This table shows common materials used in buildings. They are arranged in their correct order of THERMAL CONDUCTIVITY. Those at the top allow heat to pass through them quickly. Those at the bottom allow heat to pass through them much more slowly.
Copper – Good Conductor, Poor Insulator
Aluminum
Iron
Glass
Concrete
Brick
Water
Wood
Cork
Still Air – Poor Conductor, Good Insulator
What is meant by Thermal Conductivity?
A measure of the speed at which heat travels through a material.
If a blowlamp flame is directed against of steel not only does the area in front of the flame become hot but also in a short time, the entire piece of steel becomes hot. Heat travels through all materials but some will allow it to travel through quickly while through others it may be very slow. This heat movement is known as COONDUCTION.
Materials, which allow fast movement of heat, are known as good Conductors. When the movement is slow they are called Poor Conductors. Copper is a very good conductor of heat and is called a material of high TTHERMAL CONDUCTIVITY (Thermal means heat). In comparison, still air is a poor conductor of heat so has a low thermal conductivity.
The amount of heat, (Q), transmitted between the surfaces of a slab of material of uniform thickness and composition varies:
- Directly with the area (A)
-
Directly with the temperature difference between the faces (2-1)
- Directly with the time of flow (T)
- Inversely with the thickness (D)
- According to the amount of the material.
These relationships combine to form the equation
Q=kAT(2-1)
D
Where k is a ‘constant’ for the particular material, known as the Thermal Conductivity. The direction of heat flow will be from the surface at the higher temperature (2 in the equation) to that at the lower temperature (1). From the equation we see that Q will be (in S.I. units) the heat flow in joules through an area 1m2, in one second, for a temperature difference between the faces of 1 deg C and for thickness of 1m. The equation rearranges to give the thermal Conductivity:
K=Qd
AT(2-1)
And from this we can conclude that the ‘k’ value of a material is expressed in S.I units of W/m deg C (the reduced form of jm/m2s deg C which is first obtained). The thermal conductivity is an important property of material – for example with a very low ‘k’ value could usefully provide insulation against heat losses. Typical values of ‘k’ for different materials when dry are given in figure that ive produced (thermal conductivity increases with moisture content).
Find how much heat is conducted per second through 1m2 of sheet glass, 5mm thick, when the temperature difference between the faces is 10 deg C. (k=1.05W/m deg C)
Table of thermal Conductivity and Resistivity
(Typical Values for dry Materials)
Q=kAT(2-1)
D
=1.05x1x1x10 =2100J
0.005
This is a rate of heat loss of 2100W (2100J/s) per square meter.
