2.3– TRANSFEROFTHERMALENERGY
2.THERMALPHYSICS
2.3– TRANSFEROFTHERMALENERGY
Learning Outcomes
Students should be able to:
1. Describe experiments to distinguish between good and bad thermal conductors
2. Describe thermal conduction in all solids in terms of atomic or molecular lattice
vibrations and also in terms of the movement of free (delocalised) electrons in
metallic conductors
3. Explain convection in liquids and gases in terms of density changes and describe
experiments to illustrate convection
4. Describe the process of thermal energy transfer by infrared radiation and know
that it does not require a medium
5. Describe the effect of surface colour (black or white) and texture (dull or shiny) on
the emission, absorption and reflection of infrared radiation
6. Describe how the rate of emission of radiation depends on the surface temperature
and surface area of an object
7. Describe experiments to distinguish between good and bad emitters of infrared
radiation
8. Describe experiments to distinguish between good and bad absorbers of infrared
radiation
PREPARED BY ERWANDI H.
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2.3– TRANSFEROFTHERMALENERGY
9. Explain everyday applications using ideas about conduction, convection and
radiation, including:
(a) heating objects such as kitchen pans
(b) heating a room by convection
(c) measuring temperature using an infrared thermometer
(d)using thermal insulation to maintain the temperature of a liquid and to reduce
thermal energy transfer in buildings
PREPARED BY ERWANDI H.
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2.3– TRANSFEROFTHERMALENERGY
2.3.1-CONDUCTION
The handle of a metal spoon held in a hot drink soon gets warm. Heat passes along the spoon
by conduction.
Conduction: flow of thermal energy through matter from places of higher temperature to
places of lower temperature without movement of the matter as a whole.
1. Describe experiments to distinguish between good and bad thermal conductors
Some materials are good thermal conductors, whereas some are bad thermal conductors (good
insulator). Most metals are good conductors of heat; materials such as wood, glass, cork,
plastics and fabrics are bad conductors i.e. they are good insulators.
Metal objects below body temperature feel colder than those made of bad conductors because
they carry thermal energy away faster from the hand.
Below is an experiment to distinguish the conduction power of various metals:
֎
match
iron rod
using a little melted wax. The other ends are
copper rod
aluminium rod
paraffin wax
A match is fixed to one end of each rod
tripod
heated by a burner.
֎
When the temperatures of the far ends
reach the melting point of wax, the matches drop
off.
brass rod
֎
The best conductor is the one where the
match falls off first (in this case copper).
burner
PREPARED BY ERWANDI H.
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2.3– TRANSFEROFTHERMALENERGY
2. Describe thermal conduction in all solids in terms of atomic or molecular lattice vibrations and
also in terms of the movement of free (delocalised) electrons in metallic conductors
In terms of atomic or molecular lattice vibrations;
֎ When an object is heated up, thermal energy is transferred to particles, making them
vibrate more vigorously (i.e. kinetic energy increases).
֎ These particles then collide with neighbouring atoms.
֎ Energy is thus transferred to these neighbouring atoms.
However, in metals, conduction is faster. This is because, in addition to molecular lattice
vibration, they have a large number of “free-moving” (delocalised) electrons, as illustrated by
the green particles in the diagram below.
In terms of the movement of free (delocalised)
electrons ;
֎ When a metal is heated up, the particles will
vibrate vigorously and strike / collide with
the free-moving electrons.
֎ The electrons move faster (their kinetic
energy increases) and further.
֎ These
electrons
will
travel
to
distant
particles, transferring the thermal energy.
In summary:
Conduction in metals (faster)
Atomic /Molecular Lattice Vibration
Conduction in non-metals (slower)
Atomic /Molecular Lattice Vibration
Movement of electrons
PREPARED BY ERWANDI H.
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2.3– TRANSFEROFTHERMALENERGY
WORKEDEXAMPLES
1
How is heat conducted in a metal?
A. by movement of electrons through the metal only
B. by movement of atoms through the metal only
C. by vibration of atoms and movement of electrons through the metal
D. by vibration of atoms only
ANS: C.
2
A metal can and a plastic bottle, both containing liquid, are cooled by placing
melting ice, as shown in Fig. 4.1.
them
themininaa jug
jug of
of
jug
melting ice
Fig. 4.1
The liquid in the metal can cools down faster than the liquid in the plastic bottle.
Suggest why this happens.
ANS: Metal is a good conductor of heat - it carry thermal energy away faster.
3
Describe and explain, in terms of free electrons, how thermal energy is transferred
through the copper base of the saucepan.
Thermal energy is transferred to the particles, making them vibrate more [1]
...........................................................................................................................................
These particles will strike the free-moving electrons [1]
...........................................................................................................................................
