermal Expansion

An

Interestin

g Beh

avio

r �

Different m

aterials expand according to their coefficients of therm

al expansion. �

Refer to Table 14.1. �

Refer to Example 14.2

and Example 14.3.

332

Volu

me E

xpan

sion

of W

ater

�The graph at right is for the expansion of water from

0-10

oC. �

This is the property that allows the m

ercury to rise inside a therm

ometer.

333

Th

ermal E

qu

ilibriu

m

� If two objects are placed in contact and one has more heat energy than the other, heat will always

flow from hotter to colder. This will continue until

both objects are at the same tem

perature. This condition of stability is called therm

al equilibrium.

�W

hen heat flow is considered, some m

aterials like metals are good transm

itters of heat energy. We

term these m

aterials to be thermal conductors.

�M

aterials like styrofoam are poor conductors of heat

and will in fact, severely restrict heat flow (like the one described above). M

aterials that conduct of heat energy poorly are called insulators.

334

Th

e Zero

th L

aw

of T

herm

odyn

am

ics �

Systems A

, B, and C are not originally in therm

al equilibrium; A

, B, and C are insulated from

any external influence.

�In the top figure, A

and C will come

to equilibrium while at the sam

e time,

B and C will also. Eventually, all three – A

, B, and C will come to

equilibrium. (In the lower figure, only

A and B will com

e to equilibrium.)

A = C; B = C ��

A = B = C

�This is the essence of the Zeroth Law.

35

AAn

Abso

lute T

empera

ture

�Scientists

experimenting

with gases

noted a

linear behavior

between pressure

and tem

perature. Using

various gases,

the linear

plots were

all noted

to converge at the sam

e place (-273.15oC or 0 K). �

T(K) = T

oC + 273.15

�Nam

ed for it’s inventor, Lord

Kelvin (1827-

1907), the Kelvin scale took

this point

to be

the absolute zero of all tem

peratures, the point at which everything is a solid

and all

motion

ceases.

36

Calo

rimetery

�Problem

Solving Strategy 14.2.

�Surround a system

with a known amount of fluid

(therefore a known heat capacity). By measuring the

change in temperature you can solve for the heat

evolved by the system.

�Exam

ples 14.8 and 14.9

337

Meth

ods o

f movin

g hea

t energy

�Conduction – discussed earlier as a function of each given m

aterial. See Table 14.5 and examples

14.10-14.12. �

Convection – moving a heated fluid from

one place to another. O

ur real-life examples are heated air

from a furnace or heated water for a shower.

�Radiation – m

oving heat by electromagnetic

radiation. Infrared rays from a hot burner on a

stove can be felt by holding your hand over the burner. See exam

ples 14.13-14.14.

338