G 10 thermal energy, Temperature and heat original FINAL.pptx

1
Thermal Energy
Temperature & Heat
Grade 10 Physics - Understanding the
Relationship Between Temperature
and Kinetic Energy

2
Remember
The atoms and molecules that make up matter
are in continuous, random motion.
Matter in Motion
Matter is made of tiny particles—atoms and molecules.
• Particles are in constant, random motion
• Faster = More KINETIC energy
• Particles in hot objects move faster than cooler objects.

3
Thermal Energy
Kinetic energy + potential energy of an object = thermal energy of
that object.

4
1. Thermal energy relationships
a. Depends on temperature, mass, and
type of substance
b. As temperature increases, so does
thermal energy (because the kinetic
energy of the particles increased).
c. Even if the temperature doesn’t
change, the thermal energy of a more
massive substance is higher (because
it is a total measure of energy).

5
• Which beaker of water has more thermal
energy?
– B - same temperature, more mass
200 mL
80ºC
A
400 mL
80ºC
B

6
Temperature and Heat
Are temperature and
heat the same?

7
Are temperature and
heat the same?
No, because a spoonful
of boiling water
(100o
C) will have less
thermal (heat) energy
….

8
Are temperature and
heat the same?
No, because a spoonful
of boiling water
(100o
C) will have less
thermal (heat) energy
….
… than a beaker of
boiling water (at the
same temperature).

9
Temperature is a measure of the
average kinetic energy of the
individual particles in a substance.

10
A. Temperature
Temperature
– is measured with a
thermometer and
can be measured in
Kelvin, Celsius, and
Fahrenheit
Absolute zero-
temperature at which
particles stop moving
0o
K

11
SI unit for temp. is the Kelvin
a. K = C + 273 (50 C = ______K)
b. C = K – 273 (50 K = ______C)
•Example 1
•Convert 500 K to
celsius:
•Given, kelvin
temperature = 500 K
•Example 2
•Convert 750 K to
celsius:
•Given, kelvin
temperature = 750 K

12
•Question:
•Ethyl alcohol boils at 78.5o
C and freezes at -
117o
C under a pressure of 1 atm. Convert
these temperatures to Kelvin scale.

13
•What is absolute zero on the kelvin scale?
•Although obtaining negative values for
temperature on the Celsius scale is perfectly
natural, the Kelvin scale has a minimum value of
zero. Zero Kelvin is also called absolute zero.
•It is the point at which no more molecular motion,
and there is no chance of lower temperature.
Therefore, this implies that the lowest Celsius
temperature that can possibly be achieved is equal
to minus 273° C.

14
Temperature and Particle Motion

15
Interactive Activity
• Observe the motion of particles in the
simulation at different temperatures. Discuss:
• - How does particle motion change with
temperature?
• Phet simulation 1
• Phet simulation 2
• - What happens as temperature approaches
absolute zero?

16
Quick Assessment: MCQs
• 1. What does temperature measure?
• A. Total energy of particles
• B. Average kinetic energy of particles
• C. Potential energy of particles
• 2. What happens to particle motion as temperature increases?
• A. Particles move slower
• B. Particles move faster
• C. Particles stop moving

17
Quick Assessment: MCQs
• 1. What does temperature measure?
• A. Total energy of particles
• B. Average kinetic energy of particles
• C. Potential energy of particles
• Answer: B
• 2. What happens to particle motion as temperature increases?
• A. Particles move slower
• B. Particles move faster
• C. Particles stop moving
• Answer: B

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Temperature Thermal Energy Heat
The transfer of
energy between
objects that are
at different
temperatures
The total
internal energy
of molecules
A measure of the
average kinetic energy
of the particles
substance
in a
Degrees
Fahrenheit,
degrees
Celsius,
Kelvins
Joules
Joules
or
Varies with mass,
specific heat
capacity, and temp.
change
Varies with
mass and
temp.
Does NOT
vary with
mass
Thermal is achieved when all substance have reached the
same
Equilibrium
temperature. There is no more thermal energy transfer.