These electrons will gain energy, thus travelling to and hitting distant particles,
...........................................................................................................................................
transferring the thermal energy. [1]
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[3]
PREPARED BY ERWANDI H.
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2.3– TRANSFEROFTHERMALENERGY
2.3.2-CONVECTION
3. Explain convection in liquids and gases in terms of density changes and describe experiments
to illustrate convection
Convection is the usual method by which thermal energy travels through fluids (liquids and
gases).
Convection: flow of thermal energy through a fluid from places of higher temperature to
places of lower temperature by movement of the fluid itself.
Let’s look at what happens to
a. Fluids are heated up at the
b. The particles expand -
fluids when they are heated
bottom
their
up.
volume
increases.
Therefore, density decreases
c. The less dense / heated
d.
The
denser
particles will rise up
particles will sink
/ cooler e. This movement of fluids
due to difference in its
density sets up a convection
current
PREPARED BY ERWANDI H.
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2.3– TRANSFEROFTHERMALENERGY
Experiment to Illustrate Convection in Liquids.
֎ Drop a few crystals of potassium permanganate down a
tube to the bottom of a beaker / flask of water.
֎ The beaker is heated just below the crystals
֎ Purple streaks of water rise upwards and fan outwards.
Experiment to Illustrate Convection in Gases.
A laboratory demonstration of convection currents
smoke
in air can be given using the apparatus in the
opposite diagram.
lighted
touch paper
glass chimneys
֎ Touch paper is lighted, producing smoke.
box
֎ The smoke is directed down the chimney.
֎ The direction of the convection current
created by the candle is made visible by the
smoke from the touch paper
lighted
candle
glass
window
WORKEDEXAMPLE
When a refrigerator is switched on for the first time, the air surrounding the ice-box is cooled.
ice-box
What happens to the density of this air and to its position inside the refrigerator?
density
position of the air
A
decreases
sinks to the bottom
B
decreases
stays at the top
C
increases
sinks to the bottom
D
increases
stays at the top
ANS: C
PREPARED BY ERWANDI H.
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2.3– TRANSFEROFTHERMALENERGY
WORKEDEXAMPLE
A copper saucepan with a wooden handle contains cold water. The saucepan is placed on a
red-hot heating element that is a part of an electric cooker.
Fig. 4.1 shows the saucepan on the heating element.
water
saucepan
wooden handle
heating element
Fig. 4.1
Describe and explain how thermal energy is transferred upwards through the water.
....H
...e..a..te
...d...w..a..t.e..r..e
..x..p..a..n..d..s...a..n..d...i.s...le
..s..s...d..e..n..s..e...[.1..]..................................................................
The heated / less dense water will rise [1]
...........................................................................................................................................
Denser / colder water will sink [1]
...........................................................................................................................................
2.3.3-RADIATION
4. Describe the process of thermal energy transfer by infrared radiation and know that it does not
require a medium
Whereas conduction and convection both need matter to be present, radiation can occur in a
vacuum (region of space void of any particles of matter).
֎ Thermal radiation is infrared radiation and it does not require a medium.
֎ It travels with the speed of light and has all the properties of electromagnetic waves
(topic 3.3)
All objects emit radiation – when it falls on an object; it is partly reflected, partly transmitted
and partly absorbed.
o
An object can cool down as it emits radiation
o
An object can heat up as it absorbs radiation
PREPARED BY ERWANDI H.
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2.3– TRANSFEROFTHERMALENERGY
5. Describe the effect of surface colour (black or white) and texture (dull or shiny) on the
emission, absorption and reflection of infrared radiation
8. Describe experiments to distinguish between good and bad absorbers of infrared radiation
shiny
surface
electric
heater
Some surfaces absorb radiation better than others,
dull black
surface
as may be shown in this experiment. The inside
surface of one lid is shiny and the other dull black.
The coins are stuck on the outside with candle wax.
coin
The heater is midway so each lid receives the same
candle
wax
tin lid
amount of radiation.
After a few minutes the wax on the black lid melts
and the coin falls off. The shiny lid stays cool and
the wax unmelted.
֎ Dull black surfaces are better absorbers of radiation than shiny white surfaces.
֎ Shiny white surfaces are good reflectors of radiation.
5. Describe the effect of surface colour (black or white) and texture (dull or shiny) on the
emission, absorption and reflection of infrared radiation
7. Describe experiments to distinguish between good and bad emitters of infrared radiation
Some surfaces also emit radiation better than others
when they are hot.
hot copper sheet with one
side polished and the
other blackened
back of hands
towards sheet
One side of a hot copper sheet is polished and the other
side blackened.
If you hold the backs of your hands on either side, it
will be found that the dull black surface is warmer i.e.
it emits more heat.
֎ Dull black surfaces are better emitters of radiation than shiny white surfaces.
֎ If dull black surfaces emits more heat (loses more heat), they will cool down faster.
