C 16- T E H

Mechanics

Oct 27, 2013 (4 years and 7 months ago)

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C
H

16
-

T
HERMAL

E
NERGY

AND

H
EAT

1

S
ECTION

16.1: T
HERMAL

E
NERGY

AND

M
ATTER

Heat
is the transfer of thermal energy from
one object to another because of a
temperature difference.

Heat flows spontaneously from hot
objects to cold objects.

Temperature is related to the average
kinetic energy of the particles in an object
due to their random motions through
space.

2

Temperature
is a measure of how hot or cold an
object is compared to a reference point.

On the Celsius scale, the reference points
are the freezing and boiling points of
water.

On the Kelvin scale,
absolute zero

is
defined as a temperature of 0
kelvins
.

As an object heats up, its particles
move faster, on average. The average
kinetic energy of the particles
increases.

3

W
HAT

TWO

VARIABLES

IS

THERMAL

ENERGY

RELATED

TO
?

Thermal energy is the total potential and
kinetic energy of all the particles in an object.

Thermal energy depends on the mass,
temperature, and phase (solid, liquid, or gas)
of an object.

4

W
HAT

CAUSES

THERMAL

EXPANSION
?

Thermal expansion
is an increase in the volume of a
material due to a temperature increase.

Thermal expansion occurs when particles of matter move
farther apart as temperature increases.

Ex. If you take a balloon outside on a cold winter day,
it shrinks in a process of thermal contraction.

As temperature decreases, the particles that make up
the air inside the balloon move more slowly, on average.

Slower particles collide less often and exert less force.

Gas pressure decreases and the balloon contracts.

In general, gases expand more than liquids and liquids
expand more than solids.

5

H
OW

IS

A

CHANGE

IN

TEMPERATURE

RELATED

TO

SPECIFIC

HEAT
?

Specific heat
is the amount of heat needed to
raise the temperature of one gram of a material
by one degree Celsius.

The lower a material’s specific heat, the more its
temperature rises when a given amount of
energy is absorbed by a given mass.

Ex.
When a car is heated by the sun, the temperature
of the metal door increases more than the
temperature of the plastic bumper.

The iron in the door has a lower specific heat than the
plastic in the bumper.

6

7

C
ALCULATING

S
PECIFIC

H
EAT

In this formula, heat is in joules, mass is in
grams, specific heat is in J/g•
°
C, and the
temperature change is in degrees Celsius.

8

C
ALCULATING

S
PECIFIC

H
EAT

An iron skillet has a mass of 500.0
grams. The specific heat of iron is
0.449 J/g•
°
C. How much heat must
be absorbed to raise the skillet’s
temperature by 95.0
°
C?

9

W
HAT

INFORMATION

ARE

YOU

GIVEN
?

10

P
LAN

AND

SOLVE

What unknown are you trying to calculate?

What formula contains the given quantities and the
unknown?

11

R
EPLACE

EACH

VARIABLE

WITH

ITS

KNOWN

VALUE

12

S
AMPLE

PROBLEM

How much heat is needed to raise the
temperature of 100.0 g of water by 85.0
°
C?
(remember, the specific heat of water is
4.18
J/g•
°
C )

Answer:

Q = m * c * ∆T

= (100.0 g)(4.18 J/g•
°
C)(85.0
°
C)

= 35.5 kJ

13

O
N

WHAT

PRINCIPLE

DOES

A

CALORIMETER

OPERATE
?

A
calorimeter

is an
instrument used to measure
changes in thermal energy.

A calorimeter is used to
measure specific heat. A
sample is heated and placed
in the calorimeter. The
temperature change is
observed.

14

S
ECTION

16.2: H
EAT

AND

T
HERMODYNAMICS

Conduction, Convection
and Radiation

15

C
ONDUCTION

Conduction occurs within a material or between
materials that are touching.

In conduction, collisions between particles transfer
thermal energy, without any overall transfer of
matter.

