Thermal Energy, Work and Heat

1
Thermal Energy, Work and Heat In the energy
economy which we are studying, energy flows
through the society changing form as it
goes. The general law of conservation of
energy tells us that none of this energy ever
disappears but it does change form. These
changes of form are what the energy technologies
are doing. In the process, they create
convenience and power labor saving devices, cost
you money and (often) create environmental
problems.
2
The forms of energy introduced so far
were kinetic energy, gravitational potential
energy and thermal energy. Other forms
which will come later include chemical energy,
electrical energy and nuclear energy. You can
see how these enter in the energy flows through
the society by looking again at Figure 1.9 of
your book.
3
(No Transcript)
4
From the input side About 80 of all the
energy input to the system comes from fossil
fuels. Essentially ALL of the fossil fuels
are burned to convert the chemical energy , which
is stored mainly in the carbon-carbon bonds of
the hydrocarbons in the fuels, into thermal
energy, which is then converted into kinetic
energy in some form of engine or turbine OR it is
used directly to keep buildings warm in cold
climates. C O2 -gt CO2 thermal energy
5
Thus the role of thermal energy in the system is
a major one. From a policy and economic this
intensive use of energy gleaned from burning
fossil fuels to get thermal energy has at least
the following negative consequences It
creates a huge amount of CO2 which is
altering the climate. (We will study this
later.) It is using a finite resource which will
run out. (Discussed earlier.) It is
intrinsically inefficient, because of the second
law of thermodynamics. (We will discuss this
shortly.)
6
Thermal energy is the kinetic and potential
energy associated with the random motion of the
atoms inside a material that is heated. All
ordinary matter is made of atoms. (In
extraordinary environments, such as the interior
of the sun, the atoms are broken up into
constituent parts.) Atoms have approximate size
10-10 meters and cannot be seen with our eyes. A
kind of potential energy (of electromagnetic origi
n) associated with the forces between atoms is
involved.
7
The most direct way to see that thermal energy
is a form of energy is to make a direct transfer
of energy from one body to another by rubbing one
body against another in the presence of friction
forces at the interface between the two bodies.
Then the first body exerts a force on the second
body and does work on it. Some or all of the
work done is transferred to the surface of the
second body in the form of thermal energy. We are
seeing three examples of this so far 1) the
experiment you did last week in the laboratory 2)
the skier in the problem for homework 3 and 3)
the block being pushed across the table as in the
next slide.
8
thermometer
Fext
Ff
d
1. The block is pushed with force Fext which
could, for example come from my hand. A
frictional force Ff acts to the left and exactly
balances the external force so that the block
moves at steady speed. Fext does work Fextd.
Where does the energy transferred go? a. Kinetic
energy of the block. b. Gravitational energy of
the block c. Thermal energy d. Kinetic energy of
the surface over which block is pushed
9
Answer c. There is nowhere else for the energy
to go because neither the gravitational potential
nor the kinetic energy of the block are
changing. This is also consistent with the fact
that the action of frictional forces always
results in the production of thermal energy.
10
By analysis of many such experiments, one finds
that in most cases, the rise in temperature is
proportional to the work done and inversely
proportional to the mass of the object heated
(here the block in the lab experiment, the
water) Thermal energy gain Constant x
(temperature rise)x (mass of object
gaining
thermal
energy)
11
The constant is called the specific heat of the
material in the object gaining thermal energy so
that Thermal energy increase Specific heat x
mass x temperature rise The specific heat is
different for different materials
CAUTION This equation is only true if
the material to which thermal energy is being
added is not condensing,boiling, freezing or
melting. We will return to this.
12
(No Transcript)
13

• 2. Notice that water has the largest specific
  heat
• of the common substances. Which of the
• following is a practical consequence of
• that fact
• water tends to get hot easily
• water is a good coolant
• Water boils easily
• Water melts easily

