Capter 10

Thermal Energy
 A burning candle
1

 A measure
of the
average
kinetic
energy of
molecule
2

Why is it colder in the
night than in the day?
 The sun is the
greatest heat
source.
 As the sun
comes up, it
warms earth.
 As the sun goes
down, the heat is
taken away and
it cools off.
3

What will happen to this cold
snowman throughout the day
as the sun warms it up?
4

What would happen to the
temperature of the boiling water
in this kettle if I added ice cubes?
5

How is the change in
temperature of the snowman and
the boiling water related?
 The temperature of
both the snowman
and the boiling
water changed to a
temperature that
was not really cold
or really hot, but
rather somewhere
in between.
6

“Temperature”
How hot or cold an object is.
Measured in degrees Celsius.
Temperature and heat are
NOT THE SAME.
It’s a measure of the average
kinetic energy of an individual
molecule.
7

Heat
The amount of thermal energy an
object has. It’s measured in joules or J.
A cup of hot tea has
heat energy , it is due
to the kinetic energy
(internal energy) of its
particles.
8

The small beaker of water
boils first
The large beaker
contains more water
molecules so it
needs more thermal
energy (heat) to
reach the same
temperature (100°C)
as the small one.
9

A swimming pool at 30 C is
at a lower temperature than
a cup of tea at 100 C.
BUT
the swimming pool
contains more water,
so it stores more
thermal energy or
heat.
10

Thermometers
 A thermometer (in Greek: thermos
means "hot" and metron “meansure")
 It’s a device that measures temperature
using a physical Principle.
 The Galilean thermometer:
The first thermometer (1953) was
really a thermoscope. It is not called
a thermometer, because the scale was
arbitrary. The egg-sized globe at the
top is the sensor. The gas within it
expands or contracts, and the liquid
level rises and falls.
http://www.youtube.com/watch?v=W_xc-6662f8 11

Types of Thermometer
1- Contact thermometers:
(1) Liquid-in-glass thermometers.
(2) Electric thermometers:
a) Thermistors.
b) Thermocouples.
(3) Liquid crystal thermometers.
*2- Non-contact thermometers:
Infrared (IR) thermometers.
12

1. Liquid-in-glass thermometer
 Liquid thermometers have been around
for almost 300 years.
 These rely on the expansion of a liquid
(Mercury or Alcohol) with temperature.
13

Liquid-in-glass thermometer
 The liquid is contained in a sealed
glass bulb and it expands into the
fine bore in the thermometer stem.
 Temperature is read using a scale etched
along the stem.
 The relationship between the temperature
and the column's height is linear over the
small temperature range for which the
thermometer is used.
14

Disadvantages:
 Limited temperature-range
covered
 Not very accurate.
 Requires visual reading
and is not easy to be
automated.
16

Thermistors, invented by Samuel Ruben
1930, are very temperature sensitive.
Their resistance decreases
as the temperature
increases, so the
Electric current
increases,
(and vice versa).
2. Electrical thermometers
a) Thermistors = THERMal resISTORS
18

Resistance versus Temperature Graph
For a Thermistor
19

Thermistors
 Example Applications:
1. Temperature measurement.
2. Time delay (self heating from large current „opens‟ the
thermistor so it can be used as a slow switch). Heating = I2 R,
where R is the resistance and I is the current.
3. Surge suppression when a circuit is first energized. Current
needs to flow through the thermistor for awhile to heat it so
that it „opens‟, and acts again as a switch.
20

Thermistors can be classified into two types
depending on the sign of k.
a) If k is negative, the resistance decreases with
increasing temperature, and the device is called a
negative temperature coefficient (NTC)
thermistor.
b) If k is positive, the resistance increases with
increasing temperature, and the device is called a
positive temperature coefficient (PTC) thermistor,
Posistor.
21