PREPARED BY ERWANDI H.
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2.3– TRANSFEROFTHERMALENERGY
Absorber
Emitter
Dull black surfaces
Better (heats up faster)
Better (cools down faster)
Shiny white surfaces
Poor (heats up slower)
Poor (cools down slower)
WORKEDEXAMPLES
1.
The tubes inside solar heating panels use the Sun’s radiation to warm water.
Why are the tubes painted black?
A.
Black surfaces absorb radiation well.
B.
Black surfaces conduct heat well.
C. Black surfaces emit radiation well.
D. Black surfaces reflect radiation well.
ANS: A.
Make sure you understand the question! Even though black surfaces emit radiation well (c),
the tubes are painted black because we want to heat up the water – and so we want it to
absorb more radiation.
2.
A slice of bread is placed under a red-hot electric grill to make toast.
grill
slice of bread
How does heat energy reach the bread?
A. conduction and convection
B. conduction only
C. convection and radiation
D. radiation only
ANS: D.
In convection, thermal energy is carried upwards (heated particles rise because they are less
dense). Since the slice of bread is beneath the grill, the heated gas particles cannot reach the
bread via convection.
PREPARED BY ERWANDI H.
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0
2.3– TRANSFEROFTHERMALENERGY
6. Describe how the rate of emission of radiation depends on the surface temperature and surface
area of an object
Rate of Coolling
A thermometer is placed in hot water. The temperature is recorded every
30s as the water cools away.
A student recorded the following as the water is cooled in air and plotted a
temperature against time graph:
Time/s
0
30
60
90
120
150
180
Temp/oC
80
63
51
42
36
31
28
Temperature falls down
faster (more emission)
If the surface temperature of an object is higher than its surroundings, it emits radiation at a
faster rate than it absorbs radiation from its surrounding i.e. it cools down. From the
experiment above, radiation is emitted faster when the temperature is higher.
Rate of cooling down / rate of emission is affected by two factors:
֎ Surface temperature: the higher it is, the greater the quantity of radiation it emits (it
cools down faster)
֎ Surface area: the higher it is, the greater the quantity of radiation it emits (it cools
down faster)
PREPARED BY ERWANDI H.
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1
2.3– TRANSFEROFTHERMALENERGY
9. Explain everyday applications using ideas about conduction, convection and radiation,
including: (a) heating objects such as kitchen pans (b) heating a room by convection (c) measuring
temperature using an infrared thermometer (d) using thermal insulation to maintain the
temperature of a liquid and to reduce thermal energy transfer in buildings
2.3.4– CONSEQUENCESOFTHERMALENERGYTRANSFER
a) Heating objects such as kitchen pans
Good conductors are used whenever heat is required to travel quickly through something.
Saucepans, boilers, and radiators are made of metals such as
copper which are all good conductors that transfer thermal
energy quickly.
The handles of saucepans are made of insulators (bad
thermal conductors) so thermal energy does not flow easily.
b) Heating a room by convection
Convection currents set up by electric, gas and oil heaters help to warm
our homes.
Warm air produced by the heater rises (less dense), and the cold air
sinks. The cold air will then be warmed up by the heater and a convection
current is set up.
c) Measuring temperature using an infrared thermometer
An infrared thermometer detects the thermal radiation emitted by an
object and converts it into an electrical signal.
PREPARED BY ERWANDI H.
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2.3– TRANSFEROFTHERMALENERGY
d) Using thermal insulation to maintain the temperature of a liquid and to reduce thermal
energy transfer in buildings
Thermal insulators are used when a slow transfer of thermal energy is required.
A polysterene cup will help keep hot liquids warm or cold
liquids cool.
Single-glazed
Double-glazed
Air is one of the worst conductors (best insulator). This is why
houses with cavity walls (two layers of bricks separated by an
air space) and double-glazed windows keep warmer in winter
and cooler in summer i.e. thermal energy is not transferred
away easily.
Materials that trap air, such as wool, felt, fur, feathers,
polystyrene foam, fibreglass, are also poor conductors. Some
of these materials are used to insulate water pipes, hot water
cylinders, ovens, refrigerators and the walls and roofs of
houses so thermal energy is not easily lost.
PREPARED BY ERWANDI H.
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2.3– TRANSFEROFTHERMALENERGY
EXAM-STYLEQUESTIONS
1
(a) No thermal energy (heat) is transferred from the surface of the Sun to the Earth by
either conduction or convection.
Explain why this is so.
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(b) In a certain country, the climate is very sunny and hot during the day and extremely cold
during the night.
Explain how painting the houses white helps to maintain a comfortable temperature
both during the day and during the night.
during the day: .................................................................................................................
..........................................................................................................................................
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during the night: ...............................................................................................................
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[3]
PREPARED BY ERWANDI H.
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