Lesson 2.8 Conduction &
Convection
Overview
This lesson deals with heat transfer by conduction and
convection. On completion of the lesson, you should be
able to describe the factors that influence the transfer
of heat by conduction and convection. You should also be
able to use thermal conductivity data to calculate the
rate at which heat is transferred through a solid.
MINI LAB
By holding two objects
– a piece of wood and a metal object - try to
estimate the difference in temperature between a piece of
metal at room temperature and a piece of wood that has
been placed in a refrigerator for some time.
The wood will quickly
"feel" warmer because it has a much lower
thermal conductivity that the metal.
Heat Transfer
Heat can be transferred from one point to another by
conduction, convection and/or radiation. The rate at
which heat is transferred in each of these processes is
related to the difference in temperature between the two
points.
Conductors &
Insulators
The temperature of a material is related to the movement
of the particles that the material consists of. The
average kinetic energy of the particles increases as the
temperature increases. When heat is transferred by
conduction, particles with higher levels of energy
collide with or influence adjacent particles. This causes
an increase in the kinetic energies of the adjacent
particles. These in turn influence other particles
causing energy to be transferred along the material.
Thermal Conductivity
The thermal conductivity of a material is a measure of
its ability to conduct heat. Metals generally have higher
thermal conductivities than non-metals. Solids usually
have better thermal conductivities than liquids and gases
have very low thermal conductivities. Some typical
thermal conductivities are given in the table below.
The ability of our skin to
sense temperature depends very much on the rate at which
our warm skin transfers heat to whatever our skin is in
contact with. Metals feel colder than insulators do at
the same temperature because they absorb thermal energy
from our skin more readily.
Diamonds, for example,
have a relatively high thermal conductivity and absorb
energy rapidly from the fingers of the person holding the
diamond. This makes a diamond feel cold.
Factors That Affect
Conductivity
There are two factors that influence the ease with which
thermal energy moves through a material. Particles that
are closely packed together, as in solids, influence each
other more readily than particles that are further apart
– such as in liquids and gases. Solids are usually
better conductors of heat than liquids. Gases are
generally very poor conductors of heat. Fibrous materials
are generally very good insulators because they trap gas
between the fibers and limit convection currents that
would normally assist the gas to transfer heat. The
material that the fibers consist of does influence the
insulating property of the material. Metal fibers for
example would assist the transfer of heat and be less
efficient in providing insulation.
The bonds that hold the
particles together also influence the ease with which
energy is transferred from particle to particle. If the
particles have strong forces of attraction between them,
they do not move as easily as particles that are not as
strongly held together. To some extent, this limits their
ability to influence adjacent particles and the material
can be expected to be a poorer conductor than one that
has weaker forces of attraction between its particles.
Metals
Metal particles in the solid phase are held together in
such a way that some of the electrons in the outer
orbitals are very loosely bound to the metal atoms. These
electrons can move with relative freedom from atom to
atom and make metals good conductors of electricity.
These electrons are also capable of influencing the
kinetic energies of adjacent atoms with the result that
metals are also good conductors of thermal energy. The
outer electrons of non-metals are tightly bound to the
atoms and as a result, non-metals are generally poorer
conductors of heat.
Metals have a 'sea' of
electrons that move randomly inside them. When one part
of a metal is heated, electrons there move faster and
travel further. As a result they can quickly pass on
their kinetic energy to cooler parts, raising the
temperature.
In non-metals, conduction
occurs because the atoms themselves make 'colder' parts
of the non-metal vibrate. There are no free electrons to
do the conduction quickly, so the process is slow.
Typical
Thermal Conductivities
Material
|
Thermal conductivity
(W.m-1.ēC-1)
|
Silver
|
406.0
|
Copper
|
385.0
|
Brass
|
109.0
|
Aluminum
|
205.0
|
Steel
|
50.2
|
Lead
|
34.7
|
Mercury
|
8.3
|
Glass
|
0.8
|
Concrete
|
0.8
|
Fiberglass
|
0.04
|
Brick
|
0.6
|
Cork
|
0.04
|
Styrofoam
|
0.01
|
Wood
|
0.12-0.04
|
Air at 0 C
|
0.024
|
- Newton's Law Of
Cooling
The rate of cooling of an object is proportional
to the temperature difference between the object
and its surroundings.
Factors That Affect
Heat Transfer by Conduction
Conduction is controlled by 4 things: D T, distance,
area and thermal conductivity.
The rate is directly
proportional to the difference in temperature. Twice as
much heat is transferred if the temperature difference is
doubled.
The rate at which heat
travels by conduction is directly proportional to the
area. In other words, if you have twice the area through
which the heat can be conducted, you have twice the
amount of heat if the temperature difference is the same.
One of the main factors in
heat transfer by conduction is the thermal conductivity
of the material.
Convection in Fluids
The rate at which heat moves through fluids (gases and
liquids) is increased significantly by the physical
migration of particles. Physical movement and mixing of
the material has a significant influence of the rate of
heat transfer. Forced movement and mixing of fluids in
order to improve heat transfer is often referred to as
forced convection.
Natural convection occurs
when the local differences in the densities of different
parts of the fluid create buoyant forces that move less
dense material upward and allow cooler, more dense
material to move towards the source of heat.
Convection currents caused
by flames can be very easily detected. For example, when
a person holds his or her finger above a candle, it is in
the path of the natural convection current caused by the
candle. It will quickly feel hot or get burned. At the
side of the candle’s flame, there is no conduction
or significant convection current that will transfer the
candle’s heat to the person’s finger.
Winds and Weather
Winds are the result of convection currents in the
atmosphere. The simplest illustrations of this are the
breezes that occur near large bodies of water. Water has
a much higher heat capacity than the solid materials on
the earth’s surface. After sunset, the land cools
down faster than the large body of water nearby. The air
in contact with the water is thus less dense than the air
in contact with the cooler land. As a result, the air
moves from above the land towards the mass of water and
creates an offshore breeze.
During the day, air in
contact with the land heats up faster than air in contact
with the water. This causes more dense air to move
towards the land and after noon there is usually an
onshore wind.
Q = (k/d).A.D t : Heat = thermal conductivity x
distance x area x temperature difference
Where: Q = Heat (W)
k = Thermal conductivity of
material (W. m-1.ēC-1)
A = area through which heat travels (m2)
D t
= temperature difference (ēC)
and d = distance that heat
travels (thickness of material) (m)
Example
2.8.1
A cork board is used to close the gap left by a broken
window. The gap has an area of 0.5 m2 and the
thickness of the board is 1 cm. The thermal conductivity
of the board = 0.04 W.m-1.ēC-1
How much heat is lost
through the board if the difference in temperature
between the inside and the outside is 30ēC?
Solution
Using the equation:
Q = (k/d).A.D t
k = 0.04 W.m-1.ēC-1
d = (1/100) m
A = 0.5 m2
D t = 30ēC
The heat transferred: Q =
[0.04 ¸ (1/100)] x 0.5 x 30 = 60 Watts.
Questions
- What is the
difference between internal energy and heat?
- If two solid objects
with different temperatures are in contact, will
heat always flow from the object with the higher
temperature to that with the lower temperature?
- If one region of a
solid object is heated for an extended period,
will heat be transferred to all parts of the
object?
- A person holds the
ends of two metal bars in a fire. Each bar is 50
centimeter long. If one bar consists of copper
and the other steel, which bar will be the first
to feel hot? Why?
- Is it true that
metals are generally better conductors than non-metals?
- How do electrons play
a part in conduction?
- Why do some people
believe that diamonds are cold?
- Are liquids generally
poorer conductors than solids?
- Are there any liquids
that are better conductors than steel? Give an
example.
- Are gases generally
poorer conductors than liquids and solids?
- What is the
difference between a poor conductor and an
insulator?
- Convection occurs in
liquids and gases. What happens when the bottom
of a container with liquid in it is heated at one
end? Why does this occur?
- Which will be less
comfortable: Holding a finger 1 cm away from the
side of a candle flame or holding a finger 5 cm
above a candle flame? Why?
- Why are surfing
conditions usually better in the morning than in
the afternoon?
- Why are fibrous
materials with relatively large spaces between
the fibers generally good insulators?
- Would a layer of
glass wool be a better insulator than a layer of
steel wool with the same thickness? Why?
Answers
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