# How fast is the thermal conductivity

## Thermal conductivity

You have probably already noticed that different substances conduct heat at different speeds. This is due to the thermal conductivity and that is exactly what this post is about.

• Definition: thermal conductivity
in the text
• Table: Characteristic values ​​of thermal conductivity
in the text
• Mechanisms of heat conduction in different groups of substances
in the text
• Calculation of the heat flow
in the text

### Definition: thermal conductivity

The thermal conductivity , also Thermal conductivity or Coefficient of thermal conductivity called, measures the ability warmth within a substance - i.e. a solid, a gas or a liquid - to transport.

Your unit W / mK indicates the amount of heat in watts per hour or J / s by 1m² a meter thick material, for one Temperature difference is transmitted by one Kelvin.
Mostly increases as the temperature rises, so does the Thermal conductivity of a material easily. However, as soon as there is a phase transition or a modification of State of aggregation comes - for example from solid to liquid - the thermal conductivity often changes suddenly.

### Table: Characteristic values ​​of thermal conductivity

You can find a few here Characteristic values ​​for thermal conductivity divided into solids, gases and liquids.

materiallambda in W / mK
SOLIDS
aluminum200-240
concrete2,1
diamond2300
stainless steel15
Glass0,75-1,0
rubber0,16
Wood0,13
copper380-400
Brass120
steel42-58
Brick masonry0,5-1,4
GASES
air0,024
hydrogen0,186
LIQUIDS
water0,6
oil0,13

### Mechanisms of heat conduction in different groups of substances

The thermal conductivity is dependent of the porosity, the Water content and the density.
The phenomenon of Conduction in compact, non-metallic solids is largely based on the mechanical coupling of neighboring atoms and the associated transition from Vibrational energies. The rule of thumb therefore applies that substances with a low bulk density are small Thermal conductivity coefficients exhibit and substances with high bulk density large.

At Metals it is also the Conduction electronsthat besides electric charge also mechanical thermal energy transport.

However, it is different with the liquids. Here it is convectionwhich prevails if the temperature differences are sufficiently large.

Let’s consider last Gases. With them it's them Moleculesthat are responsible for the transport of energy - kinetic, Vibration- and Rotational energy - are responsible. Heavy molecules don't move as fast as light molecules. This also explains the difference in the thermal conductivity of hydrogen and air.

The low thermal conductivity of gases is a popular attraction Multi-pane insulating glass windows advantage. However, one must note that other heat transfer methods such as Thermal radiation and convection should be restricted as far as possible. This can be achieved, for example, by using the optimal distance between them of the panes in order to suppress the convection for the static layer of air between the panes.

### Calculation of the heat flow

We set ourselves one l = 25cm long cuboid with cross-sectional area A. There is one along the entire length of the cuboid Temperature difference of . Measure the height of the cuboid 5 cmthat measures width 4 cm.
We also assume that our cuboid completely insulated is. This means that the cuboid does not lose any energy through the side surfaces. The condition is stationary and the material is isotropice.g. copper. Consequently the flows Heat flow across cross section A.

The heat flow can be calculated using the formula

determine. The Thermal conductivity of copper is included .
With this we get:

So the heat flow is included 192 W.

Now you know what thermal conductivity is and how it can be explained and calculated.