Introduction to heat energy transfer by conduction (including thermal conductivity), convection and radiation
school physics revision notes: GCSE physics, IGCSE physics, O level
physics, ~US grades 8, 9 and 10 school science courses or equivalent for ~14-16 year old
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sources, types, properties, uses, including infrared dangers
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Introduction to heat transfer
Heat transfer by
conduction - movement of fluid
Heat transfer by convection -
vibration of particles
Heat transfer by
thermal radiation (electromagnetic infrared radiation)
More on applications of heat transfer
science including the thermos flask
to heat transfer
Energy can be transferred
from one place to another by work or by heating processes.
You need to know
and understand how this
energy is transferred and which heating processes are most important in a
When energy is transferred to an
object by some means or other, the energy is stored in the object's energy
stored. This energy store may be thermal (dealt with here), chemical,
kinetic, magnetic, gravitational potential, elastic potential or nuclear.
Here we are interested in energy
transfer by heating (conduction, convection) and infrared (radiation).
Heat energy must always flow
from hotter material at a higher temperature to cooler material at a lower
temperature and the bigger the temperature difference the bigger the rate of
heat energy transfer.
eg the greater the temperature
difference between a body and its surroundings like a hot object (eg mug of
coffee) standing in a cold room, the faster the heat energy is transferred
from the hotter material to the cooler material (eg surrounding air).
Systems, thermal energy stores and
states of matter:
System is a word that means a particular
object or objects that is being looked at in a particular context eg boiling
water in a kettle.
When a system changes, energy is
transferred into or out of the system, this may be between different
objects/materials in the system or perhaps between different energy stores
(same of different).
Closed systems are systems that do not
allow energy to leave or enter (lose or gain) so the ne change in the total
energy is zero.
When an object/material is heated to
raise its temperature, the thermal energy store of the object is increased.
This thermal energy is stored throughout
all the material by increasing the kinetic energy (KE) stores of the
material's individual particles eg the KE particle vibration in a solid and
the KE of the rapid movement of the freely moving particles in a liquid or
The thermal KE is distributed by either
the particles vibrating against each other in a solid or the collisions
between the freely moving particles in a gas or liquid. Higher KE particles
will on average lose energy to lower KE particles - that's the way energy
The higher the temperature of particles
the greater their average kinetic energy so they will vibrate more violently
in a solid and move faster in gases and liquids.
Once heat energy has stopped being
supplied to an object, it will distribute itself evenly to give a uniform
temperature throughout the material by conduction or convection. However, if
object's/material's surroundings are at a lower temperature, then heat
energy will drain from this thermal store until its temperature has fallen
to that of the surroundings - that's the way heat flows!
The amount of thermal energy transferred
can be calculated from the formula
Particle theory models, internal
energy, heat transfer in state changes,
latent heat and particle motion in gases
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2. Heat energy can be transferred
by conduction, convection and radiation
2a. Heat transfer by conduction
- vibration of particles
theory: In a solid the hotter particles vibrate more
strongly, having a greater kinetic energy (KE) store and bang into
neighbouring cooler lower KE particles and so transfer KE to them, so
heat energy is transferred from a higher temperature region to a cooler
region in any solid material. This kinetic energy of vibration is
referred to as thermal energy store. In other words the higher the
temperature of a material the more 'thermal
energy' it contains.
The more dense a material, generally
speaking the better the conductor - the faster heat is conducted.
Most non-metallic materials are poor
conductors (good heat insulators) eg water, fat (in animals), wood,
stone, concrete, plastics - particularly foams where poorly heat
conducting gases are trapped giving even better insulation that the bulk
solid plastic - and cheaper by using less material.
Metals are particularly good
conductors because of free moving electrons - a different heat transfer
mechanism to that described here, which applies to all solids. Because the
electrons are free to move in the solid metal, they can rapidly transfer
kinetic energy by particle movement. The 'hot' electrons in the higher
temperature region collide with neighbouring cooler electrons and so rapidly
transferring heat energy (KE) - much faster than vibrating atoms in
non-metals which are held in fixed positions.
Incidentally if you pick up a
cold poor conductor like a stone and then pick up an equally cold metal
object at the same cool temperature, the metal object feels colder (but it
isn't) because it conducts heat from your fingers faster than the stone!
