• To identify temperature as the amount of kinetic energy of atoms and
molecules.
• To identify different temperature scales and convert from one scale to
another.
• To describe heat as the transfer of energy between substances at different
temperature.
• To identify the three methods of heat transfer.
• To apply the principle of energy conservation to calculate changes in
potential, kinetic and internal energy.
• To identify latent and specific heat of substances.
• To calculate phase changes of different substances.
• To describe thermal expansion.
In every day life, temperature is the measure of how hot or cold something is.
Many properties of matter change with it. For example, most materials expand
when heated.
To
measure
temperature
quantitatively we use numerical
scales. The most common scale
today is Celsius scale. In USA the
Fahrenheit scale is also common.
And the most important scale for
scientific work is the absolute, or
Kelvin, scale.
The conversion between the two
temperature scales Celsius and
Fahrenheit can be written as:
𝑻 °𝑪 =
𝟓
𝟗
𝑻 ℉ − 𝟑𝟐 𝐨𝐫 𝑻 ℉ = 𝑻 ℃ + 𝟑𝟐
𝟗
𝟓
Robert Boyle (1627-1691) established a relation known as Boyle’s Law
that states that the volume of a gas is inversely proportional to the absolute
pressure applied to it when the temperature is kept constant.
Temperature also affects the volume of a gas. Jacques Charles (17461823) found that when the pressure is kept not too high and constant, the
volume of a gas increases with temperature at a nearly constant rate.
The graph of volume vs. temperature is a straight line that if projected to low
temperatures it crosses the axis at about -273̊C. Such a graph can be drawn
for any gas, and the straight line always projects back to -273̊C at zero
volume. At lower temperatures the gas would have a negative volume,
which makes no sense. It could be argued that -273̊C is the lowest
temperature possible. This temperature is called the ABSOLUTE ZERO of
temperature and is the basis of the Kelvin scale.
T( ̊ K) = T( ̊ C) + 273.15
Among the highest and lowest temperatures recorded are 136̊F in the Libyan
desert and -129̊F in Antarctica. What are these temperatures on the Celsius
scale?
15̊ below zero on Celsius scale is what Fahrenheit temperature?
15̊ below zero on Fahrenheit scale is what Celsius temperature?
Absolute zero is what temperature on the Fahrenheit scale?
Typical temperatures in the interior of the Earth and the Sun are about 4000̊C
and 15x106 ̊C respectively. What are this temperatures in kelvins?
If two objects at different temperatures are placed in thermal contact (meaning
thermal energy can transfer from one to the other) the two objects will eventually
reach the same temperature. They are then said to be in thermal equilibrium.
Amedeo Avogadro (1776-1856) stated that equal volumes of a gas at the
same pressure and temperature contain equal numbers of molecules. The
number of molecules in one mole of any pure substance is known as
Avogadro’s number 𝑵𝑨 = 𝟔. 𝟎𝟐𝒙𝟏𝟎𝟐𝟑 .
Using the Ideal Gas Law and the number of molecules N related to Avogadro’s
number we can get the following relation:
PV=NkT
where (P) Pressure, (V) Volume, (N) Number of molecules, (T) Temperature
and (k) is called Boltzmann’s constant.
The analysis of matter in terms of atoms in continuous random motion is called
the Kinetic Theory.
Comparing the equations for the average kinetic energy of the molecules in
the gas with the Ideal Gas Law we see the two agree if:
𝑲𝑬 =
𝟏
𝟑
𝒎𝒗𝟐 = 𝒌𝑻
𝟐
𝟐
This equation tells us that the average translational kinetic energy of the
molecules in random motion in an ideal gas is directly proportional to the
absolute temperature of the gas.
Heat is defined as the transferred energy from one object to another because
of a difference in temperature.
The common unit for heat is the calorie (cal) and is defined as the amount of
heat needed to raise the temperature of one gram of water by 1 Celsius
degree.
More often used than the calorie is the kilocalorie (kcal) equivalent to 1000
calories. It is also called a Calorie (With a capital C), and it is by this unit that
the energy value of food is specified.
In the British system of units the Btu (British Thermal Unit) is used and
corresponds to 0.252 kcal = 1055J.
James Prescott Joule (1818-1889)
and others performed a number of
experiments that were crucial for
establishing the view that heat, like
work, represents a transfer of energy.
Joule determined that a given amount
of work done was always equivalent to
a particular amount of heat input.
Quantitatively 4.186 joules of work
was found to be the equivalent to 1
calorie of heat. This is known as the
mechanical equivalent of heat.
Suppose you throw your caution to the wind and eat too much ice cream and
cake on the order of 500 Calories. To compensate, you want to do an
equivalent amount of work climbing stairs or a mountain. How much total
height must you climb?
