The **main difference between thermal conductivity and thermal diffusivity** is that thermal conductivity of a material is a measure of the ability of that material to conduct heat through it, while thermal diffusivity of a material is the thermal inertia of that material.

Thermal conductivity and thermal diffusivity are two terms used in thermal and statistical physics. Thermal conductivity is a frequently used term in physics, whereas thermal diffusivity is a rarely used term in thermal physics. Moreover, thermal conductivity is closely related to the thermal diffusivity. The relationship between the two quantities can be expressed as an equation.

### Key Areas Covered

**1. What is Thermal Conductivity
**

*– Definition, Unit of Measurement, Formula, Properties of Thermal Conductors*

**2. What is Thermal Diffusivity**

**– Definition, Unit of Measurement, Formula, Properties**

**3. Difference Between Thermal Conductivity and Thermal Diffusivity**

*– Comparison of Key Differences*

### Key Terms

*Thermal Conductivity, Thermal Diffusivity*

## What is Thermal Conductivity

In physics, thermal conductivity is the ability of a material to conduct heat. Thermal conductivity is denoted by the symbol K. The SI unit for measuring thermal conductivity is Watts per meter Kelvin (W/mK). The thermal conductivity of a given material often depends on the temperature and even the direction of heat transfer. According to the second law of thermodynamics, heat always flows from a hot region to a cold region. In other words, a net heat transfer needs a temperature gradient. The higher the thermal conductivity of a material, the higher the rate of heat transfer across that material will be.

The reciprocal of the thermal conductivity of a given material is known as the **thermal resistivity** of that material. That means the higher the thermal conductivity, the lower the thermal resistivity. The thermal conductivity (K) of a material can be expressed as;

K(T) = α (T)_{p }(T) C_{p}(T)

Where, α(T)- Thermal diffusivity, p(T) – density, C_{p}T- specific heat capacity

### Thermal Conducters

Materials such as diamond, copper, aluminium, and silver have high thermal conductivities and are considered good thermal conductors. Aluminum alloys are widely used as heat sinks, especially in electronics. Materials such as wood, polyurethane, Alumina and polystyrene, on the other hand, have low thermal conductivity. Therefore, such materials are used as thermal insulators.

The thermal conductivity of a material can change when the phase of the material changes from solid to liquid, liquid to gas or vice versa. For instance, the thermal conductivity of ice changes when ice melts into water.

Good electrical conductors are usually good thermal conductors. However, silver is a relatively weak thermal conductor even though it is a good electrical conductor.

Electrons are the main contributor to the thermal conductivity of metals, whereas lattice vibrations or phonons are the main contributors to the thermal conductivity of nonmetals. In metals, the thermal conductivity is approximately proportional to the product of the electrical conductivity and the absolute temperature. However, the electrical conductivity of pure metals decreases when the temperature increases as the electrical resistance of pure metals increase with increasing temperature. As a result, the product of the electrical resistance and the absolute temperature, as well as the thermal conductivity, remain approximately constant with increasing or decreasing temperature.

## What is Thermal Diffusivity

Thermal diffusivity of a material is the thermal inertia of that material. It can be understood as the ability of a material to conduct heat, relative to the heat stored per unit volume.

Moreover, the thermal diffusivity of a material can be defined as the thermal conductivity divided by the product of specific heat capacity and density. It can be expressed mathematically as;

α(T)= K(T)/(_{p}(T)C_{p}(T))

α(T) = Thermal diffusivity

That means the higher the thermal diffusivity, the higher the thermal conductivity. Therefore, materials having higher thermal diffusivity conduct heat quickly through them. The thermal diffusivity of a gas is highly sensitive to temperature as well as pressure. The SI unit for measuring thermal diffusivity is m^{2}s^{-1}.

Unlike thermal conductivity, thermal diffusivity is not a frequently used term. However, it is an important physical property of materials which helps to understand the ability of a material to conduct heat relative to the heat stored per unit volume.

## Difference Between Thermal conductivity and Thermal Diffusivity

### Definition:

Thermal conductivity of a material is a measure of the ability of that material to conduct heat through it, while thermal diffusivity is the ability of a material to conduct heat relative to the heat stored per unit volume.

### Formula for Calculation

Thermal conductivity (K) of a material can be expressed as:

K(T) = α(T)ρ(T) Cp (T)

Where α(T) – Thermal diffusivity, ρ(T)- density, Cp (T)- specific heat capacity

Thermal diffusivity (α) of a material can be expressed in terms of thermal conductivity as:

α(T)=K(T)/( ρ(T) Cp (T) )

### Denoted by:

Thermal conductivity is denoted by K, while thermal diffusivity is denoted by α

### SI unit:

The SI unit of thermal conductivity is W/mK, while the SI unit of thermal diffusivity is m^{2}.

### Dimensions

The dimensions of thermal conductivity are M^{1}L^{1}T^{−3}Θ^{−1, }while the dimensions of thermal diffusivity are L^{2}.

### Conclusion

The main difference between thermal conductivity and thermal diffusivity is that thermal conductivity of a material is a measure of the ability of that material to conduct heat through it, while thermal diffusivity of a material is the thermal inertia of that material.

##### Image Courtesy:

1. “Rough Diamond” By Unknown USGS employee – Original source: USGS “Minerals in Your World” website. (Public Domain) via Commons Wikimedia

2. “Pyrolytic graphite” (CC BY-SA 3.0) via Commons Wikimedia