B.1 Thermal Energy
Transfers
B. The
Particulate
Nature of
Matter
Syllabus Content
Syllabus Content
Syllabus Content
Syllabus Content
Thermal
Energy Transfer
• Demonstration - Balloon
• What is going on inside the balloon?
• Why does it keep its shape?
• What makes matter solid, liquids, or gases?
What are their properties?
Thermal
Energy Transfer
6 assumptions in the
Kinetic Model/Kinetic Molecular Theory:
1.Gases are composed of a large number of particles that behave like hard,
spherical objects in a state of constant, random motion.
2.These particles move in a straight line until they collide with another particle
or the walls of the container. Particles are in constant random motion.
3.These particles are much smaller than the distance between particles. Most of
the volume of a gas is therefore empty space.
Thermal
Energy Transfer
6 assumptions in the
Kinetic Model/Kinetic Molecular Theory:
4.There is no force of attraction between gas particles or between the
particles and the walls of the container.
5.Collisions between gas particles or collisions with the walls of the
container are perfectly elastic. None of the energy of a gas particle is lost
when it collides with another particle or with the walls of the container.
6.The average kinetic energy of a collection of gas particles depends
on the temperature of the gas and nothing else.
Thermal
Energy Transfer
Properties
of Solids,
Liquids, and
Gases
https://phet.colorado.edu/sims/html/states-of-matter-basics/latest/states-of-matter-basics_en.html
Phases of Matter and Their Properties
Density
When solids and liquids are heated to higher
temperatures, they typically undergo a slight
expansion (as long as the container allows that to
happen) as the particles within them experience a
small increase in separation. Consequently, this
results in a minor reduction in their densities.
Typical
Values for
Densities
Thermal
Energy Transfer
Answer questions
1-5, p178
Thermal Energy
Transfer
HEAT
TEMPERATURE
Using a Venn diagram, write the differences between
heat and temperature based on the given
characteristics below
A. Transfer of thermal energy between objects due to
difference in temperature
B. Measures the average kinetic energy of the
particles.
C. Form of energy
D. Not a form of energy
E. Cannot be measured directly
F. Can be measured directly
G. Unit J/kgoC
H. Joule (J)
I. Deals with energy and matter
J. Dependent on molecules
Thermal Energy
Transfer
HEAT
Answer
TEMPERATURE
C
A
J
E
G
F
B
D
H
I
A. Transfer of thermal energy between objects due to
difference in temperature
B. Measures the average kinetic energy of the
particles.
C. Form of energy
D. Not a form of energy
E. Cannot be measured directly
F. Can be measured directly
G. Unit Joule (J)
H. Unit OC or K or OF
I. Deals with energy and matter
J. Dependent on molecules
Thermal Energy
Transfer
The resultant flow of thermal energy is always from higher
temperature to lower temperature
Thermal Equilibrim
All temperatures within a system are constant.
If there is no net transfer of thermal energy between A and
B, then they must be at the same temperature.
Figure B1.5 Two objects (A and B) at different temperatures, insulated from their
surroundings but not from each other, will reach thermal equilibrium
Thermal Energy
Transfer
Celsius (scale of temperature):
Temperature scale based on the melting
point (0°C) and boiling point (100°C) of pure
water.
Kelvin scale of temperature:
Also known as the absolute temperature
scale. The kelvin, K, is the fundamental SI
unit of temperature. T (in K) = 0 °C + 273.
Absolute zero
Temperature at which (almost) all molecular
motion has stopped (0 K or −273 °C).
Thermal Energy
Transfer
• Answer questions 8-11 on
page 182
TASK
Thermal Energy
Transfer
All gases, at the same temperature, contain particles
with the same average translational kinetic energy
The particles in most gases are molecules, which means
that they also have other forms of kinetic energy (not
just translational), for example, rotational kinetic energy
and vibrational kinetic energy
Thermal Energy
Transfer
Particles can have potential energy as well as kinetic energy.
In solids and liquids, it is the electrical forces (between charged particles) that keep particles from
moving apart or moving closer together. Wherever there are electrical forces there will be electrical
potential energy in a system.
Thermal Energy
Transfer
Answer questions 12-15 on
page 184
TASK
Thermal
Energy
Thermal energy is the transfer of energy because of a temperature
difference: a net flow from hotter to colder.
Three principal ways in which thermal energy can be transferred:
• Thermal conduction. In which kinetic energy is transferred between
particles.
• Convection. In which differences in the densities of liquids and gases result in
their movement.
• Thermal radiation. In which electromagnetic radiation is emitted by surfaces.
Thermal
Conduction
Thermal conduction occurs because of the transfer of kinetic energy
between particles.
Solids are generally better thermal conductors than liquids, and liquids conduct better
than gases considering the closeness of particles and the strength of forces between them
Thermal
Conduction
A larger number means that the
substance is better at conducting
thermal energy: more energy is
transferred under similar conditions.
(Metals are good conductors because
they contain many free / delocalized
electrons.)
Thermal
Conduction
Task
Answer questions
16-20 page 187
Quantitative
treatment of
thermal
conductivity
k is a constant, different for each substance.