19
Recap and Homework
• Recap:
• - Thermal energy is the total kinetic energy of particles.
• - Temperature measures average kinetic energy.
• - Absolute zero is the minimum kinetic energy state.
• Homework:
• Research a real-world application of temperature and
kinetic energy, and prepare a short presentation.

20
Heat
a. The Flow of
thermal energy
from one object to
another.
b. Heat always
flows from warmer
to cooler objects.
Ice gets warmer
while hand gets
cooler
Cup gets cooler while
hand gets warmer

21
Heat
The movement of heat from a high
temperature to low temperature – when two
substances at different temperatures are
mixed together, heat flows from the warmer
body to the cooler body until they reach the
same temperature or equilibrium
(Zeroth Law of Thermodynamics – Thermal
Equilibrium).

22
Heat transfer only when there is a
difference in temperature between to
objects

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How does heat transfer ?
Heat transfer by three ways:
1. Conduction
2. Convection
3. Radiation

24
A. How is heat transferred?
• What type of HEAT TRANSFER is occurring in the
pictures? Conduction, convection or radiation?
CONDUCTION –
The transfer of thermal energy with NO transfer of matter.
Occurs because particles that make up matter are in
constant motion and have collisions

27
Heat transfer by convetion
Learning objective:
• Describe the three modes of heat transfer: convection
and radiation,
• Provide examples of each
By the end of the lesson:
• Students can describe the processes of convection and
radiation and provide basic examples of each. By
explain in writing how a campfire transfers heat
through radiation and how a pot of boiling water
demonstrates convection.

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HEAT TRANSFER
CONVECTION –
The transfer of thermal energy when particles of a liquid
or gas move from one place to another

29
How convection occur ?
• As the water above the flame heats up,
• it expands,
• becomes less dense,
• and is pushed upwards,
• while the cooler, more dense water sinks to take its place.

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Convection Currents
Stream of warm moving fluids are called
convection currents

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Applications on convection
(The heater)
 What about the Air conditioner!

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Convection currents inside Earth
Tectonic drift

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HEAT TRANSFER
Convection currents– in the earth and sun
The circular flow of hot and cold creates convection currents

34
HEAT TRANSFER
RADIATION –
The transfer of thermal energy by electromagnetic waves
(IR) moving through (vacuum) space. ALL OBJECTS
radiate energy!

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Radiation
Heat radiation is the infra-red radiation

36
Check point
Heat transfer by radiation
a) is not possible from human beings to their
environment.
b) does not occur from light bulbs - they are too
bright.
c) does not require any material between the
radiator and the object receiving the radiation.
d) none of the above.

37
Check point
Heat transfer by radiation
a) is not possible from human beings to their
environment.
b) does not occur from light bulbs - they are too
bright.
*c) does not require any material between the
radiator and the object receiving the radiation.
d) none of the above.

38
Absorption and reflection
Absorbtion experiment
When an object is heated by an external source of heat by radiation,
The amount of heat reflected and absorbed heat radiation (infrared)
varies according to the color of the surface of the objects.

39
Check point
• Which object cools fastest at night?
a) White rock
b) Black soil
c) Silver car
d) Green grass

40
Check point
• Which object cools fastest at night?
a) White rock
*b) Black soil
c) Silver car
d) Green grass

41
Emission
Emission experiment
• When an object is hotter than surround it radiates infrared
(emits heat) to the surround.
• the amount of radiated (emitted) heat radiation (infra-red)
also varies according to the color of the surface.

42
• Two objects at the same temperature but
different colors will:
a) Emit the same radiation
b) Emit different radiation
c) Absorb the same heat
d) Reflect the same light

43
Check point
• Two objects at the same temperature but
different colors will:
a) Emit the same radiation
*b) Emit different radiation
c) Absorb the same heat
d) Reflect the same light

44
Factors affect emission
• If the surface area of the container increased,
the amount of emitted heat radiation will also increase.
• If objects have the same temperature and same
color but different surface areas, the greater surface
area will emit larger amount of heat radiation and cools
faster.
• Black color is good emitter (radiator) of heat
radiation (infrared).
• White color is bad emitter (radiator) heat radiation.

