E NE RG Y MAT TER S
FACT SHEET 3: HEAT
Around 20-25% of energy consumption in
industrialised countries is used in the
heating and cooling of buildings. In
households, the figures is as high as 75%.
It is therefore an area where substantial
savings in energy consumption can be
made. A short introduction to heat, its
properties, behaviour and means of
transfer is appropriate, so that the various
energy conservation processes centred on
heat can be understood.
What is heat?
Heat was originally thought to be an actual
substance, known as caloric, which when
transferring from a hot body to a colder
one, would increase its temperature
(which it does) and also its mass (which it
doesn’t).
In the 18th century, it was found that
heat is a form of energy. The English
physicist James Joule found that an
increase in the temperature of a beaker of
water could be achieved by either adding
heat from a candle, or by performing work,
for example, by turning a paddle-wheel in
it.
At the atomic level, heat is indicated
by increased movement of the atoms or
molecules, so it is like a form of invisible
kinetic energy.
Heat and temperature
We tend to think of heat and temperature
as the same thing, but they aren’t.
Temperature is a measurable property of a
substance, in the same way as shape or
colour. A hot object is one that has a
higher temperature than another.
Heat is the internal property of an
object that is externally measurable as
temperature.
EXERCISE 1
Does an object with a higher temperature
have more heat energy than one with a
lower temperature?
EXERCISE 2
When heat energy is added to an object,
its temperature usually increases. What is
the exception?
Apart from the exception outlined above,
there is a simple relationship between heat
energy added (or removed), as shown
below.
∆T =
Q
mc
where ∆T is the change in temperature, Q
is the amount of heat of energy added or
removed, mass is the mass of the object
and c is the heat capacity of the
substance.
The heat capacity, also known as the
specific heat, is the amount of energy
required to raise 1 mass unit (g, kg, lb etc)
by one degree (Celsius or Fahrenheit). In
the unit for specific heat in Table 1, the
specific heat is the heat energy in Joules
required to raise 1 kg of the substance by
1°C. This means that if you had 1 kg of
water and 1 kg of iron, it would take almost
10 times as much energy to raise the
temperature of the water by the same
amount as the iron.
(b)
What temperature would be reached
by 2 L of water at room temperature
(20°C), if heated for 1 minute by a
2000W kettle? Assume 80% of the
energy is converted to heat that is
put into the water.
TABLE 1 Heat capacities of some common
materials (J/kg °C)
Substance
Heat
Capacity
Water
4186
Ice
2090
Air
1004
Concrete
960
Aluminium
900
Iron
448
Wood
2800
Ethanol
2460
EXAMPLE 1
A 500 g block of aluminium at room
temperature (25°C) is heated by burning
fuel which generates 20 kJ of energy.
What temperature is reached?
The unit for heat capacity requires energy
in J (20 kJ = 20000 J) and mass in kg (500
g = 0.5 kg). Using the equation on the
previous page:
∆T =
20000
= 44.4 °C
0.5 x 900
Therefore the final temperature would be
69.4°C.
EXERCISE 3
(a) Which of the substances in Table 1
would show the (i) greatest and (ii)
least temperature rise given the
same amount of energy and the
same mass?
The rate of temperature change for a
given object depends not only on the rate
of heating (or cooling), but also on the
mass of material and its specific heat.
Just as a material with a high heat
capacity increases most slowly, the rate at
which it cools will also be slower. For
example, when you remove a pie from the
oven, the contents of the pie, which have a
high proportion of water, will remain hot
for much longer than the aluminium foil.
Building materials with high heat
capacity take a long while to heat up and
also lose heat more slowly, stabilising the
temperature inside.
EXERCISE 4
(a) Have you ever noticed how hot the
tomato in a toasted sandwich stays?
Why should this be?
(b)
Which of the following objects will
increase in temperature at the (i)
fastest and (ii) slowest rate, given
the same rate of heating?