Water in an electric kettle is a two
part system, but even though the kettle contains the water, it is NOT a
closed system because electrical energy is coming in, and being changed
to heat energy by the electrical resistance of the heating element.
heat energy conducts through the heating element and into the water
whose temperature rises as its thermal energy store increases. However,
the heat is then transferred to all the water by convection currents
coming from the hottest least dense water by the heating element which
then rises and circulates around (see convection below).
BUT, you cannot stop some thermal
energy escaping from the kettle by conduction through the case,
convection through top opening and radiation from the kettle's surface.
These heat losses will be minimised by the design of the kettle e.g.
In a toasted sandwich maker the heat is transferred to the bread by
conduction. In the toaster the electrical energy is converted into heat and the
thermal energy stores of the toaster and the sandwich are increased to
effect the cooking.
A measure of good/bad a material is
at conducting heat is called its thermal conductivity.
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2b. Heat transfer by convection
- movement of fluid
Heating a gas or liquid increases its
thermal energy store which will distributes itself through the kinetic
energy stores of the particles. Although from the point of heating,
conduction will be slow, most heat will be transferred to the bulk of
the fluid by convection.
Convection occurs when
hotter/warmer less dense fluid (gas/liquid) flows and rises, is replaced by cooler
more dense fluid moving (flowing) downwards.
This cycle of events is called a
and its 'mechanism' is explained below.
a material is heated the particles have more KE, move faster and tend to
push each other further apart, ie the material expands, becoming less dense.
It is this change in density that cause convection to happen as gases and
liquids are fluids - they can flow in convection currents.
This is how hot water is
produced in the hot water tank in the home, or the heating of water in a
kettle where convection currents flow from the heating element so enabling
all the water to be heated up. Note that the heating element must be near
the bottom of the tank or kettle to produce the convection circulation to
heat up all the water! If you put the heating element at the top there is no
convection and all you do is heat up the top layer of water!
Despite the name, radiators on
the walls heat rooms up mainly by convection (there is some radiation too).
Heat is transferred to the air particles when they collide with the
radiator, and warm less dense air rises from the radiator towards the ceiling.
The elevated air cools as the heat is distributed to the cooler air,
which falls on the
other side of the room. At the same time cooler air is drawn in at the base of the
radiator to replace it - hence you get a convections current situation that
gradually warms up all of the room.
The hot gases from a gas fire will always rise due to the immediate
formation of a rising convection current (recycling air flow) which carries the heat
around the room. The chemical energy store of the fuel gas increases the
thermal energy of the contents of the room.
The energy is transferred in several
(i) The hot flame gases will heat the
immediate surrounding air by conduction.
This will cause lots of hot air to
rise towards the ceiling.
Heat is dissipated to the contents of
the room and surroundings and the risen air begins to cool and become
The more dense air falls and
eventually circulate back round near the fire where cool air is drawn in
to the fire.
The process repeats itself
continuously as long as the fire is lit - hence the formation of the
convection current flow of air.
(ii) The heat is also
transferred by infrared radiation emitted from the hot flame - from the
higher temperature flame to the lower temperature regions of the room.
Hot water or electrical radiators
work in exactly the same way.
The surrounding air next to the
radiator is heated by conduction.
The thermal energy from the hot water
or electrical element heats the particles of radiator case.
The increased kinetic energy of the
hot metal/concrete particles is transferred to the air particles in
contact with the radiator i.e. those that collide with the radiator.
The air particles move faster and
spread out to decrease the density.
The less dense air rises to start the
convection current and are replaced by cooler air coming up and passing
by the radiator surface.
Again, some energy will be
transferred to the room from the radiator surface by infrared radiation
- you can feel this for yourself by placing your hand near, but not
touching the radiator.
More on methods of reducing heat transfer
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2c. Heat transfer
by thermal radiation (electromagnetic infrared radiation)
Thermal (heat) radiation
is emitted by all materials, gases, liquids or solids and the hotter the
material the more strongly it gives out heat radiation which is called
infrared radiation (IR).
(a) All objects continuously emit and absorb
infrared radiation from their surface, whatever their temperature
- increasing or decreasing their thermal energy stores.
(b) The hotter an object
is the more infrared radiation it radiates in a given time, the higher the
temperature of the material, the more intense is the infrared radiation.
An object that is hotter (higher
temperature) than its surroundings will emit more radiation than it absorbs
and an object that is cooler than its surroundings will absorb more
radiation than it emits.