14
Answer c. A large specific heat means that water
can absorb a lot of thermal energy with a small
temperature rise. If water is put in contact
with a material of smaller specific heat which is
hotter, then because the specific heat of water
is large it can absorb thermal energy from the
hot object without itself experiencing a very
large increase in temperature.
15
We have seen one way to add thermal energy to an
object or material by rubbing another object
along its surface. Another way to add thermal
energy to an object or material is to put the
object or material into contact with another
object or material which is at higher
temperature. This is what you are doing when you
heat an object on a stove. The hotter burning
gas is being put in contact with the cooler pan,
and thermal energy passes into the pan from the
hot gas. General rule If two objects at
different temperature are put into contact so
that energy can flow between them, then energy
will flow from the hotter one to the cooler one
until the two objects are at the same
temperature.
16

• 3.A can containing 500gm of water at 100 o C is
• put in contact with another can containing
• 500 gm of water at 30 o C and the two touching
• cans are wrapped in insulation so that no
• thermal energy can escape. What is the final
• temperature of the water in the cans?
• (Ignore any thermal effects of the cans
  themselves)
• 100 C
• 70 C
• 65 C
• 30 C

17
Answer c. Let the final temperature be denoted
by the symbol T in order to set up an equation
for it. Let the specific heat of water be
Cwater. The thermal energy which flows out of the
hot water is .5kgxCwater(100-T) The thermal
energy flowing into the cold water is
.5kgCwater(T-30) Conservation of energy requires
that these be the same .5kgxCwater(100-T)
.5kgCwater(T-30) Solve for T (Cwater cancels
out) T(10030)/265 C
18

• 4, A can containing 500gm of water at 100 o C is
• put in contact with another can containing
• 1000 gm of water at 30 o C and the two touching
• cans are wrapped in insulation so that no
• thermal energy can escape. What is the final
• temperature of the water and in the cans?
• (Ignore any thermal effects of the cans
  themselves)
• 130/1.586.7 C
• 115/1.576.7 C
• 80/1.553.3 C
• 130/2 65 C

19
Answer b. The reasoning is the same as in the
preceding example but the mass of hot water
is twice as much, giving the equation
.5kgxCwater(100-T) 1.0kgCwater(T-30)
Solving for T (1.5)T0.5x10030 T80/1.5 C
20
5.A can containing 500gm of water at 100 o C put
in contact with another can containing 1000 gm of
water at 30 o C and the two touching cans are
wrapped in insulation so that no thermal energy
can escape. The final temperature was 53.3 C.
How much energy was transferred from the Hotter
to the cooler can? Specifiic heat of water 4186
J/Ckg. a 4186(53.3-30)97659.38
J b.4186(100-30)293,020J c.4186(53.3-30)(.5)4876
6.9 J d.4186(100-30)(.5)146,510J
21
Answer a. QCwaterx1.kgx (53.3C-30C)
22
6.The rooms in a 10 room house are all the
same size. One room is closed off (say a well
insulated door is closed) and the heat is shut
off in that room so that its temperature drops to
10 C while the rest of the house stays at 30 C.
Now assuming that the house is well insulated in
its outside walls, how much does the temperature
drop in the house if the door to the 10th room is
opened, assuming that no heat is coming in
through a furnace or other heating system? a. 2.0
C b. 10 C c. 20 C d. 1 C
23
Answer a. Let the specific heat of air be Cair
and the mass of air in one room be M and The
final temperature T The thermal energy lost by
the 9 warm rooms is 9xMxCairx(30-T) The energy
gained by the cold room is 1xMxCair(T-10) These
must be the same (conservation of
energy) 9xMxCairx(30-T)1xMxCair(T-10) (M and
Cair cancel) 9(30-T)(T-10) Solve for T
T(27010)/1028, 30-282C drop