45
Summary
Dark and dull surfaces are ________
absorbers and ________ emitters of infra-
red radiation.
Light and shiny surfaces are ________
absorbers and ________ emitters of infra-
red radiation, they are ________
reflectors.

46
Summary
Dark and dull surfaces are GOOD
absorbers and GOOD emitters of infra-
red radiation.
Light and shiny surfaces are POOR
absorbers and POOR emitters of infra-
red radiation, they are GOOD reflectors.

47
Application
• A vacuum or Thermos
flask keeps hot liquids
hot or cold liquids cold
• It is very difficult for
heat to travel into or
out of the flask

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Preventing losing heat by conduction
• The flask is made of a double-walled vessel with vacuum between
the walls
• Vacuum is a good insulator so there is no heat conduction
between the hot water inside and the cold air outside.
Preventing losing heat by radiation
• Radiation is reduced by plating the walls with silvery color from
inside
• so heat radiation that is emitted from the hot liquid, will reflect
back again
Preventing losing heat by convection
• The hot water vapour that rises up by convection is prevented
from going out by the stopper,
• Therefore, the liquid will be hot as long as possible.

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Heat Transfer Video
• https://www.generationgenius.com/videoless
ons/heat-transfer-of-thermal-energy-video-for
-kids/?gclid=CjwKCAjwhaaKBhBcEiwA8acsHC
ME7fLqjSl5OjwqwJnWjmKglMa5ezXQ4I-fTz-ae
_hClgEUU2Vh5hoCk34QAvD_BwE

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Heat Transfer
1.Specific Heat Capacity (Cp)
• amount of energy required to
raise the temp. of 1 kg of
material by 1 Kelvin
• units:
J/(kg·K) or J/(kg·°C) or J/(g·°C)
• MUST PAY ATTENTION TO
UNITS
SpecificHeatValues
(J/(kg·K))
Water
Alcohol
Aluminum
Carbon(graphite)
Sand
Iron
Copper
Silver
4184
2450
920
710
664
450
380
235

53
Heat Transfer
• Which sample will take
longer to heat to 100°C?
SpecificHeatValues
(J/(kg·K))
Water
Alcohol
Aluminum
Carbon(graphite)
Sand
Iron
Copper
Silver
4184
2450
920
710
664
450
380
235
50 g Al 50 g Cu

54
Heat Transfer
• Which sample will take
longer to heat to 100°C?
SpecificHeatValues
(J/(kg·K))
Water
Alcohol
Aluminum
Carbon(graphite)
Sand
Iron
Copper
Silver
4184
2450
920
710
664
450
380
235
50 g Al 50 g Cu
• Al - It has a higher specific heat.
• Al will also take longer to cool down.

55
What is specific heat capacity?
• This is a measure of how much energy a material can
store.
• It is the energy needed to raise the temperature of
1kg of a material by 1o
C. It is measured in J/kg o
C
• If we heated these materials for 10 minutes , which
would get hottest?
copper limestone water
http://www.youtube.com/watch?v=D3CwpfBzF94

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SHC of different materials
• The copper became hottest because it has the
lowest SHC.
• The water has the highest SHC as it absorbed
the energy without becoming very hot.
82o
C 65o
C 43o
C

57
Heat Transfer
Q = m  T  Cp
Q: heat (J)
m: mass (kg)
T: change in temperature (°C)
Cp: specific heat (J/kg·°C)
T = Tf - Ti
– Q = heat loss
+ Q = heat gain

58
m =
ΔQ
c x T
T
=
ΔQ
c x m c = ΔQ
c x T
Rearranging the equation:
Energy transferred (J)
mass
kg
temperature
change
o
C
specific heat
capacity
J/kg o
C
x x