Conduction
is the transfer of thermal energy with
no overall transfer of matter.

Conduction in gases is slower than in liquids and
solids because the particles in a gas collide less
often.

16

T
HERMAL

C
ONDUCTORS

A
thermal conductor
is a material that conducts
thermal energy well.
Metals are good thermal
conductors.

The red arrows
show how
thermal energy
is conducted
away from the
heat source in a
metal frying pan.

17

T
HERMAL

I
NSULATORS

A material that
conducts thermal
energy poorly is called
a
thermal insulator.

Air is a very good
insulator. Wool
garments and plastic
foam cups use trapped
air to slow down
conduction.

18

C
ONVECTION

Convection
is the transfer of
thermal energy when particles of a
fluid (including gases like air!) move
from one place to another.

Convection currents are important
in many natural cycles, such as
ocean currents, weather systems,
and movements of hot rock in
Earth’s interior.

19

C
ONVECTION

A
convection current
occurs when a fluid
(including gases like air!) circulates in a loop
as it alternately heats up and cools down.

Air at the bottom of an oven heats up,
expands, and becomes less dense. The hot air
rises.

Rising hot air cools as it moves away from the
heat source.

As a result, the coolest air is at the top of the
oven.

20

R
ADIATION

Radiation
is the transfer of energy
by waves moving through space.

All objects radiate energy. As an
object’s temperature increases, the
rate at which it radiates energy
increases.

21

R
ADIATION

When you stand to the side of a
charcoal grill, heat reaches you
without convection or conduction.

The sun warms you by radiation on
a clear day. The space between the
sun and Earth has no air to transfer
thermal energy.

Heat lamps used in restaurants are
another example of radiation.

22

T
HERMODYNAMICS

The study of conversions
between thermal energy and
other forms of energy is called
thermodynamics.

There are three laws of
thermodynamics…

23

F
IRST

L
AW

OF

T
HERMODYNAMICS
:

The first law of
thermodynamics states that
energy is conserved.

E
nergy cannot be created or destroyed, but it
can be converted into different forms.

Added energy increases the thermal energy of
a system or does work on the system. In
either case, energy is conserved.

24

S
ECOND

L
AW

OF

T
HERMODYNAMICS
:

The second law of thermodynamics states that
thermal energy can flow from colder objects to
hotter objects only if work is done on the
system.

Thermal energy flows spontaneously only
from hotter to colder objects.

A refrigerator must do work to transfer
thermal energy from the cold food
compartment to the warm room air.

The thermal energy is released by coils at
the bottom or in the back of the refrigerator.

25

E
NTROPY

Spontaneous changes will always
make a system less orderly, unless
work is done on the system.

Disorder in the universe as a whole is
always increasing.

Entropy is the measure of the
disorder of a system.

26

T
HIRD

L
AW

OF

T
HERMODYNAMICS

The third law of
thermodynamics
states that absolute
zero cannot be
reached.

27

S
ECTION

16.3: U
SING

H
EAT

The two main types of heat engines are
the external combustion engine and the
internal combustion engine.

A steam engine is an
external combustion
engine

an engine that burns fuel outside
the engine

An
internal combustion engine
is a heat
engine in which the fuel burns inside the
engine. Most cars have internal combustion
engines.

28

H
EAT

E
NGINES

In an internal combustion engine,
the cooling system and exhaust
transfer heat from the engine to the
environment.

Gasoline engines are more efficient
than old
-
fashioned steam engines,
but they still are not very efficient.
About one third of the energy in a
gasoline engine is converted to
work.

29

H
OME

H
EATING

Most heating systems use
convection

to distribute
thermal energy.

A
central heating system
heats many rooms
from one central location.

The most commonly used energy sources for
central heating systems are electrical energy,
natural gas, oil, and coal.

Heating systems differ in how they transfer
thermal energy to the rest of the building.

Different methods of home heating are
available. Some more efficient than others!

30