• 500 g of water
• 250 g of iron
• 250 g of aluminium
• 500 g of aluminium
• 1 kg of ice
• 2 kg of air
Heat transfer
There is one absolute truth about the
relationship between heat and
temperature:
Heat energy will ALWAYS flow from a
hot object to a colder one, regardless
of the sizes of the two objects.
EXERCISE 5
If a cup of hot coffee and a cup of iced
coffee are placed on a table and left, what
happens?
While all materials can conduct heat, some
– for example, metals – are far better than
others. In general, conduction only occurs
to any great extent in solids, because this
is the state where atoms and molecules are
closest together.
The rate at which heat is conducted
through a material is dependent on:
• the type of material
• the difference in temperature in the
two objects
• the surface area of contact
• the thickness of the material
Rate of conduction =
k A (T2 − T1 )
w
where A is the surface area of contact, T1
and T2 the temperatures of the two objects,
w the thickness of the material through
which the heat is being transferred and k
the thermal conductivity of the material.
How does the heat go from one object to
another? There are three basic
mechanisms for the transfer of heat:
• conduction
• convection
• radiation
TABLE 2 Thermal conductivities of selected
materials
Substance
Thermal
Conductivity
Water
0.35
Glass
0.5
ANALOGY
To make the difference between the three
forms simpler, imagine how you could get
a cricket ball from one end of the room to
the other.
Air
Conduction
If you hold a metal spoon in a cup of hot
coffee, fairly quickly you get a burning
sensation in your fingers, because the heat
in the drink has transferred itself along the
metal.
This is known as conduction, and is
where the heat energy moves through the
collisions of atoms and molecules,
“bumping” into one another and so
transferring the energy along the material.
Wood
0.06-0.08
Rock
0.8-2.4
ANALOGY
Passing the ball from one person to the
next.
Concrete
0.015
0.5-0.75
Aluminium
136
Iron
47
The division between metals and other
materials, in terms of heat conduction, is
very clear. Non-metals are known as
insulators.
EXERCISE 6
(a) A small mammal, such as a mouse,
eats almost 20 times the mass of
food (on a per kg body weight basis)
as a human? Part of the reason is
the need to maintain body
temperature. Why should this be
so?
(b)
Given that the conductivity of air is
much less than solid insulators, why
bother purchasing insulation “batts”
for the ceiling and wall spaces?
Convection
In a gas or liquid, the atoms or molecules
are too far apart for effective conduction
by collision. Movement of the liquid or gas
occurs through differences in density
between hot and cold zones. A hot liquid
(or gas) is less dense than a cool one, and
therefore will rise. In doing so, the cool
material will sink, therefore setting up a
convection current, moving the heat away
from the heat source.
Radiation of heat energy – also called
infrared radiation – occurs for any
material above absolute zero. The higher
the temperature, the more rapid the rate of
radiation. This type of heat transfer does
not depend on temperature differences,
though a cooler object may heat up
because it is absorbing more heat than it is
radiating. Therefore, a bitumen road on a
hot day absorbs heat from the sun, and retransmits some of that back into the
atmosphere. Once the sun goes down, the
tar continues to radiate heat until it loses
enough to reach an equilibrium
temperature with the surroundings.
ANALOGY
Throwing the ball to the other end of the
room.
A oil-filled radiator provides an example of
all three types of heat transfer (given the
name of the device, it is perhaps not
surprising which is the main source of
heating):
• heat energy is conducted through the
metal from the oil to the air in contact
with the radiator (or your hand if you
touch it)
• the motion of the warmer air via
convection currents caries heat around
the room
• the hot radiator radiates heat directly
See Figure 1 for a comparison of the three.
ANALOGY
Carrying the ball from one end of the room
to the other.
Radiation
Conduction and convection both require
matter – known as the propagating
medium – to allow heat transfer. If all
heat transfer required a medium, then the
earth would be a very cold place indeed,
since the heat component of solar
radiation would never reach us through
the near vacuum that is space.
FIGURE 1 Heat transfer modes (from
http://www.williams.edu/resources/sustainability/
index.php )