15.1 Conduction
We will learn
• What is conduction?
• How can we change the amount of
conduction?
• Where do we make use of conduction in the
home?
Energy
Heat
• Heat is a form of energy.
• http://www.bbc.co.uk/schools/gcsebitesize/sc
ience/aqa_pre_2011/energy/heatrev4.shtml
• Heat always travels from hotter places to
cooler places.
• http://www.bbc.co.uk/learningzone/clips/con
duction-convection-and-radiation/10589.html
• In solids heat is transferred by conduction.
Heat is transferred more quickly if
• There is a large temperature difference.
• The object has a large cross section.
• The object is made from a good heat
conductor.
Food
• Cook food at high temperatures
• Store foods at low temperatures.
• Fridges and freezers are lined with insulating
material.
Insulators
•
•
•
•
Materials that do not conduct heat well.
Cloth
Plastic
Wood
Oven gloves
•
•
•
•
Padded cloth
Take hot food from the oven
Air trapped between the fibres
Slows down the heat transfer from the hot
dish to your hand
Saucepans
• Made from metal
• Metals are good heat conductors
• Heat spreads quickly and evenly when it is on
a hob
• Food heats up quickly
• http://www.bbc.co.uk/learningzone/clips/hea
t-transfers/10669.html
Good conductors
•
•
•
•
•
•
Copper?
Plastic?
Aluminium?
Wood?
Carpet?
Iron?
Fish and chips
• Why does wrapping them in paper keep them
warm?
Heating a solid
• Particles vibrate in fixed
positions.
• Vibrate more in the part of the
solid being heated.
• These bigger vibrations pass
energy onto next door particles.
• So energy passes through the
solid, warming up parts further
from the heat source.
Metals
• Good heat conductors
• Particles are in fixed positions, but some
electrons are free to move
• When the metal is heated the free electrons
spread heat quickly through the material.
• http://www.gcse.com/energy/conduction.htm
Non-metals
• Most are good insulators
• Do not have free electrons so conduction is
slow
Materials with trapped air pockets
make good insulators
•
•
•
•
Breeze blocks
Double glazing
Duvets
Padded clothing
Drawing the curtains
• Why does it help keep the house warmer?
How is heat wasted in our homes?
•
•
•
•
Walls
Windows
Floors
roofs
Breeze blocks
• Many homes are built from breeze blocks
• Trapped air in the concrete reduce conduction
Cavity walls
• Many homes
have cavity walls
• Inner wall, outer
wall separated by
a few
centimetres.
• Trapped air
reduces
conduction
• Some people fill
this with foam for
extra insulation.
Double glazed windows
• Two layers of glass with air trapped between
them to reduce conduction.
A sectioned diagram of a fixed
Insulated Glazed Unit (IGU),
indicating the numbering
convention used in this article.
Surface #1 is facing outside, Surface
#2 is the inside surface of the
exterior pane, Surface #3 is the
outside surface of the interior
pane, and Surface #4 is the inside
surface of interior pane. The
window frame is labelled #5, a
spacer is indicated as #6, seals are
shown in red (#7), the internal
reveal is on the right hand side (#8)
and the exterior windowsill on the
left (#9
Carpets and rugs
• Reduce conduction through floors
Loft insulation
• Thick layers
of padding
• Rolled
between the
roof joists
• Reduces
conduction
through the
ceiling
Did you know
• People can walk
safely over hot
coals
• The skin on our
feet and the ash
on top of the coal
conduct heat very
slowly
• This allows people
to walk quickly
over the hot coals
that are about
500 Cͦ
Key points
• Conduction is when heat is transferred
between neighbouring particles
• Metals are good heat conductors
• Non-metals and gases are poor heat
conductors, or good insulators
Look at this
• http://www.bbc.co.uk/learningzone/clips/hea
t-transfers-and-the-vacuum-flask/10670.html
Think about this
• http://www.bbc.co.uk/learningzone/clips/con
duction-of-heat-pt-1-2/231.html
• http://www.bbc.co.uk/learningzone/clips/con
duction-of-heat-pt-2-2/234.html