ELECTRIC SIGNAL
HAIR DRIER
THERMISTOR
22

b) Thermocouples
A thermocouple is a pair of junctions formed
from two dissimilar metals. One at a reference
temperature (e.g. 20⁰C) and the other junction
at the temperature to be measured.
A temperature
difference will
produce an
electric voltage
that depends
on this temp.
difference.
23

Why use thermocouples to
measure temperature?
◦ They are inexpensive.
◦ They are rugged and reliable.
◦ They can be used over a wide
temperature range.
24

3. Liquid crystal thermometers
 A liquid crystal thermometer (or plastic strip
thermometer) is a type of thermometer that contains
heat-sensitive liquid crystals in a plastic strip that
change color to indicate different temperatures.
 In medical applications, liquid crystal
thermometers may be used to read body
temperature by placing against the forehead, usually
called “forehead thermometers”.
 When it’s cold 
 When it’s used to
measure temp. 
25

Temperature scales
 Celsius Scale:
 Celsius is the metric scale for
measuring temperature.
 Water freezes at 0ºC and boils at
100ºC.
Fahrenheit Scale: water freezes
at 32 °F, and boils at 212 °F
[F = 1.8C + 32]
26

Kelvin scale
 In the Kelvin scale temperature is
measured in Kelvin units (K)
 Formula (273+ºC)= Kelvin
 Absolute zero (0 K) :
The temperature in which all
molecular motion stops
27

Units for measuring heat
 Like any type of energy, the SI unit for heat is the
Joule.
Calorie: the name calorie is used for two units of
energy:
a) The small calorie or gram calorie (symbol: cal) is
the amount of energy needed to raise the temperature
of one gram of water 1⁰C.
b) the large Calorie, or ”Kg calorie”, “nutritionist's
calorie”, (symbol: Cal) is the amount of energy needed
to raise the temperature of 1 Kg of water by 1⁰C.
 The large calorie is thus equal to 1000 small calories
or one kilocalorie (Cal = 1000 cal =1 Kcal ).
 1 Cal is about 4.2 kilojoules (4.186 KJ), and
1 cal = 4.2 J
28

Specific Heat Capacity
S.H.C. is defined as the amount of thermal energy needed to
raise a unit mass of substance a unit of temperature. Its symbol
is C (or s.h.c.).
For example, the specific heat of water is : C = 1 cal /(g·ºC),
or 4.186 J/(g·ºC), or 4186 J/Kg ºC ≈ 4200 J/Kg ºC)
Water has a very high specific heat, so it takes more energy to
heat up water than it would to heat up most other substances, of
the same mass, by the same amount of temp. Oceans and lakes
act like “heat sinks” storing thermal energy absorbed in the
summer and slowly releasing it during the winter. Large bodies
of water thereby help to make local climates less extreme in
temperature from season to season.
29

Specific Heat Equation
Q = m C T
Q = thermal energy (J, or KJ, or cal, or Cal …)
m = mass (g, or Kg …)
T = change in temp. (ºC, or ºF, or Kelvin)
C = specific heat capacity
Example1: The specific heat of silicon is 703 J / (kg · ºC).
How much energy is needed to raise the temperature
of a 7 kg chunk of silicon by 10ºC ?
Solution:
703 J
kg · ºCQ = 7 kg * [ ]*10 ºC = 49 210 J
☺ Visual experiments: Measure Specific Heat Capacity of Ethanol:
http://www.chm.davidson.edu/vce/calorimetry/SpecificHeatCapacityofEthanol.html
30

 Example2:
How much energy does it take to raise the temperature
of 50 g of copper by 10 ºC?
 Example 3:
If we add 30 J of heat to 10 g of aluminum, by how
much will its temperature increase?
 Example 4:
216 J of energy is required to raise the temperature of
aluminum from 15o to 35oC. Calculate the mass of
aluminum.(S.H.C. of aluminum is 0.90 JoC-1g-1).
 Example 5:
The initial temperature of 150g of ethanol was 22oC.
What will be the final temperature of the ethanol if 3240 J
was needed to raise the temperature of the ethanol?
(Specific heat capacity of ethanol is 2.44 JoC-1g-1).
 Answers: 192.5 J, 3 ºC, 12 g, 30.9 ºC. 31

