HEAT ENERGY AND TRANSFER 219(i) Since convection causes hot air to rise it is
important to insulate the roof space, which is
probably the greatest source of heat loss in the
home. This can be achieved by laying fibre-
glass between the wooden joists in the roof
space.
(ii) Glass is a poor conductor of heat. However,
large losses can occur through thin panes
of glass and such losses can be reduced by
using double-glazing. Two sheets of glass,
separated by air, are used. Air is a very good
insulator but the air space must not be too
large otherwise convection currents can occur
which would carry heat across the space.(iii) Hot water tanks should be lagged to prevent
conduction and convection of heat to the sur-
rounding air.
(iv) Brick, concrete, plaster and wood are all poor
conductors of heat. A house is made from two
walls with an air gap between them. Air is
a poor conductor and trapped air minimises
losses through the wall. Heat losses through
the walls can be prevented almost completely
by using cavity wall insulation, i.e. plastic-
foam.
Besides changing temperature, the effects of sup-
plying heat to a material can involve changes in
dimensions, as well as in colour, state and electrical
resistance.
Most substances expand when heated and contract
when cooled, and there are many practical applica-
tions and design implications of thermal movement
as explained in Chapter 20 following.
Now try the following exercise
Exercise 100 Short answer questions on
heat energy- Differentiate between temperature and
heat. - Name two scales on which temperature
is measured. - Name any four temperature measuring
devices. - Define specific heat capacity and name
its unit. - Differentiate between sensible and latent
heat. - The quantity of heat,Q, required to raise
a mass m kg from temperaturet 1 °Cto
t 2 °C, the specific heat capacity beingc,
is given byQ=......- What is meant by the specific latent heat
of fusion? - Define the specific latent heat of vapor-
isation. - Explain briefly the principle of operation
of a simple refrigerator. - State three methods of heat transfer.
- Define conduction and state two prac-
tical examples of heat transfer by this
method. - Define convection and give three exam-
ples of heat transfer by this method. - What is meant by radiation? Give three
uses. - How can insulation conserve fuel in a
typical house?
Exercise 101 Multi-choice questions on
heat energy (Answers on
page 285)- Heat energy is measured in:
(a) kelvin (b) watts
(c) kilograms (d) joules- A change of temperature of 20°Cis
equivalent to a change in thermody-
namic temperature of:
(a) 293 K (b) 20 K
(c) 80 K (d) 120 K- A temperature of 20°C is equivalent to:
(a) 293 K (b) 20 K
(c) 80 K (d) 120 K- The unit of specific heat capacity is:
(a) joules per kilogram
(b) joules
(c) joules per kilogram kelvin
(d) cubic metres - The quantity of heat required to raise the
temperature of 500 g of iron by 2°C,
given that the specific heat capacity is
500 J/(kg°C), is: