Thermal Conductivity

Different materials have different conducting properties. Thermal conductivity measures the rate at which thermal energy can flow through a substance. It is dependent on the type of substance, the cross-sectional area and the thickness of the substance as well as the temperature difference across the object/s.  The unit of thermal conductivity is Wm-1K-1.

Calculations involving thermal conductivity: where: is the rate of heat energy transferred (Js-1) and (W) is the thermal conductivity of the material (Wm-1K-1) is the surface area perpendicular to the heat flow (m2) is the temperature difference across the material (°C or K) is the thickness of the material through which the heat is being transferred (m)

Thermal conductivities of some common materials:

• silver = 420 Wm-1K-1
• copper = 386 Wm-1K-1
• gold = 315Wm-1K-1
• aluminium = 204 Wm-1K-1
• concrete = 1 Wm-1K-1
• water = 0.6 Wm-1K-1
• wood = 0.15 Wm-1K-1
• air = 0.025 Wm-1K-1

Example 1:

a) What is the rate that energy is transferred through a 70cm long piece of copper with a cross-sectional area of 0.15m2 with one end at 20°C and the other end at 55°C?

b) how much energy is transferred in 250 seconds?

a) = 386Wm-1K-1 = 0.15m2 = 55 − 20 = 35°C = 70cm = 0.7m

using:   Js-1

b)  J