Question

Thermal diffusivity is a

• A. function of temperature
• B. physical property of a substance
• C. dimensionless parameter
• D. function of pressure

Hint:

Thermal diffusivity ($\alpha = \frac{k}{\rho c_p}$) has the same units as kinematic viscosity ($\nu = \frac{\mu}{\rho}$) and mass diffusivity ($D$): $\text{m}^2/\text{s}$.
These three transport properties represent the diffusion of thermal energy, momentum, and mass, respectively.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The question asks to classify the thermodynamic term "thermal diffusivity" ($\alpha$) in terms of its physical nature and properties.

Step 2: Key Formula or Approach:

• Thermal diffusivity ($\alpha$) is defined mathematically as: \[ \alpha = \frac{k}{\rho c_p} \] where:
$k$ is the thermal conductivity of the material ($\text{W/m-K}$),
$\rho$ is the density ($\text{kg/m}^3$),
$c_p$ is the specific heat capacity at constant pressure ($\text{J/kg-K}$).

• The SI unit of thermal diffusivity is $\text{m}^2/\text{s}$.

Step 3: Detailed Explanation:

• Thermal diffusivity measures the rate of heat transfer through a material relative to its heat storage capacity.

• A material with high thermal diffusivity (like copper or silver) responds rapidly to temperature changes, diffusing thermal energy quickly.

• Since it is defined using other material properties ($k$, $\rho$, $c_p$), thermal diffusivity itself is a fundamental physical property of a substance.

• Let us evaluate the options:
- While $\alpha$ can vary slightly with temperature, it is not fundamentally defined as a "function of temperature".
- It has physical dimensions of $\text{m}^2/\text{s}$, so it is not a dimensionless parameter.
- It is not classified primarily as a function of pressure.
- Option (B) correctly identifies it as a physical property of a substance.

Step 4: Final Answer:
Thermal diffusivity is a physical property of a substance.