It is called the thermal conductivity of the substance (as
shown in Table B1.4). Unit:Wm−1K−1.
∆𝑸
is a flow of energy per second (a power) so it is
∆𝒕
measured in watts.
Insulation
The best insulator for limiting
thermal energy flowing out of, or
into, homes is air.
If the air can move, thermal
energy can also be transferred by
convection currents .
• Parallel sheets of glass (known as double glazing), can be used to trap air and limit
thermal energy flow through a window.
• Double glazing has the added benefit of reducing the transfer of sound.
Task
Task
Answer questions
24-25 page 190
Thermal
Convection
• When part of a fluid (gas or liquid)
is heated, there will be a localized
decrease in density.
• Because of increased buoyancy,
the warmer part of the fluid will
then rise and flow above the
cooler part of the fluid, which has
a slightly greater density.
• This movement of thermal energy
in a fluid is called thermal
convection.
Group Task (Poster)
• In pairs, explain how convection
happens in:
• Group 1: Ovens
• Group 2: Air conditioners
• Group 3: heaters/radiators
• Group 4: sea and land breeze
• Group 5: Thunderstorm
• Group 6: Refrigerators
• You will be assessed according to the
following criteria:
• Accuracy of the explanation [3
marks]
• Diagrams/illustrations [3 marks]
• Ability to answer questions [3
marks]
Task
Answer questions 27-29
p191
Thermal
Radiation
Thermal radiation
Electromagnetic
radiation
emitted
because of the movement of charged
particles in the atoms of all matter at all
temperatures. Most commonly, infrared.
Infrared Radiation
is an electromagnetic radiation emitted by
all objects (depending on temperature) with
wavelengths longer than visible light
Convection
http://www.pinterest.com/pin/452400725040236330/
During daytime, as
the Sun shines,
land heats up more
quickly than the
sea. This results in
air being warmed
and rising. This
warmer air is
replaced by cooler
air coming in from
the sea.
The reverse happens during nighttime. The sea remains warmer than the land,
which cools down more quickly. Above the sea the warm air rises, and it is
replaced by cooler air coming in from the land.
Convection
Convection in the home
1. As the convector
heater gets warmer, it
heats air immediately
above it.
1
http://imgarcade.com/1/air-convection-currents/
Convection
Convection in the home
1. As the convector
heater gets warmer, it
heats air immediately
above it.
2. The warm air rises,
carrying thermal
energy all around the
room.
2
1
http://imgarcade.com/1/air-convection-currents/
Convection
Convection in the home
1. As the convector
heater gets warmer, it
heats air immediately
above it.
2. The warm air rises,
carrying thermal
energy all around the
room.
3. As the air cools, so it
falls towards the floor
before being heated
again.
2
1
http://imgarcade.com/1/air-convection-currents/
3
Convection
Convection in the home
http://chowlaiwan-physicsproject2009.blogspot.co.uk/
In the refrigerator, the cool air sinks below the freezer compartment. This
sets up a circulating current of air which cools all food in the refrigerator.
Thermal
Radiation
The power of the emitted radiation from any surface depends on:
1 Surface temperature.
The radiated power is proportional to the fourth power of the
surface temperature (in kelvin), T4.
This means, for example, a metal bar at 600 K (323 °C) will emit
24 = 16 times as much radiation as the same bar at 300 K (23 °C).
2 Surface area
The radiated power is proportional to the area, A.
Note that the emitted power is not
dependent on the chemical nature
of the material.
3 Nature of the surface
Thermal
Radiation
Black bodies
• Black surfaces
emit and absorb
radiation well;
• white surfaces are
poor at absorbing
and emitting.
https://www.youtube.com/watch?v=BBwULYIOkfk
Black-body emission spectra at three different
temperatures
The total power, P, emitted (across all wavelengths)
from a perfect black body of surface area A can be
calculated from the Stefan–Boltzmann law:
σ is known as the Stefan–Boltzmann constant.
It has the value of 5.67 × 10−8Wm−2K−4
Thermal
Radiation
Thermal
Radiation
Wien’s displacement law
Relationship between absolute temperature and the wavelength emitted with maximum
power by a black body at that temperature
https://www.youtube.com/watc
h?v=qjM73TlVkTo
Thermal
Radiation
Task
Answer questions 32 – 35 p196
Thermal Radiation and Stars
Luminosity of a star (or other body),
L=
4
σAT
◆ Luminosity (stellar) Total power of electromagnetic radiation emitted by a star
(SI unit: W).
Thermal Radiation and Stars
Thermal Radiation and Stars
We can write Polaris’s luminosity as L = 1600 L⊙
Thermal Radiation and Stars
Task
Betelguese is our nearest red giant star. It has a luminosity
of 4.49 × 1031 W and emits radiation with a peak
wavelength of 850 nm.
Calculate the ratio of the radius of Betelgeuse rB to the
radius of the Sun rs.
Radius of the Sun rs = 6.95 × 108 m
Answer
Apparent Brightness, b
and Intensity
https://www.youtube.com/watch?v=GnFF
NcJ5Aks
Apparent Brightness, b
and Intensity