Definition of Thermal Conductivity

All the materials can transfer or conduct heat as a natural quality or nature. The symbol of kinetic energy is mainly denoted as “k”. The thermal conduction unit helps express the unit of thermal conductivity of materials with some dimensions. All the measurements or tests are represented with the SI unit.

The SI unit of thermal conductivity of materials is calculated with the dimensional formula of thermal conductivity. To get the unit of thermal conduction, people need to follow the mathematical calculation method for thermal conductivity. The thermal conduction unit is expressed using power as the main term.

How is thermal conductivity measured?

The thermal conductivity is measured with a specific mathematical formula. All the materials have different natural and heat-producing characteristics. Therefore, the thermal conductivity of all the materials would vary. Molecular movement is one of the vital components to measure the thermal conductivity of materials.

Loosely packed materials will have lower thermal energy as compared to tightly packed materials. The following formula is used for the measurement of thermal conductivity.

The formula for thermal conductivity is,

Q/t= kAΔT/d

Q stands for representing the amount of heat transferred

t sands for representing the time taken for the amount of heat transfer

A stands for the cross-sectional area of any material.

ΔT stands for the difference in temperature of both sides of the material.

d stands for the thickness of the heat-producing material.

The measurement of thermal conductivity can be done in two ways. These two ways are Steady-state techniques and transient techniques. Let’s understand each of these techniques properly to get a better knowledge about thermal conductivity and unit of thermal conductivity.

Steady measurement techniques

In these techniques, the materials used for measurement need to be in their constant or steady state. This means the material will not change its nature and state over time. The temperature stays constant for the materials in steady measurement techniques.

The analysis of thermal conductivity is easy in the steady measurement techniques as no change in temperature happens. A professional setup is required for the steady measurement techniques. Lee’s disc method is an example of steady measurement techniques.

Transient measurement techniques

The transient measurement techniques occur when the heating process is happening. In such a measurement process, the measurements are considerably taken faster. The analysis of data is difficult in transient measurement techniques. All the samples should not be in their constant or steady form. The temperature is not constant in the transient measurement techniques.

Both techniques have their pros and cons. You can try out the best techniques according to your preferences. The thermal conductivity analysis of solids is easier than fluid due to their molecular structures.

Unit of thermal conductivity

Watt per metre Kelvin is the Standard International unit of thermal conduction unit. Joseph Fourier brought up the concept of thermal conductivity. Joseph used various mathematical techniques for finding the thermal conductivity of materials. There is a specific dimensional formula calculated with some factors that affect the thermal conductivity.

The formula (dimensional formula) for the thermal conduction unit is M¹L¹T^-2ϴ^-1. The thermal conductivity is measured or represented in the form of an imperial unit. The imperial unit of thermal conductivity is BTU.h^-1.ft^-1.F^-1.

Summary:

• The SI unit of thermal conduction unit is represented as M¹L¹T^-2ϴ^-1.
• The imperial unit of thermal conduction is represented as BTU.h^-1.ft^-1.F^-1

Factors affecting thermal conductivity

Materials

The energy flow gets affected by the materials. Energy flow is directly proportional to the thermal conductivity of materials. This means the energy flows increase with an increase in the thermal conductivity.

Length

The energy flow of any material gets affected by the length of materials. If the length of the material is longer than the energy flow will be slower. The short-length materials will have a faster flow of energy.

The short-length materials will see a faster increase in thermal conductivity. The rate of thermal concavity in long materials may be slow but happens.

Difference in temperature

The temperature of the material plays a huge role in predicting the thermal conductivity of materials. Temperature differences will directly affect the thermal conductivity of the material.

The materials with huge temperature differences will see a faster increase in thermal conductivity. The difference in temperature increases the energy flow and thermal conductivity.

Form of materials

The form of materials affects thermal conductivity. There is some material with very high thermal conductivity. The thermal conductivity of gases is lower than solids. The main reason behind the low thermal conductivity of gases is due to their molecular movement and structure of materials.

The gases have loose-packed molecules which is the main reason behind their low thermal conductivity. The thermal conductivity of solids is higher than gases due to their packed molecular structure and less molecular movement. Similarly, the thermal conductivity of metallic solid is higher than non-metallic solid. The metals tend to have higher thermal conductivity than non-metals.

How thermal conductivity affects metals and non-metals?

The temperature has a direct influence on the thermal conductivity of metal and non-metal.

For metals: The thermal conductivity of metals is not directly proportional to the temperature. An increase in temperature would increase the thermal conductivity of metals.

For non-metals: The thermal conductivity of non-metals stays in constant mode if the temperature is high. If the temperature is low (below the Debye temperature), then the thermal conductivity would start decreasing.

Conclusion

Thermal conductivity is the natural nature of the material to transfer or conduct heat. Thermal energy is seen in the daily life activities of humans. The unit of thermal conductivity helps in expressing the dimensions of factors influencing the thermal conduction. The thermal conductivity SI unit is Wm^-1K^-1.