Remember W = ΔPE = mgh
NOTE: The human body does
not transform energy with
100% efficiency it is more like
20% efficient. So you will
actually have to climb 20% of
the way (Which is still a lot)
Using the kinetic theory, we can make a clear distinction between
temperature, heat and internal energy (sometimes referred as thermal
energy).
Temperature (in kelvins) is a measure of the average kinetic energy of
individual molecules.
Internal Energy refers to the total energy of all the molecules in the object.
Heat refers to a transfer of energy from one object to another because of a
difference in temperature.
Material Needed:
Two identical glasses.
Hot Water.
Cold Water.
Liquid Food Coloring.
Fill your glasses. One should have hot water in it, the other cold water. Pick a
color of food coloring.
Put three drops of food coloring in each glass.
Share your observations
If heat flows into an object, the object
temperature rises (assuming no phase
change). But how much does the
temperature rise? It depends. The
amount of heat Q required to change
the temperature of a given material is
proportional to the mass and the
change in temperature.
Q = mcΔT
Where c is a quantity characteristic of
the material called its specific heat.
How much heat input is needed to raise the temperature of an empty 20 kg
vat made of iron from 10̊C to 90̊C? What if the vat is filled with 20 kg of
water?
When different parts of an isolated system are at different temperatures, heat
will flow from the part at higher temperature to the part at lower temperature,
that is within the system. If the system is isolated no energy is transferred
into or out of it.
So the conservation of energy again plays an important role. The heat lost by
one part of the system is equal to the heat gained by the other part.
Heat lost = Heat gained
Energy out of one part = energy into another part.
If 0.20kg of tea at 95̊C is poured into a 150g glass cup initially at 25̊C what will
be the common final temperature T of the tea and cup when equilibrium is
reached, assuming no heat flows to the surroundings.
Solution: 86̊C
* An automobile cooling system holds 16kg of water. How much heat does it
absorb if its temperature rises from 20̊C to 90̊C?
Solution: 4.7 ∗ 106 J
* A 35g glass thermometer reads 21.6̊C before it is placed in 135g of water.
When the water and the thermometer come to equilibrium, the thermometer
reads 39.2̊C. What was the original temperature of the water?
Solution: 40.1̊C
• The students are assigned in groups of three persons. The sorting method
will be by student ID number as per the assistance list.
• Each team will assign one member to be the captain of the team.
• A second member will be the spoke person and will present the results.
• The third member will be the closer and responsible for presenting the
group conclusions on the activity.
• The teams will have 25 minutes to complete the research.
• Then in 10 minutes the teams will present the work to the evaluators.
• And after that 5 minutes to present your conclusions.
The Students must construct a collage of images containing the three (3)
types of Heat Transfer
1. Conduction
2. Convection
3. Radiation
Clearly define in three sections of the collage these different types.
CATEGORY
Distinguished (4) Proficient (3)
Apprentice (2)
Novice (1)
Physic Concepts Show complete
understanding of
30%
the physics
concepts used.
Clearly
demonstrates
deep
understanding of
the lesson overall.
Show substantial
understanding of
the physics
concepts used.
Demonstrates
substantial
understanding of
the overall lesson.
Show partial
understanding of
the physics
concepts used.
Demonstrates
partial
understanding of
the overall lesson.
Show complete
lack or very
limited
understanding of
the underlying
concepts
needed OR are
not provided.
Demonstrates
complete lack of
understanding of
the overall lesson.
Real World
Models
50%
Real world
models shows
substantial
evidence of
creativity and
imagination.
Real world
models show
partial evidence
of creativity and
imagination.
Models are
messy, difficult to
understand, OR
missing. Real
world models
lack creativity OR
are not provided.
Real world
models indicate
strong evidence
of creativity and
imagination.
CATEGORY
Verbal and nonverbal
communication
20%
Distinguished (4)
Accomplishes all
of the following:
Good tone and
voice volume,
fluid speaking,
adequate body
stance, good use
of space and
resources.
Proficient (3)
Accomplishes
most of the
following: Good
tone and voice
volume, fluid
speaking, body
stance, good use
of space and
resources.
.
Apprentice (2)
Accomplishes just
three or two of
the following:
Good tone and
voice volume,
fluid speaking,
body stance,
good use of
space and
resources.
Novice (1)
Accomplishes
none or just one
of the following:
Good tone and
voice volume,
fluid speaking,
body stance,
good use of
space and
resources.
When a material changes phase from solid to liquid, or from liquid to gas, a
certain amount of energy is involved in this change of phase.
The heat required to change 1kg of a substance from solid to liquid state is
called HEAT OF FUSION; it is noted with 𝑳𝑭 .
The heat required to change from liquid to gas phase is called the HEAT OF
VAPORIZATION; it is noted with 𝑳𝑽 .
