3 Thermal Physics (11h)
© Kari Eloranta
2015
Jyväskylän Lyseon lukio
International Baccalaureate
November 10, 2015
© Kari Eloranta 2015 (Jyväskylän Lyseon lukio 3International
Thermal Physics
Baccalaureate)
(11h)
November 10, 2015
1 / 10
3.1 Thermal Concepts
3.1 Thermodynamic System and Phases of Matter
In this topic we study the physical properties of thermodynamic
systems.
The physical behaviour of a thermodynamic system is described by
four state variables: pressure p , volume V , amount of substance n ,
and temperature T .
In thermodynamics, the phases of matter are: solid, liquid and gas.
In solids, atoms and molecules have fixed positions. A solid maintains
a fixed shape and size.
In liquids, atoms and molecules move easily inside the liquid. A liquid
takes the shape of its container, but it is not readily compressible.
In gases, atoms and molecules move randomly in all directions in
space. The particles interact very weakly with each other. A gas
expands and fills its container and is easily compressible.
© Kari Eloranta 2015 (Jyväskylän Lyseon lukio 3International
Thermal Physics
Baccalaureate)
(11h)
November 10, 2015
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3.1 Thermal Concepts
3.1 Absolute Temperature T
Matter particles are in constant motion. The faster they move, the
higher the temperature of an object.
Absolute Temperature T
Absolute temperature (measured in kelvins) is a measure of average kinetic
energy of the molecules in a substance.
Conversion from degrees Celsius to kelvins
If the temperature t is measured in degrees Celsius, the temperature in
kelvins is
t
T
+ 273.15
=
K ◦C
(1)
A change in absolute temperature equals the change in degrees
celsius. For example, if the temperature of air rises from 0 ◦C to 20 ◦C
(∆t = 20 ◦C), the change in absolute temperature is
∆T = (20 + 273.15)K − (0 + 273.15)K = 20 K.
© Kari Eloranta 2015 (Jyväskylän Lyseon lukio 3International
Thermal Physics
Baccalaureate)
(11h)
November 10, 2015
3 / 10
3.1 Thermal Concepts
3.1 Absolute Temperature T
Temperature is a statistical quantity. In a container filled with gas, the
velocity of gas molecules vary. The average kinetic energy of gas
molecules, however, is proportional to the absolute temperature.
The lower limit of temperature is absolute zero: 0 K=−273.15 ◦C, in
which the thermal motion of matter particles would have ceased.
The cosmic microwave background radiation is the thermal radiation
left from the "Big Bang". It corresponds to 2.73 K.
Temperature measured in degrees Celsius is denoted by t or θ in a
case where t is used for time (such as measuring the temperature of
an object as a function of time). Absolute temperature is usually
denoted by T .
We speak about degrees Celsius and kelvins, not degrees Kelvin.
© Kari Eloranta 2015 (Jyväskylän Lyseon lukio 3International
Thermal Physics
Baccalaureate)
(11h)
November 10, 2015
4 / 10
3.1 Thermal Concepts
Thermal Energy Q
Thermal Energy
Energy that is transferred between a system and its surroundings by
non-mechanical means is called thermal energy.
Definition of Thermal Energy
Thermal energy is energy that is transferred from one system to another
due to difference in temperature.
Thermal energy relates always to energy transfer between systems.
When two systems are in thermal contact with each other, thermal
energy flows from hotter system to the colder system.
Definition of Temperature
Temperature is a quantity that defines the direction of thermal energy flow.
© Kari Eloranta 2015 (Jyväskylän Lyseon lukio 3International
Thermal Physics
Baccalaureate)
(11h)
November 10, 2015
5 / 10
3.1 Thermal Concepts
Internal Energy U
Transfer mechanisms for thermal energy are conduction, convection
and radiation.
In many textbooks, heat is the synonym for thermal energy.
Definition of Internal Energy of a System
Internal energy of a system is the total potential energy and random kinetic
energy of the molecules comprising the system.
Internal Energy of Substance
Internal energy of a substance is the total potential energy and random
kinetic energy of the molecules of the substance.
Potential energy refers to energy associated to intermolecular forces.
In ideal gases the forces are negligible (internal potential energy is
zero). In liquids and solids the internal potential energy cannot be
neglected.
© Kari Eloranta 2015 (Jyväskylän Lyseon lukio 3International
Thermal Physics
Baccalaureate)
(11h)
November 10, 2015
6 / 10
3.1 Thermal Concepts
3.2 Closed System
A closed system can exchange energy, but cannot exchange matter
with its surroundings.
If the system is heated, the internal energy of the system increases.
The experiments have shown that for homogeneous systems, if no
phase changes occur inside the system, the temperature change ∆T of
the system is directly proportional to the thermal energy Q transferred
to the system.
© Kari Eloranta 2015 (Jyväskylän Lyseon lukio 3International
Thermal Physics
Baccalaureate)
(11h)
November 10, 2015
7 / 10
3.1 Thermal Concepts
3.2 Thermal Capacity C
Definition of Thermal Capacity
Thermal capacity is the energy required to raise the temperature of a body
by 1K.
Equation of Thermal Capacity
The thermal capacity C of a body is
C=
Q
∆T
(2)
where Q is thermal energy transferred into the body, and ∆T the absolute
temperature change of the body.
© Kari Eloranta 2015 (Jyväskylän Lyseon lukio 3International
Thermal Physics
Baccalaureate)
(11h)
November 10, 2015
8 / 10
3.1 Thermal Concepts
3.2 Thermal Capacity C
Equation of Thermal Capacity
The thermal energy Q transferred into a system is proportional to the
change in the absolute temperature of the system ∆T
(3)
Q = C ∆T,
where the proportionality constant C is the thermal capacity of the system.
Thermal capacity is a property of a system.
© Kari Eloranta 2015 (Jyväskylän Lyseon lukio 3International
Thermal Physics
Baccalaureate)
(11h)
November 10, 2015
9 / 10
3.1 Thermal Concepts
3.2 Specific Heat Capacity c
Definition of Specific Heat Capacity
Specific heat capacity is the energy per unit mass required to raise the
temperature of a substance by 1K.
Equation of Specific Heat Capacity
The specific heat capacity c of a substance is
c=
Q
m∆T
(4)
where Q is thermal energy transferred into the system, m mass of the
object made of a particular substance, and ∆T the absolute temperature
change of the object.
© Kari Eloranta 2015 (Jyväskylän Lyseon lukio 3International
Thermal Physics
Baccalaureate)
(11h)
November 10, 2015
10 / 10
3.1 Thermal Concepts
3.2 Specific Heat Capacity C
Equation of Thermal Capacity
The thermal energy Q transferred into a homogeneous system is
proportional to the mass of the system, and the change in the absolute
temperature of the system ∆T
(5)
Q = cm∆T,
where the proportionality constant c is the specific heat capacity of the
system.
Thermal capacity is a property of the substance the system is made of.
© Kari Eloranta 2015 (Jyväskylän Lyseon lukio 3International
Thermal Physics
Baccalaureate)
(11h)
November 10, 2015
11 / 10