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Applications
• Engineering
• Chemistry
• Earth Science
• Astrophysics
• Everyday life

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Applications
To design pressure cookers

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Applications
Automobile engine

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Applications

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Applications
Household refrigerator

8. Power Plant
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Applications

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Applications
Solar Heater

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1 Introduction
• In the study of thermal physics there is a
need to approach it in two levels.
– macroscopic level
– microscopic level

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1 Introduction
Macroscopic Approach
• Look at macroscopic variables that can be
measured by simple experiments:
– temperature
– pressure
– volume
• Derive general relationships and equations that
can describe the experiments.
– Advantage:
• easily related to experiment
– Disadvantage:
• lacks fundamental explanations

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1 Introduction
Microscopic Approach
• Explain macroscopic properties by looking
closely into the atoms and molecules that make
up matter to understand what is happening .
– Advantage:
• Gives a real explanation of what is actually
happening.
– Disadvantage:
• Too many particles will require the use of complex
computation.

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1 Introduction
• Lets begin by asking ourselves,
– What is Temperature?
– What is Heat?

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2. Temperature
• In a macroscopic view:
– Temperature is the physical property which
determines the direction flow of heat.
heat
• e.g Heat flows from higher temperature to lower
temperature
Hot Cold
Heat flows

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2. Temperature
• In a macroscopic view:
– Temperature is a physical property of a
system that provide a measure of hotness or
coldness.

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2. Temperature
• In a microscopic view:
– Temperature is a measure of the average
kinetic energy of molecules in a body.

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2.1 Heat and Thermal Equilibrium
• When two bodies are in thermal contact,
energy can be transferred between them.
Hot Cold

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2.1 Heat and Thermal Equilibrium
• Microscopically the energy transfer occurs
in two ways.
Higher kinetic Lower kinetic
energy molecules energy molecules
Hot Cold

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2.1 Heat and Thermal Equilibrium
• But the rate of transfer of energy from a
hotter body is always greater than that from
a cooler body.
Higher kinetic Lower kinetic
energy molecules energy molecules
Hot Cold
Energy Energy

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2.1 Heat and Thermal Equilibrium
• This net energy transfer from a body of a
higher temperature to a lower temperature
is known as heat.
Hot Cold
Heat flows

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2.1 Heat and Thermal Equilibrium
• Heat will flow between two bodies as long
as there is temperature difference
between them.
Hot Cold
Heat flows

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2.1 Heat and Thermal Equilibrium
• When two bodies are in thermal contact and
there is no flow of heat from one body to
another, they are said to be in thermal
equilibrium.
• At thermal equilibrium (microscopically)
 The rate of transfer of energy is the same from both
bodies.
 Thus these two bodies are said to be at the same
temperature.

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2.1 Heat and Thermal Equilibrium
Consider the 2 systems, X and Y:
insulator
No thermal contact
No flow of heat between
X and Y
X Y
Fig 1

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2.1 Heat and Thermal Equilibrium
X and Y are in thermal contact
Temperature of X > Temperature of Y
Rate of transfer of energy from X to Y >
Rate of transfer of energy from Y to X
X Y
Heat flows from X to Y
Fig 2
heat Temperature of X decreases and
Temperature of Y increases

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2.1 Heat and Thermal Equilibrium
X and Y in thermal equilibrium
Rate of transfer of energy from X to Y =
Rate of transfer of energy from Y to X
X Y No flow of heat between X and Y
Fig 3 Temperature of X = Temperature of Y

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2.2 Zeroth Law (law of thermal equilibrium)
Zeroth law of thermodynamics states that if bodies
A and B are in thermal equilibrium with a third
body C, then A and B are in thermal equilibrium
with each other.
C
A
⇒ TA = TB C
B
TA = T C TB = TC

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2.2 Zeroth Law (law of thermal equilibrium)
• The significance of the Zeroth Law:
– It allows us to claim that two objects in
thermal equilibrium with each other must be at
the same temperature.
– It allows us to know whether objects are at the
same temperature, even when we can’t place
them in thermal contact.
– It allows temperature to become reproducible,
and quantifiable. (Temperature can be a
physical quantity)

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2.2 Zeroth Law (law of thermal equilibrium)
In other words
We could create a thermometer to
measure temperature.

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Example 1
A solid X is in thermal equilibrium with a solid Y, which is at the
same temperature as a third solid Z. The three bodies are of
different materials and masses. Which one of the following
statements is certainly true?
A X and Y have the same heat capacity.
B There is no net transfer of energy if X is placed in
thermal contact with Z.
C It is not necessary that Y should be in
thermal equilibrium with Z.
D It is not necessary that X should be at the same
temperature as Z.
Tx = Ty Therefore Tx = Tz Ans: B
Ty = Tz

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2.3 Thermodynamic Temperature Scale
• To measure temperature quantitatively, we need
to have a scale.
Empirical
Scale
2 Scales
Thermodynamic Temperature
Scale
• An empirical temperature scale is a temperature
scale based on experimental results.
– e.g. Centigrade Scale

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2.3 Thermodynamic Temperature Scale
• An empirical scale requires a
thermometric property and two fixed
points (i.e ice point and steam point)
• Thermometric Property:
A physical property that changes in a
known way with temperature.