59
m =
ΔQ
c x ΔT
ΔT =
ΔQ
c x m c = ΔQ
c x ΔT
mass
kg
temperature
change
o
C
specific heat
capacity
J/kg o
C
x x

60
m =
ΔE
c x Δθ
Δθ =
ΔE
c x m c = ΔE
c x ΔT
mass
kg
temperature
change
o
C
specific heat
capacity
J/kg o
C
x x

61
Practice Problem
What is the specific heat of silver
if the temperature of a 15.4 g
sample of silver is increased from
20.0o
C to 31.2o
C when 40.5 J of
heat is added?

62
Givens:
m = 15.4 g
Ti = 20.0o
C
Tf = 31.2o
C
Q = 40.5 J
Q = mC∆T
40.5=15.4(C)(31.2-20.0)
40.5=15.4(C)(11.2)
40.5=172.48(C)
C = 0.235 J/g(o
C)

63
Practice Problem
What is the final temp of silver
if the temperature of a 5.8 g
sample of silver starts out at
30.0o
C and 40.5 J of heat is
added? The specific heat of
silver is .235 J/g(o
C).

64
Givens:
m = 5.8 g
Ti = 30.0o
C
Q = 40.5 J
C = 0.235
Tf = ???
Q = mC∆T
40.5=5.8(0.235)(Tf -30.0)
40.5=1.363(Tf-30.0)
40.5=1.363Tf – 40.89
81.39=1.363Tf
Tf = 59.7139

65
• What quantity of heat is required
to raise the temperature of 100
mL of water from 45.6C to
52.8C? The specific heat of
water is 4.184 J/g(C) and water
has a density of 1.00 grams/mL.
Practice Problem

66
Givens:
Q = ?
V = 100. mL
Ti = 45.6o
C
Tf = 52.8o
C
C = 4.184
dwater=1.00
g/
100 mL = 100 g
Q = mC∆T
Q=100(4.184)(52.8-45.6)
Q=3012.48
Q=3010 J

67
Heat Transfer
 A 32-g silver spoon cools from 60°C to 20°C.
How much heat is lost by the spoon.
Cp = 240 J/kg·°C
?
GIVEN:
m = 32 g
Ti = 60°C
Tf = 20°C
Q = ?
Cp = 240 J/kg·°C
WORK:
Q = m·T·Cp
m = 32 g = 0.032 kg
T = 20°C - 60°C = – 40°C
Q = (0.032kg)(-40°C)(240 J/kg·°C)
Q = – 301 J

68
Heat Transfer
How much heat is required to warm 230 g of
water from 12°C to 90°C? Cp= 4184 J/kg·°C
GIVEN:
m = 230 g
Ti = 12°C
Tf = 90°C
Q = ?
Cp= 4184 J/kg·°C
WORK:
Q = m·T·Cp
m = 230 g = 0.23 kg
T = 90°C - 12°C = 78°C
Q = (0.23kg)(78°C)(4184 J/kg·°C)
Q = 75,061 J

69
Specific Heat
2. Some things heat up or cool
down faster than others.
Land heats up and cools down faster than water

70
b. Specific heat is the amount of
heat required to raise the
temperature of 1 kg of a material by
one degree (C or K).
1) C water = 4184 J / kg C
2) C sand = 664 J / kg C
This is why land heats up quickly
during the day and cools quickly at
night and why water takes longer.

72
Heat Transfer from object to surroundings
• The heat transfer from an object to its
surroundings and from the surroundings to an
object is equal. One will experience heat loss,
one will experience heat gain.
Hloss = Hgain
m·T·Cp (object) = Q = m·T·Cp (surroundings)

73
Exit Slip
• A wooden spoon has a mass of 20 kg and a
specific heat of 1,700 J/(kg·C). Find the heat
change of the block as it warms from 15°C to
25°C.