Some Materials’ Specific Heat Capacity (J/g ºC)
 Water 4.18 6 Air 1.01
 Ice 2.03 Glass 0.84
 Al 0.385C Aluminum 0.902
 Graphite 0.720 NaCl 0.864
 Mercury 0.14 Granite 0.79
 Fe 0.451 Concrete 0.88
 Cu 0.385 Wood 1.76
 Au 0.128
 C2H5OH (ethanol) 2.46
 (CH2OH)2 (antifreeze) 2.42
32

 H.C. of an object is the energy required to change
the temperature of this object by 1 degree.
 Equation: or
 Thermal capacity  Q, Thermal capacity  1/ T
 The SI unit for heat capacity is J/K
 It can also be expressed in KJ/K, J/ºC, Cal/ºC …
 E.g. Night storage heaters (page 106 in your Course book).
Exercise: Solve the previous examples (1-5) to calculate the
thermal capacity for each object.
Answers: 4921 J/ºC, 19.25 J/ºC, 10 J/ ºC, 10.8 ºC, 364 J/ ºC .
Homework: make a comparison table for the thermal (heat) capacity
and the s.h.c. (C).
Heat capacity = Q / T Heat capacity = m * C
☺ Visual experiment: T vs. Q to calculate heat capacity:
http://www.chm.davidson.edu/vce/calorimetry/heatcapacity.html
33

Latent Heat
The word “latent” comes from a Latin word that means
“Hidden.” When a substance changes phases (liquid  solid
or gas  liquid) energy is transferred without a change in
temperature. For example, to turn water ice into liquid water,
energy must be added to bring the water to its melting point,
0ºC. This is not enough, additional energy is required to
change 0 ºC ice into 0 ºC water. The energy supplied to water
increases its internal energy but does not raise its temp., it
breaks down the bonds between, the particles. This energy is
called latent heat of fusion.
34

Latent Heat (L)
 Latent heat of fusion: the energy needed to
convert a solid into a liquid at the melting
temperature.
 Specific Latent heat of fusion: the energy needed
to convert 1Kg of a solid into liquid at the melting
temperature.
 Latent heat of vaporization: the energy needed to
change a liquid into vapor (gas) at the boiling
temperature.
 Specific Latent heat of vaporization: the energy
needed to change 1Kg of a liquid into vapor (gas)
at the boiling temperature.
35

Latent Heat Formula
S.L.H. = energy supplied / mass
i.e. Lf = Q/m & Lv = Q/m, or Q = m Lf & Q = m Lv
Q = thermal energy Units examples: J, KJ, Cal., Kcal.
m = mass Kg, g
Lf = Specific Latent heat of fusion J/Kg, KJ/Kg, J/g, Cal/Kg
Lv = Specific Latent heat of vaporization J/Kg, KJ/Kg, Cal./Kg
Example: Gold melts at 1063 ºC, what is the amount of heat
needed to melt 5 grams of solid gold at this temp. given that Lf
(the specific latent heat of fusion) for gold is 6440 J / kg. ?
Answer: Q = 32 J.
Homework: make a comparison table for the thermal (heat)
capacity, the s.h.c. (C), the latent heat & the specific latent heat.
36

Specific Latent Heat & Specific Heat
Substance Specific Heat (in J/kg·ºC)
Ice 2090
Liquid water 4186
Steam 1970
Example: Superman can vaporize a 1800 kg ice-monster
with his heat ray vision. The ice-monster was at -20 ºC.
After being vaporized the steam was heated to 135 ºC.
How much energy did Superman expend?
For Water: Lf = 3.33 105 J/kg; Lv = 2.26 106 J/kg
Information you will need:
ICE
-20 ºC
ICE
0 ºC
WATER
0 ºC
VAPOR
100 ºC
WATER
100 ºC
VAPOR
135 ºC
Heating Melting Heating Boiling Heating
S.H.C. Lf S.H.C. Lv S.H.C.
37