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2.3 Thermodynamic Temperature Scale
Examples of thermometric properties used in various
thermometers:
Type of Thermometer Thermometric Property
liquid-in-glass length of mercury in a
thermometer capillary tube
Resistance resistance of platinum wire
thermometer
Thermocouple EMF of a copper-
thermometer constantan thermocouple
constant volume gas pressure of a fixed mass
thermometer of gas at constant volume

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Thermomete Constant volume
gas thermometer
rs
Thermocouple Resistance Thermometers
mV
iron
iron
constantan
Junction 2
Junction 1

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2.3 Thermodynamic Temperature Scale
• Problem with empirical
scales based on particular
thermometers:
– scales agree only at
calibration points. Ideal
• A need to have a scale
which is independent of
any thermometric property
(absolute scale).
– reliable
– reproducible

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2.3 Thermodynamic Temperature Scale
• The THERMODYNAMIC temperature
scale is theoretical and is independent
of the properties of any particular
thermometric substance.
• The scale is also known as the Absolute
Temperature Scale.

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2.3 Thermodynamic Temperature Scale
The two fixed points in the Thermodynamic
Temperature Scale are:
(a) absolute zero which is the temperature at
which the pressure of an ideal gas becomes
zero. It is arbitrarily given the value 0 K.
(b) the triple point of water which is the
temperature at which ice, water and water
vapour coexist in dynamic equilibrium.

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2.3 Thermodynamic Temperature Scale
Phase diagram for water

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2.3 Thermodynamic Temperature Scale
Diagram of a triple point cell
Thermometer
well
Water
Dewar
vapour
vessel
Ice Ice water
sheath mixture
Thermal
contact
liquid Water

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2.3 Thermodynamic Temperature Scale
• The triple point of water is chosen
because it
– is unique, invariant and occurs only at one
definite temperature and pressure.
(T = 273.16 K and Pressure = 611.73 Pa)
– can be easily and accurately reproduced
using a triple point cell.

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2.3 Thermodynamic Temperature Scale
The unit of temperature in the thermodynamic scale
is the kelvin, symbol K.
Kelvin is also the S.I. unit of temperature
One kelvin is defined to be 1 of the
273.16
thermodynamic temperature of the triple point of water
1
If, × Ttr = 1 K
273.16
Ttr = 273.16 K

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2.4 The Celsius Scale
The Celsius scale is related to the Thermodynamic
scale by the exact equation:
t/oC = T/K – 273.15
The unit for this scale is degree Celsius, symbol oC
(same symbol as for degree Centigrade).

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Example 2
What is the change in temperature in kelvin when
the temperature falls from 540.85 °C to 502.02 °C?
A 38.83 K
B 311.98 K
C 273.15 K
D 228.85 K
Ans: A

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What we have covered
• Definition of Temperature
– Macroscopic
– Microscopic
• Heat and Thermal Equilibrium
• Zeroth Law and its significance
– Thermodynamic Temperature Scale
– Celsius Scale

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What happen when you heat a substance?
Expands
Temperature increase

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3 Heat Capacity
Heat Capacity
HEAT CAPACITY, C, of a body is defined as the
quantity of heat absorbed / liberated, Q, by the body
per unit temperature change.
Q = C ∆θ
S.I. unit for heat capacity is J K-1

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3 Heat Capacity
Specific Heat Capacity
SPECIFIC HEAT CAPACITY, c, of a material, is
defined as the quantity of heat absorbed / liberated,
Q, per unit mass of the material per unit temperature
change.
Q = m c ∆θ
The S.I. unit of specific heat capacity is J kg-1 K-1
C = mc

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Example 3
The specific heat capacity of copper is 400 J kg -1 K-1.
(a) What is the heat capacity of 5 kg of copper?
(b) If the copper temperature rises by 10 oC, what
would be the heat gained?
(a) heat capacity C = m c
= 5 x 400
= 2,000 J K-1

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Example 3
The specific heat capacity of copper is 400 J kg -1 K-1.
(a) What is the heat capacity of 5 kg of copper?
(b) If the copper temperature rises by 10 oC, what
would be the heat gained?
(b)Heat gained, Q = m c ∆θ
= 5 x 400 x 10
= 20,000 J

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3.1.1 Determination of specific heat capacity, c
(Electrical Method)
For Solid Conductors:
(Insulation)

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By the principle of conservation of energy,
Assuming negligible heat loss to the surroundings.
Electrical energy supplied = Heat absorbed by block
VIt = m c (θ2 - θ1)
V It
c=
m(θ 2 − θ1 )
What happens to the value of c if heat loss is not
negligible?

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By the principle of conservation of energy,
Assuming negligible heat loss to the surroundings.
Electrical energy supplied = Heat absorbed by block
V I t2 = m cL (θ2 - θ1) + h t2
t2 would be longer than t (ideal time taken)
cL = V It 2
m(θ 2 − θ1 )
The calculated cL will be higher than the actual
value c.