Chapter 11
Heat &
Temperature
Heat & Temperature
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Both words are used interchangeably in day to –day conversation, but they have different
scientific definitions.
Temperature is a state or a number that
decides the direction of heat flow.
Heat is the energy transferred from one
object to another because of a temperature
difference between them.
Thermometers

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
Used to measure the temperature of an
object or a system
Make use of physical properties that change
with temperature
Many physical properties can be used
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–
–
–
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Volume of a liquid
Length of a solid
Pressure of a gas held at constant volume
Volume of a gas held at constant pressure
Electric resistance of a conductor
Color of a very hot object
Thermometers, cont



A mercury thermometer
is an example of a
common thermometer
The level of the mercury
rises due to thermal
expansion
Temperature can be
defined by the height of
the mercury column
Temperature Scales

Thermometers can be calibrated by placing
them in thermal contact with an environment
that remains at constant temperature
–
–
Environment could be mixture of ice and water in
thermal equilibrium
Also commonly used is water and steam in
thermal equilibrium
Celsius Scale

Temperature of an ice-water mixture is
defined as 0º C
–

Temperature of a water-steam mixture is
defined as 100º C
–

This is the freezing point of water
This is the boiling point of water
Distance between these points is divided into
100 segments or degrees
Kelvin Scale

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When the pressure of a gas goes to zero, its
temperature is –273.15º C
This temperature is called absolute zero
This is the zero point of the Kelvin scale
–

–273.15º C = 0 K
To convert: TC = T – 273.15
–
The size of the degree in the Kelvin scale is the
same as the size of a Celsius degree
Fahrenheit Scales
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Most common scale used in the US
Temperature of the freezing point is 32º
Temperature of the boiling point is 212º
180 divisions between the points
Comparing Temperature Scales
Temperature Scales
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Fahrenheit  F
Celsius C
Kelvin K
In order to covert temperatures
TF  1.8TC  32
TK  TC  273
Questions And In class problems


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Convert the following Fahrenheit temp. to
Kelvin :
120⁰ F
-456 ⁰ F
At what temperature is the Celsius and
Fahrenheit value the same?
The Atomic Basis of Temperature
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Temperature is related to the random motion
of atoms and molecules in a substance.
Temperature is actually a measure of the
magnitude of the average speed of atoms
and molecules.
Temperature is directly proportional with
average translational kinetic energy of
random molecular motion.
The Atomic Basis of Temperature

Molecules may also rotate or vibrate, with
associated rotational or vibrational kinetic
energy but these motions are not
translational and don’t define temperature.
Check Point :
 True
or False ?
 Temperature is a measure
of the total kinetic energy in
a substance.
Answer:


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False, Temperature is a measure of the
average translational kinetic energy of
molecules in a substance.
If you have 1L of boiling water A
2L of boiling water B
Total kinetic energy in B=2 total kinetic
energy in A
They have the same temperature.
Thermal Expansion

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The length of a solid changes as
temperature changes.
The change in length is proportional to the
change in temperature.
L2  L1 (1  T )
L2 the length of the material at temp. T2
L1 the length of the material at temp. T1
 Coefficient of Linear Expansion
Example


The coefficient of expansion for steel is
0.000056 per degree c . A particular bridge is
100 meters long when the temp. is 0 ⁰ C
How long will that bridge be in the summer when
the temp. climbs to 40 ⁰ C ?
Solution
L2  L1 (1  T )
L2  100m  1  0.000056C  40C 
L2  100.22m
Application

In times past , railroad
tracks were laid in 39foot segments
connected by joint bars,
with gaps for thermal
expansion. In summer
months, the tracks
expanded and the gaps
were narrow. In winter,
the gap widened.
Application

Even in routine daily activities such as
cooking, thermal stress is often is a serious
consideration. Pyrex glass, for instance, is
now often used to make dishes that can be
placed directly into ovens because this
material expands only about 1/3 as much as
regular glass. It is therefore much less likely
to crack when subjected to thermal stress
inside the oven.
Applications of Thermal
Expansion – Bimetallic Strip

Thermostats
–
–
Use a bimetallic strip
Two metals expand differently

Since they have different coefficients of expansion
Application : Bimetal strip
More Applications of Thermal
Expansion