75
Conductors and Insulators
Materials are either conductors or insulators.
A conductor transfers thermal energy
Ex:metals-silver and steel, tile floors takes heat
away from your
An insulator does not transfer thermal energy
well.
Ex: wood, wool, straw, paper

76
Thermodynamics
• Study of the relationships between thermal
energy, heat and work
– Heat and work increase thermal energy
– Heat – warming hands by a fire
– Work – warming hands by rubbing them together

77
The Zeroth Law of Thermodynamics
• If two thermodynamic systems are each in thermal
equilibrium with a third, then they are in thermal
equilibrium with each other

78
The Zeroth Law of Thermodynamics
• If C is initially in thermal equilibrium with both A and B,
then A and B are in thermal equilibrium with each other.

79
Linear Thermal Expansion
• Increasing the temperature of a rod causes it to expand.
• For moderate changes in temperature, the change in length is
given by:

80
1) A metal rod is 2 meters long at
20°C. If its coefficient of linear
expansion is 1.2 × 10⁻⁵ °C⁻¹, find its
length when heated to 50°C.

81
Answer
ΔL = L₀αΔT
= (2)(1.2 × 10⁻⁵)(50 - 20)
= 0.00072 m
= 0.72 mm.
New length = 2.00072 m.

82
2) A glass rod expands by 0.3 mm
when heated from 25°C to 75°C. If
its original length is 1.5 m, find the
coefficient of linear expansion.

83
Answer
α = ΔL / (L₀ΔT)
= 0.0003 / (1.5 × 50)
= 4 × 10⁻⁶ °C⁻¹.

84
3) A steel rod has a coefficient of
linear expansion of 1.1 × 10⁻⁵ °C⁻¹. If
it is 3 m long at 20°C, what is the
change in length when heated to
80°C?

85
Answer
ΔL = (3)(1.1 × 10⁻⁵)(80 - 20)
= 0.00198 m
= 1.98 mm.

86
4) If a brass rod expands by 2 mm when
heated by 100°C, what would be its
expansion if the temperature change
were only 50°C?

87
Answer
Since expansion is proportional to
temperature change:
New expansion = (50/100) × 2
= 1 mm.

88
5) A railway track is made of steel rails,
each 12 m long at 15°C. If the
temperature rises to 45°C, and the
coefficient of linear expansion for steel
is 1.2 × 10⁻⁵ °C⁻¹, find the expansion
per rail.

89
Answer
ΔL = (12)(1.2 × 10⁻⁵)(45 - 15)
= 4.32 mm

90
6) A rod expands from 1.2 m to
1.202 m when the temperature
increases from 25°C to 125°C. Find
the coefficient of linear expansion.

91
Answer
α = (0.002) / (1.2 × 100)
= 1.67 × 10⁻⁵ °C⁻¹

92
7) A rod expands from 100 cm to
100.5 cm when heated from 20°C
to 120°C. Find the coefficient of
linear expansion.

93
Answer
α = (0.5) / (100 × 100)
= 5 × 10⁻⁶ °C⁻¹

94
9) A bridge made of steel is 500 m
long at 10°C. How much will it
expand at 40°C if the coefficient of
linear expansion for steel is 1.2 ×
10⁻⁵ °C⁻¹?

95
Answer
ΔL = (500)(1.2 × 10⁻⁵)(40 - 10)
= 18 cm

96
10) A rod expands by 0.002 m
when heated by 100°C. If its
original length was 2 m, find its

97
Answer
α = (0.002) / (2 × 100)
= 1 × 10⁻⁵ °C⁻¹.

98
12) A rod of unknown material
expands by 0.01 m when its
temperature increases by 150°C. If its
original length was 4 m, determine its

99
Answer
α = (0.01) / (4 × 150)
= 1.67 × 10⁻⁵ °C⁻¹.

100
13) A copper rod of length 2.5 m is
heated from 30°C to 200°C. If the
coefficient of linear expansion of
copper is 1.7 × 10⁻⁵ °C⁻¹, calculate
the final length of the rod.