You notice this effect in bright
sunlight by feeling the warmth on your hand or standing near a fire.
When an object cools down to the
same temperature as its surroundings emitted infrared radiation equals the
absorbed heat radiation.
(c) Dark, matt surfaces
are good absorbers and good emitters of infrared radiation eg rough black
Solar panels for hot water
comprise of pipes carrying water to be heated set under a black surface to
efficiently absorb the infrared radiation from the Sun. You can even just
use matt black painted water pipes. You may even have a silvered surface
under the pipes so more infrared ins reflected onto the black surface rather
than becoming waste heat radiation. The pipes are made of copper which
allows efficient conduction of the surface heat energy to the incoming cold
water., so the hot water can be used as part of the households domestic
heating or washing etc.
(d) Light, shiny surfaces are poor
absorbers and poor emitters of infrared radiation eg white gloss paint,
silver surface used in vacuum flask. See 'thermos flask'
in section (c).
(e) Light, shiny surfaces are good reflectors
of infrared radiation, this maybe to keep heat in to keep things warm or to
minimise heat radiation in to keep things cool eg a vacuum flask. See 'thermos flask'
in section (c).
(e) Another domestic case of infrared radiation!
Unlike 'modern' LED bulbs, 'old fashioned' filament bulbs emit quite a
bit of IR heat radiation. You can detect this with a frosty car where
the central portion of the ice melts first on the transparent headlamp cover. Filament bulbs only
convert ~10% of the electrical energy into visible light energy, most of the
rest is converted into infrared radiation. The ice on the headlamp cover absorbs infrared
equivalent to the latent heat of fusion (melting) and changes the ice to liquid
Energy store changes: The chemical
energy store of the battery decreases as it is converted into
electrical energy. The electrical energy increases the thermal
energy store of the metal filament of the bulb. The thermal energy
store of the filament decreases as it emits visible and infrared EM
radiation. The absorbed EM radiation increases the thermal energy
store of the headlamp cover and ice - causing the latter to melt.
Eventually all the energy involved from the battery increases the
thermal energy store of the surroundings.
A 'green' note: If there is, and
its happening now in the UK and other countries, a change from
very inefficient filament light bulbs to very efficient low
energy LED light bulbs, there will be quite a reduction in the
domestic demand for electricity. This reduced demand will help,
on closure fossil fuel power stations, reduce CO2
emissions, reducing the greenhouse effect, and allow renewable
energy resources to take over more of our electricity
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3. More on applications of heat transfer
More on methods of reducing heat transfer eg in a house
and investigating insulating properties of materials
of good insulators OR good conductors
The thermal conductivity of a
material is a measure of efficiently heat is transferred through a
material by conduction.
Materials like metals are very good
heat conductors and transfer thermal energy very quickly.
Materials like stone, brick, wood and
concrete etc. are poor heat conductors and have low thermal
Thermal conductivity data is
important when considering the material required to fulfil a particular
application e.g. in heating systems when in one situation you might want
good insulation (e.g. in loft) and in another rapid heat transfer
(copper piping inside a hot water tank).
The design of a vacuum flask and other examples of a 'thermos flask'
The 'thermos' vacuum flask is a container
designed to keep hot liquids hot and cold liquids cold. Diagram on the
It is designed to minimise thermal energy
(heat) transfer by conduction, convection or radiation, both in an out of
The flask is double walled with a vacuum
(of nothing!) between the walls.
The vacuum ensures there is no thermal
energy transfer by conduction - no material to vibrate.
If the double-walled flask is made of
glass, the inner surfaces exposed to the vacuum are silvered to reflect back
any thermal radiation (infrared).
If the flask is steel, the surfaces are
shiny and reflect infrared in the same way.
The top cap should be a poor conductor of
heat, and is often made of plastic or incorporates a plastic seal.
Both double walls and the cap help
minimise losses by convection. The design ensures no air can pass over any
surface that is in contact with the fluid.
Insulated cups, flasks and jugs come in
all sizes - illustrated by the pictures below.
A double walled plastic jug
Several steel vacuum flasks
Plastic insulated coffee cup,
sometimes with an added cardboard hand holder for extra insulation.
Your knowledge of examples of
heat transfer situations should include the ...
design of a vacuum flask
how to reduce the energy
transfer from a building
how humans and other animals
cope with low temperatures
Homeostasis - thermoregulation, control of temperature
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