Pyrex Glass
–

Thermal stresses are smaller than for ordinary
glass
Sea levels
–
Warming the oceans will increase the volume of
the oceans
Unusual Behavior of Water
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Most liquids have a quite simple behavior when they are cooled
(at a fixed pressure): they shrink. The liquid contracts as it is
cooled; because the molecules are moving slower they are less
able to overcome the attractive intermolecular forces drawing
them closer to each other. Then the freezing temperature is
reached, and the substance solidifies, which causes it to
contract some more because crystalline solids are usually
tightly packed.
Unusual Behavior of Water


Water is one of the few exceptions to this behavior. When liquid
water is cooled, it contracts like one would expect until a
temperature of approximately 4 degrees Celsius is reached.
After that, it expands slightly until it reaches the freezing point,
and then when it freezes it expands by approximately 9%.
This unusual behavior has its origin in the structure of the water
molecule. There is a strong tendency to form a network of
hydrogen bonds, where each hydrogen atom is in a line
between two oxygen atoms. This hydrogen bonding tendency
gets stronger as the temperature gets lower (because there is
less thermal energy to shake the hydrogen bonds out of
position). The ice structure is completely hydrogen bonded, and
these bonds force the crystalline structure to be very "open", as
shown in the following picture:
Unusual Behavior of Water
It is this open solid structure that causes ice to be less
dense than liquid water. That is why ice floats on water,
for which we should all be thankful because if water
behaved "normally" many bodies of water would freeze
solid in the winter, killing all the life within them.
Water's "density maximum" is a product of the same
phenomenon. Close to the freezing point, the water
molecules start to arrange locally into ice-like structures.
This creates some "openness" in the liquid water, which
tends to decrease its density. This is opposed by the
normal tendency for cooling to increase the density; it is
at approximately 4 degrees Celsius that these opposing
tendencies are balanced, producing the density
maximum.
Unusual Behavior of Water
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As the temperature of water increases from 0ºC to 4
ºC, it contracts and its density increases
Above 4 ºC, water exhibits the expected expansion
with increasing temperature
Maximum density of water is 1000 kg/m3 at 4 ºC
Heat & Internal Energy
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1.
2.
3.
4.
Matter does not contain heat.
Heat is energy transferred when there is a difference in
temperature.
Internal energy is the grand total of energies inside the
substance;
Translational Kinetic Energy.
Rotational Kinetic Energy.
Kinetic Energy due to internal movement of atoms within
molecules.
Potential energy due to the forces between molecules.
Heat & Internal Energy


When a substance absorbs or gives off heat,
the internal energy of the substance
increases or decreases.
When ice is melting, the added heat does not
increase molecular kinetic energy, therefore
no increase in temperature.
Heat & Internal Energy
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For two things in thermal contact, heat flows
from the higher temperature substances to
the lower temperature substances.
This is not necessarily a flow from more
internal to less internal
How much heat flows depends on
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Temperature difference ∆T.
Amount of material m.
The substances used.
Q  m  c  T
Heat Units
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Jules(J)
Calorie (c )(small c) :Amount of heat required
to change the temperature of 1 gram of water
by 1 Celsius degree.1 calorie=4.184 J
Calorie (kilocalorie in fact)( C) energy rating :
amount of heat required to change
temperature of 1 kg of water by 1 Celsius
degree.
Heat in Customary units (USA)

The British thermal unit (BTU or Btu) is a
traditional unit of energy equal to about 1.06
kilojoules. It is approximately the amount of
energy needed to heat one pound of water
one degree Fahrenheit. It is used in the
power, steam generation, heating and air
conditioning industries.

A BTU is the amount of energy necessary to raise
the temperature of 1 lb of water from 63° F to 64° F

1 btu = 1 055.05585 joules
Check Point

An iron thumbtack and a big bolt are
removed from a hot oven . Both are red-hot
and have the same temperature .When
dropped into identical containers of water of
equal temperature .Which one raises the
water temperature more?
Check your answer!