101
Answer
ΔL = (2.5)(1.7 × 10⁻⁵)(200 - 30)
= 7.225 mm.
Final length = 2.507225 m

102
14) A laboratory experiment
determines that a metal rod expands
by 0.004 m when heated from 25°C to
225°C. The original length was 5 m.
Identify the metal using the given
table of expansion coefficients.

103
Answer
α = (0.004) / (5 × 200)
= 4 × 10⁻⁶ °C⁻¹.
This does not match the listed metals, so it
is likely an unknown material.

104
15) A steel pipeline of length 800 m is
installed at 15°C. If it is expected to reach
a maximum of 45°C, calculate how much
expansion should be allowed in the
design
(coefficient of expansion for steel: 1.2 ×
10⁻⁵ °C⁻¹).

105
Answer
ΔL = (800)(1.2 × 10⁻⁵)(45 - 15)
= 28.8 cm

106
Applications of thermal expansion

111
C) Another application of bimetallic strip
Fire alarm

112
Example of Thermal Expansion
• This railroad track has a gap between segments to allow for
thermal expansion.
• On hot days, the segments expand and fill in the gap.
• If there were no gaps, the track could buckle under very hot
conditions.

113
8) Why do gaps need to be left
between railway tracks?

114
Answer
To allow for the expansion of metal due to
temperature increases. Without these
gaps, the rails could bend or break due to
thermal expansion.

115
11) A metal wire
is stretched
tightly between
two poles.
Explain why it
may sag in the
summer and
become tight in
the winter?

116
Answer
In summer, the wire expands and sags. In
winter, it contracts and becomes tighter.

117
Bridges due to thermal expansion
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.

118
Concrete due to thermal 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.

119
What is meant by a thermostat?
A thermostat is temperature operated electrical switch that uses
the expansion properties of a bimetal strip.

120
Some applications to thermostat in
industry
1.electric irons
2.fish tanks
3.home heating/cooling systems
4.Ovens, refrigerators,
5.fire alarms, car flashers

121
Thermal expansion of solids,
liquids and gases
The particles in gases and liquids move more freely
than in solids, so their volume increases when
heated.

122
Summary
•Matter expands when heated and contracts
when cooled
•Liquids expand and contract more than solids
•Gases expand and contract more than liquids

123
Molecular Basis for Thermal Expansion (1 of 2)
• We can understand linear
expansion if we model the
atoms as being held together
by springs.
• When the temperature
increases, the average
distance between atoms also
increases.
• As the atoms get farther apart,
every dimension increases.

124
Molecular Basis for Thermal Expansion (2 of 2)
• A graph of the “spring”
potential energy versus
distance between
neighboring atoms is not
symmetrical.
• As the energy increases
and the atoms oscillate
with greater amplitude,
the average distance
increases.

125
Expanding Holes and Volume Expansion
• If an object has a hole in it, the
hole also expands with the
object, as shown.
• The hole does not shrink.
• The change in volume due to
thermal expansion is given by
0
V V T

  
where β is the coefficient of
volume expansion and is equal to
3α.
• Video Tutor Solution: Example 1
7.3

126
Table 17.1 Coefficients of Linear Expansion
Material
alpha in units per kelvin or degree Celsius
Aluminum
2.4 times 10 to the negative fifth
Brass
Copper
Glass
0.4 to 0.9 times 10 to the negative fifth
Invar (nickel–iron alloy)
Quartz (fused)
Steel
  
1 -1
[K or ( C) ]
5
2.4 10

5
2.0 10

5
1.7 10

5
0.4 0.9 10
 
5
0.09 10

5
0.04 10

5
1.2 10


127
Table 17.2 Coefficients of Volume Expansion
Solids
beta in units per kelvin or degree Celsius
Aluminum
Brass
Copper
Glass
1.2 to 2.7 times 10 to the negative fifth
Invar
Quartz (fused)
Steel
  
1 -1
[K or ( C) ]
5
7.2 10

5
6.0 10

5
5.1 10

5
1.2 2.7 10
 
5
0.27 10

5
0.12 10

5
3.6 10


128
Thermal Expansion of Water
• Between 0 degrees
Celsius and 4 degrees
Celsius, water decreases
in volume with
increasing temperature.
• Because of this
anomalous behavior,
lakes freeze from the
top down instead of
from the bottom up.

