**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^{-1}K^{-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^{-1}K^{-1})

is the surface area perpendicular to the heat flow (m^{2})

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
^{-1}K^{-1} - copper = 386 Wm
^{-1}K^{-1} - gold = 315Wm
^{-1}K^{-1} - aluminium = 204 Wm
^{-1}K^{-1} - concrete = 1 Wm
^{-1}K^{-1} - water = 0.6 Wm
^{-1}K^{-1} - wood = 0.15 Wm
^{-1}K^{-1} - air = 0.025 Wm
^{-1}K^{-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 0.15m^{2} with one end at 20°C and the other end at 55°C?

b) how much energy is transferred in 250 seconds?

**Answer:**

a) = 386Wm^{-1}K^{-1}

= 0.15m^{2}

= 55 − 20 = 35°C

= 70cm = 0.7m

using:

Js^{-1}

b)

J