24

• 7.An aluminum can of mass 500gm of water at 30 o
  C is
• filled with 1000 gm of water at 100 o C and the
• cans is wrapped in insulation so that no
• thermal energy can escape. What is the final
• temperature of the water and the can?
• Specific heat of water 4186J/Ckg
• Specific heat of aluminum 900J/Ckg
• (30100(4186/900))/(1(4186/900))87.6 C
• (30100)/(1(4186/450))12.6 C
• (30100(4186/450))/(1(4186/450))93.2 C
• (30100)/(1(4186/900))23.0 C

25
Answer c. Energy gained by aluminum .5kgx 900
J/kg x(T-30) Energy lost by water 1.kgx4186J/kg(1
00-T) 450(T-30)4186(100-T) T(4504186)30x4504
186x100 T 3094186/450)100 C
1(4186/450)
26
Things to notice about these problems of thermal
energy transfers between bodies at different
temperatures. The final temperature always ends
up between the two initial temperatures. The
more massive bodys temperature changes least,
other things being equal. The body with the
highest specific heat changes least, other things
being equal. You never get all the thermal
energy out of the hotter body. The closer the
starting temperatures, the less you get.
27

• 8.If you turn off the heating system and leave
• your house for a few weeks in the winter when the
  average
• temperature outside is -10 C, what will the
  average
• temperature of the interior of your house be
  while you are
• gone.
• Close to -10 C because there is more mass
• in the outside air than in the inside air.
• b. Close to the temperature at which you left it
• because the cold cant get in.
• c. Half way between the outside and inside
  temperature
• when you left.
• d. It depends on the specific heat of air.

28
Answer a. Say the mass of air in the house is m
and the mass of air outside the house Is M. Then
the balance equation is MCair(T-Toutside)mCair(T
inside-T) So (T-Toutside)(m/M)(Tinside-T) But
M is huge compared to m so Approximately T
Toutside0, TToutside
29
Summary There are two ways to transfer thermal
energy a material. Mechanically, by doing work
through a friction force By transfer from
another body at a higher temperature. in the
second case, we call the process heat transfer
and the amount of thermal energy transferred is
called the amount of heat transferred. We only
refer to heat when it is an amount of energy
transferred between bodies. We DO NOT refer to
the amount of heat in a body.
30
(No Transcript)
31
Boiling/condensation and melting/freezing. The
se are called phase changes. During these
phase changes thermal energy Is added or taken
away from the material but THE TEMPERATURE DOES
NOT CHANGE. We cannot therefore characterize
the behavior by a specific heat. The amount of
thermal energy added per kilogram to a material
during melting, for example is called the HEAT
OF FUSION. For boiling the thermal energy
required per Gram is called the HEAT OF
VAPORIZATION
32
Another energy unit, the kilocalorie In the
nineteenth century people did not understand that
what flowed from a hotter body to a colder one
was actually energy, so they used a different
unit for it called the calorie. The calorie is an
energy unit. 1 calorie4.186 joules 1
kilocalorie 1000calories4186 joules The
calories which are used to measure
the (chemical)energy content of food are actually
kilocalories
33

• 9. Use the figure to estimate the latent heat of
• fusion of ice.
• 200 kilocalories/kg
• 80 kilocalories/kg
• 640 kilocalories/kg
• 100 kilocalories/kg

34
Answer b or d. In fact the heat of fusion of Ice
is about 80J/kg
35
Temperature units and scales. There are two
metric temperature scales. They both use the
degree centigrade but they define the zero of
temperature differently. Temperature in
centigrade Temperature in kelvin -273 The
British system uses Fahrenheit degrees Temperatur
e in Fahrenheit (9/5)(temperature in centigrade)
32
36
In the centigrade system, water freezes at zero
degrees centigrade and boils (near the earths
surface) at 100 degrees centigrade The Kelvin
scale is designed so that the temperatures are
always positive. Experiment shows that no
temperature lower than -273 degrees Centigrade
0 degrees kelvin is possible, That temperature
is called the absolute zero of temperature.
(Strictly speaking there are some more decimals
after 273.)
37
Table 4-1, p. 101