129
Quantity of Heat (1 of 2)
• Sir James Joule (1818–1889) studied how water can be
warmed by vigorous stirring with a paddle wheel.

130
Quantity of Heat (2 of 2)
• The same temperature change
caused by stirring can also be
caused by putting the water in
contact with some hotter
object.
• The calorie (abbreviated cal) is
the amount of heat required
to raise the temperature of 1
gram of water from 14.5
degrees Celsius to 15.5
degrees Celsius.

131
Specific Heat
• The quantity of heat Q required to increase the
temperature of a mass m of a certain material by ΔT is:
• The specific heat c has different values for different
materials.
• The specific heat of water is approximately
4190 J/kg K.


132
Phase Changes
• The phases (or states) of matter
are solid, liquid, and gas.
• A phase change is a transition
from one phase to another.
• The temperature does not change
during a phase change.
• The latent heat, L, is the heat per
unit mass that is transferred in a
phase change.

133
Heat Added to Ice at a Constant Rate
• Video Tutor Solution: Example 17.8

134
Heat of Fusion
• The metal gallium, shown
here melting in a person’s
hand, is one of the few
elements that melts at
room temperature.
• Its melting temperature is
29.8 degrees Celsius, and
its heat of fusion is
4
8.04 10 J/kg.
f
L  

135
Heat of Vaporization
• The water may be warm and it
may be a hot day, but these
children will feel cold when they
first step out of the swimming
pool.
• That’s because as water
evaporates from their skin, it
removes the heat of
vaporization from their bodies.
• To stay warm, they will need to
dry off immediately.

136
Conduction of Heat
• In conduction, heat flows from a higher to a
lower temperature.
• Consider a solid rod of conducting material
with cross-sectional area A and length L.
• The left end of the rod is kept at a
temperature TH and the right end at a lower
temperature TC.
• The rate that heat is transferred is:

137
Thermal Conductivities of Some Common
Substances
Substance
k in watts per meter kelvin
Silver 406
Copper 385
Aluminum 205
Wood 0.12 – 0.04
Concrete 0.8
Fiberglass 0.04
Styrofoam 0.027

k(W/m K)

138
Energy Conservation in Thermodynamic
Processes
1. Introduction to Thermodynamics
Thermodynamics is the branch of physics that deals
with heat, work, and energy.
The fundamental concept in thermodynamics is the
conservation of energy, which states that:
Energy cannot be created or destroyed, only
transferred or converted from one form to another.

139
2. Principle of Energy Conservation in
Thermodynamic Processes
Definition
The principle of energy conservation states that in any
thermodynamic process, the total energy remains constant.
Energy may be transferred as heat (Q) or work (W) but the
internal energy (U) of the system changes accordingly.

140
Examples of Energy Conservation in
Thermodynamics
1. Boiling Water: Heat from a stove transfers energy to the
water, increasing its temperature.
2. Car Engine: Chemical energy in fuel is converted into heat
and mechanical work.
3. Air Conditioners: Electrical energy is used to remove heat
from a room, keeping it cool.

141
3. First Law of Thermodynamics
Mathematical Formula:
ΔU=Q−W
Where:
•ΔU = Change in internal energy (J)
•Q = Heat added to the system (J)
•W = Work done by the system (J)

142
Explanation
 If heat is added to a system (+Q), the internal energy
increases.
 If heat is removed from a system (-Q), the internal energy
decreases.
 If work is done on the system (-W), the internal energy
increases.
 If work is done by the system on the surroundings (+W), the
internal energy decreases.

