CHAPTER 7
TRANSFER OF THERMAL
ENERGY
Overview on IGCSE Syllabus
Core:
Conduction
1. Describe experiments to demonstrate the properties of good thermal conductors
and bad thermal conductors (thermal insulators)
2. Describe thermal conduction in all solids in terms of atomic or molecular lattice
vibrations and in terms of the movement of free (delocalized) electrons in metallic
conductors
3. Describe, in terms of particles, why thermal conduction is bad in gases and most
liquids
4. Know that there are many solids that conduct thermal energy better than thermal
insulators but do so less well than good thermal conductors
Convection
1. Know that convection is an important method of thermal energy transfer in liquids
and gases
2. Explain convection in liquids and gases in terms of density changes and describe
experiments to illustrate convection
Radiation
1. Know that thermal radiation is infrared radiation and that all objects emit this
radiation
2. Know that thermal energy transfer by thermal radiation does not require a medium
3. Describe the effect of surface color (black or white) and texture (dull or shiny) on
the emission, absorption and reflection of infrared radiation
4. Know that for an object to be at a constant temperature it needs to transfer energy
away from the object at the same rate that it receives energy
5. Know what happens to an object if the rate at which it receives energy is less or
more than the rate at which it transfers energy away from the object
6. Know how the temperature of the Earth is affected by factors controlling the
balance between incoming radiation and radiation emitted from the Earth’s surface
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
9. Describe how the rate of emission of radiation depends on the surface temperature
and surface area of an object
Consequences of thermal energy transfer
1. Explain some of the basic everyday applications and consequences of conduction,
convection and radiation, including:
(a) heating objects such as kitchen pans
(b) heating a room by convection
2. Explain some of the complex applications and consequences of conduction,
convection, and radiation where more than one type of thermal energy transfer is
significant, including:
(a) a fire burning wood or coal
(b) a radiator in a car
Khizar Yousaf
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CHAPTER 7
TRANSFER OF THERMAL ENERGY
Heat transfer:
Heat is the transfer of energy from a warmer object to a cooler object.
Heat can be transferred in three ways: by conduction, by convection, and
by radiation.
Thermal Conduction:
Conduction is the transfer of energy from one molecule to another by
direct contact.
How does Conduction Occur?
When a material is heated, the particles move faster, push on neighboring
particles, and speed those up too. All materials conduct like this but, in
metals, energy is transferred by another, much quicker method.
In atoms, there are tiny particles called electrons. Most are firmly attached,
but in metals, some are loose and free to drift between atoms. When metal is
heated, these free electrons speed up. As they move randomly within the
metal, they collide with and make them vibrate faster. In this way, thermal
energy is rapidly transferred to all parts.
Thermal conductor and insulators:
•
•
•
Conduction is the main method of thermal energy transfer in
solids
Metals are extremely good at conducting heat because of free
electrons
Non-metals solid are the poor conductor of heat like wood, plastic,
and rubber these are also called insulators.
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CHAPTER 7
TRANSFER OF THERMAL ENERGY
These experiment shows which metal is a best conductor
Comparing four good thermal
conductors. Ten minutes or so
after the boiling water has been
tipped into the tank, length of
melted wax shows which
material is the best conductor.
This experiment shows that
water is a poor thermal
conductor. The water at the top
of the tube can be boiled without
the ice melting.
Poor conductor or thermal insulator:
A thermal insulator or insulators of heat prevents thermal energy from
passing through them. e.g. plastic, rubber.
Air is the worst conductor of heat. Many materials are insulators because
they contain tiny pockets of trapped air.
Using insulating material:
Insulating materials are used to reduce heat
losses from a house:
1. Plastics from lagging round the hot
water storage tank.
2. Glass or mineral wool insulation in the
loft.
3. Wall cavity filled with plastic foam,
beads, or mineral wool.
4. Double glazed windows: two sheets of
glass with air between them.
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CHAPTER 7
TRANSFER OF THERMAL ENERGY
Convection:
Convective heat transfer refers to the movement of thermal energy within
fluids. Convection occurs due to the bulk movement of molecules within the
liquid, gas, or liquid-gas mixture. Initially, heat is transferred between
molecules via conduction, but the main heat transfer is via the motion of
molecules within the fluid.
Natural convection examples:
•
Hot air rising above a fire
•
Ice melting
•
Sea breeze or land breeze caused by a difference in pressure
•
Blood circulation in warm-blooded animals
Convection in a liquid
Convection in a liquid can be seen by putting a crystal of potassium
permanganate in a beaker of water and gently heating it with a Bunsen
flame.
•
Heat is initially transferred through the glass wall of the beaker
by conduction.
•
The water in the region of the Bunsen
flame is heated.
•
It expands, becomes less dense and rises.
•
It is replaced by the cooler, denser water
which surrounds it.
•
This water is in turn heated, expands
becomes less dense and rises.
•
The process continues, a convection current is set up and heat is
transferred through the liquid.
Convection in a gas:
•
•
•
•
It expands, becomes less dense and rises.
It is replaced by the cooler, denser air which
surrounds it.
This air is in turn heated, expands becomes less
dense and rises.
The process continues, a convection current is set up
and heat is transferred through the air and hence through the room.
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CHAPTER 7
TRANSFER OF THERMAL ENERGY
As the ice melts, it cools the water around it. Given that cold
water is denser than hot water, I would presume that the cold
water would sink to the bottom … but it would warm as it
sinks, reducing the density. Meanwhile, the ice is still melting
and giving off its cold to the surrounding water
Sea breeze:
Thermal Radiation:
Thermal radiation, transmitted as electromagnetic wave energy, is a form of
heat transfer generated by any matter having a temperature higher than
absolute.
In contrast to conduction and convection, thermal radiation has two unique
properties:
(1) it does not require a medium to be transferred
(2) during the emission/absorption of radiation, there is transfer between
electromagnetic energy and kinetic energy of atoms, namely, thermal energy.
Emitter and absorber:
Some surfaces are better at emitting thermal radiation than others. For
example, a black saucepan cools down more quickly than a similar white
one because it emits energy at a faster rate.
Good emitters of thermal radiation are also good absorbers. White and
silvery surfaces are poor absorbers because they reflect most of the thermal
radiation away. That is why, in hot, sunny countries, houses are often
painted white to keep them cool inside.
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CHAPTER 7
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