There are a number of factors that affect how energy flows through an object. A very important factor is what the object is made from.
The conductivity of materials can be compared by examining the time taken to transmit energy through them. A fan of rods made of different materials can be heated at one end with the same flame. Whichever rod gets hottest first at the other end is the best conductor. The material that heats the quickest is said to have a high thermal conductivity.
Some typical values of conductivities are:
|Material||Thermal conductivity (watts per metre per degree Celsius (W/m/°C))|
This means that 385 joules (J) of energy will flow per second through a cubic block of copper (1 m × 1 m × 1 m) when the temperature difference between its sides is 1°C.
When trying to keep houses warm, the choice is between materials that are poor conductors such as brick, wood, plastic and glass. A house built of conducting materials like copper would be very cold to live in as energy would be able to leave the house easily.
Referring to the table of conductivities above, why is it better to have a window made of two layers of glass with a layer of air trapped between them?
Both glass and air are insulators because they have low thermal conductivities. The layer of air has the lowest thermal conductivity and reduces the overall conductivity of the window unit. Since air and glass are both transparent people can still see through the window.