143
Example Situations:
•Expanding Gas: When a gas expands in a piston, work is done
by it on its surroundings, decreasing internal energy.
•Compressing Gas: When a gas is compressed, work is done on
it, increasing internal energy.

144
4. Work and Heat Transfer in
Thermodynamics
Work (W)
•Work is done by a system when it expands (gas pushing a piston).
•Work is done on a system when it is compressed.
•Measured in Joules (J).
Heat (Q)
•Heat is energy transferred due to temperature difference.
•Flows from hot to cold.
•Measured in Joules (J).

145
5. Practice Questions
A. Multiple-Choice Questions
1. What happens to the internal energy of a gas when work is
done on it?
A) It decreases
B) It increases
C) It remains constant
D) It depends on the heat transfer

146
1. What happens to the internal energy of a gas when work is
done on it?
A) It decreases
B) It increases ✅
C) It remains constant
D) It depends on the heat transfer

147
2. Which equation represents the first law of thermodynamics?
A) W=Q−U
B) ΔU=Q+W
C) ΔU=Q−W
D) Q=W−ΔU

148
2. Which equation represents the first law of thermodynamics?
A) W=Q−U
B) ΔU=Q+W
C) ΔU=Q−W ✅
D) Q=W−ΔU

149
3. In a heat engine, what happens to some of the input energy?
A) It is destroyed
B) It is converted into matter
C) It is converted into useful work and heat loss
D) It disappears

150
3. In a heat engine, what happens to some of the input energy?
A) It is destroyed
B) It is converted into matter
C) It is converted into useful work and heat loss ✅
D) It disappears

151
4. If a system absorbs 100J of heat and does 40J of work, what
is the change in internal energy?
A) 60J
B) 140J
C) -60J
D) 40J

152
4. If a system absorbs 100J of heat and does 40J of work, what
is the change in internal energy?
A) 60J ✅
B) 140J
C) -60J
D) 40J

153
5. What does the first law of thermodynamics primarily
describe?
A) Conservation of energy
B) The expansion of gases
C) The cooling of objects
D) The properties of solids

154
5. What does the first law of thermodynamics primarily
describe?
A) Conservation of energy ✅
B) The expansion of gases
C) The cooling of objects
D) The properties of solids

155
Free response questions
6. Explain why the first law of thermodynamics is also called
the law of energy conservation.

156
6. Explain why the first law of thermodynamics is also called
the law of energy conservation.
Answer:
The first law of thermodynamics states that the change in
internal energy of a system is equal to the heat added minus the
work done. This aligns with the principle of conservation of
energy, which states that energy cannot be created or destroyed,
only transformed.

157
7. A system receives 500J of heat and performs 200J of work.
Calculate the change in internal energy.

158
7. A system receives 500J of heat and performs 200J of work.
Calculate the change in internal energy.
Answer:
ΔU=Q−W=500J−200J=300J.

159
8. Describe a real-life example of energy conservation in a
thermodynamic process.

160
8. Describe a real-life example of energy conservation in a
thermodynamic process.
Answer:
A steam engine converts heat energy from burning fuel into
mechanical work, demonstrating the conservation of energy.

161
9. How does heat transfer occur in a thermodynamic system?

162
9. How does heat transfer occur in a thermodynamic system?
Answer:
Heat transfer occurs through conduction, convection, or
radiation, depending on the medium and temperature
difference.

163
10. What is the relationship between work and heat in
thermodynamic processes?

164
10. What is the relationship between work and heat in
thermodynamic processes?
Answer:
Heat and work are the two ways energy is transferred in
thermodynamic systems. They influence the internal energy of a
system as described by the first law of thermodynamics.

165
6. Summary
•Energy conservation means that energy cannot be created or
destroyed.
•The first law of thermodynamics states that the internal energy
change is equal to heat added minus work done.
•Work (W) is energy used to move objects or expand gases.
•Heat (Q) is energy transfer due to temperature differences.
•Applications include engines, refrigerators, and heat pumps.

G 10 thermal energy, Temperature and heat original FINAL.pptx

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