Wiedemann Franz Law
Wiedemann Franz Law
Wiedemann-Franz law is the law which relates the thermal conductivity (κ) and the electrical conductivity (σ) of a material which consists of somewhat freely moving electrons in it.
- Thermal Conductivity (κ): It is the degree (measure) of capacity of a material to conduct heat.
- Electrical Conductivity (σ): It is the degree (measure) of capacity of a material to conduct electricity.
In metals; when temperature increases, the velocity of free electrons increases and that leads to an increase in heat transfer and it also increases the collisions between the lattice ions and free electrons. This results in the drop in electrical conductivity.
The law defines the ratio of the electronic role of the thermal conductivity of a material to the electrical conductivity of a material (metal) is directly relative to the temperature.
This law is named after Gustav Wiedemann and Rudolph Franz in 1853 reported that the ratio k/σ has more or less the similar value for dissimilar metal at the same temperature.
The thermal conductivity increases with the average particle velocity since that increases the forward transport of energy. However, the electrical conductivity decreases with particle velocity increases because the collisions divert the electrons from forward transport of charge.
Limitations of Wiedemann Franz Law
- The value of L is not the same for all the materials.
- This law is not valid for intermediate temperature.
- In the pure metals, both σ and κ increases as temperature decreases.
Read article – Units of Resistivity
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