State Wiedemann-Franz law. Explain it.
ANSWER : The Wiedemann-Franz Law describes the relationship between the electrical conductivity (σ) and thermal conductivity (κ) of a material. Formulated by Gustav Wiedemann and Rudolph Franz in the mid-19th century, this law highlights the correlation between these two fundamental properties of conductive materials.
Statement of the Law:
The Wiedemann-Franz Law states that the ratio of the thermal conductivity (κ) of a metal to its electrical conductivity (σ) multiplied by the temperature (T) is proportional to a constant value, known as the Lorenz number (L). The formula for this relationship is given by:
\[ \frac{\kappa}{\sigma} = L \cdot T \]
where :
- κ (kappa) is the thermal conductivity of the material.
- σ (sigma) is the electrical conductivity of the material.
- T is the absolute temperature.
- L is the Lorenz number, which is a constant value.
Explanation:
This law suggests that metals with high electrical conductivity also tend to have high thermal conductivity, and vice versa. It implies that the mechanisms governing the flow of electricity and heat in metals are related and dependent on the behavior of free electrons within the material.
In simple terms, materials that conduct electricity well also conduct heat well because both processes rely on the movement of free electrons. Metals generally possess a high density of free electrons, allowing them to conduct both electricity and heat efficiently.
However, deviations from this law can occur in non-metallic conductors or in specific conditions, like at very low temperatures or for materials with highly structured lattices, where the movement of free electrons is restricted.
Importance:
Understanding the Wiedemann-Franz Law helps in the study of material properties and plays a crucial role in various fields such as material science, thermoelectricity, and the development of efficient conductive materials for technological applications, like in electronics and energy production.
In summary, the Wiedemann-Franz Law elucidates the close relationship between electrical and thermal conductivities in metals, highlighting the commonality in the behavior of free electrons that govern both phenomena.
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