Introduction to heat energy transfer by conduction (and thermal conductivity), convection and radiation
Doc Brown's Physics Revision
Suitable for GCSE/IGCSE Physics/Science courses or
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!
2. Heat energy can be transferred
by conduction, convection and radiation
2a. Conduction: Conduction
involves heat transfer by particles vibrating against each other in a solid
or collisions between particles in a gas or liquid. Conduction is the main
mode of heat transfer in a solid.
theory: In a solid the hotter particles vibrate
more strongly, having more kinetic energy (KE) 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 of thermal store.
In other words the higher the temperature of a material the more 'heat
energy' it contains.
The more dense the solid, generally
speaking the better the conductor. In materials where the particles are
further apart the rate of heat transfer (rate of conduction) is reduced eg
gases like air are much poorer conductors than solids like stone.
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. The
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).
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.
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