This are generally called LATENT HEATS and is defined as Q = mL. Where
m is the mass.
How much energy does a freezer have to remove from 1.5kg of water at 20 ̊ C
to make ice at -12 ̊ C.
Approach: We need to calculate the energy to reduce the water from 20 to 0
degrees, then to change it to ice, and then to reduce the ice from 0 to -12
degrees.
Solution: 660kJ.
How much heat is needed to melt 16.50kg of silver that is initially at 20 ̊ C?
Solution: 5.0 x 106 J
What mass of steam at 100 ̊ C must be added to 1 kg of ice at 0 ̊ C to yield
liquid water at 20 ̊ C?
Solution: 1.61 x 10−1 kg
The latent heat to change a liquid to a gas is needed not only at the boiling
point. Water can change from liquid to gas even at room temperature. This
process is called evaporation. The value of the heat of vaporization
increases slightly with a decrease in temperature. At 20 ̊ C for example it is
2450kJ/kg compared to 2260kJ/kg at 100 ̊ C.
When water evaporates, the remaining liquid cools because the energy
required to evaporate comes from the water itself, (Internal Energy) and
therefore the temperature must drop.
Evaporation of water from the skin is one
of the most important methods the body
uses to control temperature.
When a silver spoon is placed in a
hot bowl of soup, the end that you
hold becomes hot as well, even
though it is not directly in contact
with the source of heat. We say
the heat has been conducted
from the hot end to the cold end.
Heat conduction is carried via
molecular collisions.
Some materials are good heat
conductors, while others are bad
conductors and therefore are good
insulators.
Although liquids and gases are not
generally not very good conductors
of heat, they can transfer heat
quite rapidly by convection.
Convection
is
the
process
whereby heat flows by the mass
movement of molecules from one
place to another.
A forced air furnace, in which air is
heated and then blown by a fan
into a room is an example of
forced
convection.
Natural
convection occurs as well, one
familiar example is hot air rises.
Convection
and
Conduction
require the presence of matter as a
medium to carry heat. Radiation
occurs without any medium at all.
Radiation consists essentially of
electromagnetic waves.
The heat received from the sun is
the most common example of
radiation.
The warmth we receive from fire is
mainly radiant energy. Most of the
air heated by a fire in a fireplace
rises by convection up the chimney
and does not reach us.
Using the world wide web, research
how a thermos bottles work.
Specify how each type of heat
transfer is prevented to keep
temperature.
In groups of two, write a small report
with your findings
Watch the video and then answer the questions that follow.
http://www.youtube.com/watch?v=N354fChhjV0
1. What is the theory behind the myth of burying a six pack in the sand,
pouring gasoline over and set it on fire is supposed to cool the six pack ?
2. Why does Adam proposes checking various traditional cooling methods?
3. How is that the fire extinguisher cools the six pack?
4. Why do you think both Adam and Jamie devices use spiral forms?
5. In the end which way do you think is the most time-cost efficient method to
cool a six pack?
Most substances expand when heated and contract when cooled.
However the amount of expansion or contraction varies, depending on the
material.
Experiments indicate that the change in length of almost all solids is
approximately directly proportional to the change in temperature.
∆𝑳 = 𝜶𝑳𝒐 ∆𝑻
Where α is a proportionality constant called the coefficient of linear
expansion for the particular material.
The change in volume of a material which undergoes a temperature change is
given by a relation similar to linear expansion.
∆𝑽 = 𝜷𝑽𝒐 ∆𝑻
Where β is the coefficient of volume expansion.
Much of the energy we use in every day life makes use of a heat engine. Over
90% of the electric energy produced in the U.S. is generated at fossil-fuel
plants, where oil, coal or gas is burned to move a steam engine.
Thermal Pollution is the heat
produced by every heat engine and is
absorbed by the environment such as
rivers or lakes, or by the air using large
cooling towers.
In the case of cooling towers, the
output heat raises the temperature of
the atmosphere, which affects weather.
Chemicals released in the burning of fossil fuels gives rise to smog and other
problems.
One big problem is the buildup of Carbon Dioxide in Earth’s atmosphere.
Carbon Dioxide absorbs infrared radiation that the Earth naturally emits,
causing Global Warming, a serious problem that can be addressed by
limiting the burning of fossil fuels.
Thermal pollution however is unavoidable. Engineers can try to design and
build more efficient engines. But nature imposes a limit to efficiency defined
in the second law of thermodynamics that says 100% efficiency is not
possible to achieve.
What we can do is use less energy and conserve our fuel resources.
• Giancoli , Douglas C. Physics Sixth Edition. USA Pearson 2005
• Serway, Raymond A. Essentials of College Physics. USA Thomson 2007