The big iron bolt has more internal energy to
impart to the water and warms it more than
the thumbtack . Although they have the same
initial temperature (the same average kinetic
energy per molecule), the more massive bolt
has more molecules and therefore more total
energy-internal energy . This example
underscores the difference between
temperature and internal energy.
Specific Heat
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
Every substance requires a unique amount
of energy per unit mass to change the
temperature of that substance by 1° C
The specific heat, c, of a substance is a
measure of this amount
Q
c
m T
Units of Specific Heat
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SI units
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J / kg °C
Historical units
–
cal / g °C
Heat and Specific Heat
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Q = m c ΔT
ΔT is always the final temperature minus the
initial temperature
When the temperature increases, ΔT and ΔQ
are considered to be positive and energy
flows into the system
When the temperature decreases, ΔT and
ΔQ are considered to be negative and energy
flows out of the system
A Consequence of Different
Specific Heats
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Water has a high
specific heat
compared to land
On a hot day, the air
above the land
warms faster
The warmer air
flows upward and
cooler air moves
toward the beach
Calorimeter
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
One technique for determining the specific
heat of a substance
A calorimeter is a vessel that is a good
insulator which allows a thermal equilibrium
to be achieved between substances without
any energy loss to the environment
Calorimetry
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Analysis performed using a calorimeter
Conservation of energy applies to the
isolated system
The energy that leaves the warmer
substance equals the energy that enters the
water
–
–
Qcold = -Qhot
Negative sign keeps consistency in the sign
convention of ΔT
Phase Changes
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A phase change occurs when the physical
characteristics of the substance change from
one form to another
Common phases changes are
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Solid to liquid – melting
Liquid to gas – Evaporation
Phases changes involve a change in the
internal energy, but no change in temperature
Latent Heat
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During a phase change, the amount of heat is
given as
–

L is the latent heat of the substance
–
–

Q = ±m L
Latent means hidden
L depends on the substance and the nature of the
phase change
Choose a positive sign if you are adding
energy to the system and a negative sign if
energy is being removed from the system
Latent Heat, cont.
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SI units of latent heat are J / kg
Latent heat of fusion, Lf, is used for melting
or freezing
Latent heat of vaporization, Lv, is used for
boiling or condensing
Sublimation

Some substances will go directly from solid
to gaseous phase
–
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Without passing through the liquid phase
This process is called sublimation
–
There will be a latent heat of sublimation
associated with this phase change
Graph of Ice to Steam
Conduction
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The transfer can be viewed on an atomic
scale
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–
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It is an exchange of energy between microscopic
particles by collisions
Less energetic particles gain energy during
collisions with more energetic particles
Rate of conduction depends upon the
characteristics of the substance
Conduction example
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The molecules vibrate about
their equilibrium positions
Particles near the stove coil
vibrate with larger
amplitudes
These collide with adjacent
molecules and transfer some
energy
Eventually, the energy
travels entirely through the
pan and its handle
Conduction, cont.

In general, metals are good conductors
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–
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They contain large numbers of electrons that are
relatively free to move through the metal
They can transport energy from one region to
another
Conduction can occur only if there is a
difference in temperature between two parts
of the conducting medium
Convection

Energy transferred by the movement of a
substance
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–
When the movement results from differences in
density, it is called natural conduction
When the movement is forced by a fan or a pump,
it is called forced convection
Convection example

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Air directly above
the flame is warmed
and expands
The density of the
air decreases, and it
rises
The mass of air
warms the hand as
it moves by
Convection applications
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Boiling water
Radiators
Upwelling
Cooling automobile engines
Algal blooms in ponds and lakes
Radiation
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Radiation does not require physical contact
All objects radiate energy continuously in the
form of electromagnetic waves due to
thermal vibrations of the molecules
Radiation example
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The electromagnetic waves carry the energy
from the fire to the hands
No physical contact is necessary
Cannot be accounted for by conduction or
convection
Ideal Absorbers
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An ideal absorber is defined as an object that
absorbs all of the energy incident on it
This type of object is called a black body
An ideal absorber is also an ideal radiator of
energy
Ideal Reflector

An ideal reflector absorbs none of the energy
incident on it
Applications of Radiation

Clothing
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Thermography
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Black fabric acts as a good absorber
White fabric is a better reflector
The amount of energy radiated by an object can
be measured with a thermograph
Body temperature
–
Radiation thermometer measures the intensity of
the infrared radiation from the eardrum
Resisting Energy Transfer
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Dewar flask/thermos bottle
Designed to minimize energy
transfer to surroundings
Space between walls is
evacuated to minimize
conduction and convection
Silvered surface minimizes
radiation
Neck size is reduced
Global Warming

Greenhouse example
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–

Visible light is absorbed and re-emitted as
infrared radiation
Convection currents are inhibited by the glass
Earth’s atmosphere is also a good
transmitter of visible light and a good
absorber of infrared radiation