thermal conductivity – a material's ability to conduct heat
To remember the meaning of thermal conductivity, use the following mnemonic:
In an experiment, we tested the thermal conduct activity (thermal conductivity) of several materials and compared their ability to conduct heat.
Thermal conductivity is a measure of how well a material can conduct heat, describing the rate at which thermal energy transfers through a substance. Materials with higher thermal conductivity such as metals like copper, gold and silver allow heat to flow through them quickly and efficiently.
Metal feels cold to the touch because they are excellent conductors rapidly transferring heat away from your warmer skin. Heat moves from higher temperature regions to lower temperature regions until thermal equilibrium is reached. A metal spoon and a wooden spoon in a cold room are at the same temperature but the metal spoon feels much colder because it removes heat from your hand faster.
In contrast, materials with a low thermal conductivity, known as thermal insulators, such as wood, plastic, wool, and air, slow down the transfer of heat and are used in applications where preventing heat loss or gain is important, like insulation for houses and clothing.
Every material can be sent to a laboratory to find its thermal conductivity which is a measure of how well a material can conduct heat.
The thermal conductivity of a material is a measure which describe the rate at which thermal energy transfers through a substance.
A brick might have a thermal conductivity value of 0.5 w/mk
This means that in a steady state, 0.5 Joules per second (watts) of heat energy will pass through one metre thick of this material when there is a steady temperature difference of one degree Celsius across the thickness. The material assumes a standard surface area of 1 square metre but in practice the units are expressed as w/mk and the M stands for 1 metre thick of material only.
The rate that a brick wall in a house will transfer energy largely depends on how thick the brick wall is. So instead of 1 metre thick, a normal may only be 200mm thick (0.2m).
We can work out the rate of heat transfer per one degree Celsius across any thickness of material. This quantifies the rate of heat transfer through any building element. They call this a U value. U is for unit value and indicates that it's a measure of heat transfer per unit area per degree difference.
`\text(U value each material) = (\text (K) (\text(thermal conductivity)))/(\text(x mm) (\text(thickness material))`
U value for brick wall A:
`\text(U) = (0.5 \text(w/mk))/(0.2 metre) = 2.5 \text(w/m)^2 \text(k)`
U value for brick wall B:
`\text(U) = (0.5 \text(w/mk))/(1 metre) = 0.5 \text(w/m)^2 \text(k)`
U value for brick wall C:
`\text(U) = (0.5 \text(w/mk))/(2 metre) = 0.25 \text(w/m)^2 \text(k)`
So you can see that the thinner the wall the more heat loss there is per 1 metre square of surface area.
Temperature
If the 200mm brick wall divided the outside winter, at -5° from an inside room at 20° we can work out the heat loss from the room to the outside. We use:
Q (heat loss) watts = U value x area (1m2) x Δt
Q (heat loss) watts = 2.5 x 1m2 x (20 - -5)
= 2.5 x 1 x 25
= 62.5 watts
Answer: 1m2 of this wall loses 6.25 Joules of energy per second (watts).
If you add up all the 1m2 of heat loss in a house you can then work out the size of your boiler to heat the house.
This can all be achieved by knowing the thermal conductivity of materials.
Thermal conductivity is therefore measured in watts per metre kelvin. This is the materials ability to conduct heat, defined as the rate of heat transfer (watts) through a material per unit thickness (metre) per unit temperature different (kelvin). This is expressed in w/mk.
For example, copper's thermal conductivity is measured at 385 wm/k/ This means that 385 joules of energy will flow per second through a cubic block (1m x 1m x 1m).
In comparison brick has a thermal conductivity brick has a thermal conductivity of only 0.5 wm/k, meaning that only 0.5 joules of energy would flow per second tough a cubic metre of brick, showing that it is a much worse conductor than copper, but a better insulator.