Solution steps
1. The energy needed for heating ice from -20ºC to the melting point:
Q1 = m*C*T = (1800 kg) (2090 J/kg·ºC) (20 ºC) = 75240000 J
2. The energy needed for turning ice into water at 0ºC:
Q2 = m*Lf = (1800 kg) (3.33 105 J / kg) = 5994 105 J
3. The energy needed for heating water from 0ºC to the boiling point:
Q3 = m*C*T = (1800 kg) (4186 J/kg·ºC) (100ºC) = 753480000 J
4. The energy needed for turning water into steam at 100 ºC:
Q4 = m*Lv = (1800 kg) (2.26 106 J/kg) = 4068 106 J
5. The energy needed for heating steam to 135 ºC:
Q5= m*C*T = (1800 kg) (1970 J/kg·ºC) (135 ºC) = 478710000 J
Total energy expended by Superman = 75240000 + 5994 105 +
753480000 + 4068 106 + 478710000 = (75.24 106) + (599.4 106) +
(753.48 106) + (4068 106) + (478.71 106) = 5974.83 106 J
38

Thermal Expansion
As a material heats up its particles move or vibrate more
vigorously, and the average separation between them increases.
This results in small increases in lengths and volumes.
Buildings, railroad tracks, bridges, and highways contain
thermal expansion joints to prevent cracking and warping due
to expansion.
Factors which affect the expansion of solids:
Original length & temperature raise (direct), material type.
39

What do you see in these pictures?
What is meant be expansion?
It is the difference between the original size of an object and its size
when its heated (or cooled).
40

Some real-life problems due to expansion
 On a hot day concrete runway sections in airport
expands and this cause cracking. To solve this
problem we leave small gabs between sections.
 On a hot day concrete bridges expand. To solve this
problem, we leave small gab at one end and support the
other end with rollers.
 Telephone wire contract on cold days. To solve this
problem, we leave wires slack so that they are free
to change length.
 On a hot day railway lines expand. To solve this
problem, gaps are left between sections of railway
lines to avoid damage of the rails as the expand in
hot weather.
41

Bimetallic Strip
A bimetallic strip is a strip of two different metals — often steel on
one side and brass on the other. When heated the strip curves
because the metals have different coefficients of thermal expansion.
Brass‟s coefficient is higher, so for a given temperature change, it
expands more than steel. This causes the strip to bend toward the
steel side. The bending would be reversed if the strip were made
very cold.
cold strip 
 hot strip
handle steel (brass on
other side)
Top view
steel side
brass side
Side view
42

Thermostats
 Bimetallic strips are used in thermostats, at least in some
older ones. When the temperature changes, the strip bends,
making or breaking an electrical circuit, which causes the
furnace to turn on or shut off.
 Some applications to thermostat in industry :
electric irons, home heating/cooling systems, ovens,
refrigerators, fire alarms, fish tanks, car thermostat 
.
43

Expansion of liquids
 What do you see in the picture below?
 Explain what happen when liquids are heated?
When a liquid is heated, its molecules gain kinetic
energy and vibrate more vigorously. As the vibration
become larger, the molecules are pushed further
apart and the liquid expands slightly in all directions.
44

List the factors which affect the
expansion of liquids?
 temperature & liquid volume (direct), liquid type.
*The effect of temperature on volume and
density of water:
45

Expansion of gases
 Compare the expansion of gases to that of solids and liquids?
The expansion of gases is much more larger than that of solids or
liquids under the same rise in temperature.
 The effect of temperature on gas volume under constant pressure
The volume of a gas is directly proportional to the Kelvin
temperature under constant pressure (Charlie's Law).
When the temperature of a gas is increased, the molecules move
faster and the collisions become more violent thus they spread away
from each other causing the volume to increase.
inside and outside pressures
are balanced.
46

Capter 10

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Capter 10