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How It Works
Thermal diffusivity (α) tells you how fast temperature changes spread through a material. It combines conductivity, density, and heat capacity into one number: α = k / (ρ × cp). High diffusivity means temperature equalizes quickly; low diffusivity means the material responds slowly to thermal changes.
Metals like copper (α ≈ 1.17 × 10⁻&sup4; m²/s) heat up and cool down rapidly, while water (α ≈ 1.43 × 10⁻&sup7; m²/s) changes temperature slowly because it stores so much energy per unit mass.
Example Problem
Find the thermal diffusivity of aluminum given: k = 205 W/(m·K), ρ = 2700 kg/m³, cp = 900 J/(kg·K).
- Apply the formula: α = k / (ρ × cp)
- α = 205 / (2700 × 900)
- α ≈ 8.44 × 10⁻&sup5; m²/s
This relatively high value explains why aluminum cookware heats evenly and responds quickly to burner adjustments.
Frequently Asked Questions
What is thermal diffusivity in simple terms?
Thermal diffusivity measures how fast heat spreads through a material. A material with high diffusivity reaches thermal equilibrium quickly, while one with low diffusivity takes longer to warm up or cool down throughout its bulk.
What is the thermal diffusivity of steel?
Carbon steel has a thermal diffusivity of about 1.2 × 10⁻&sup5; m²/s, roughly 10 times lower than copper. Stainless steel is even lower at around 4 × 10⁻&sup6; m²/s because of its lower conductivity.
Why does thermal diffusivity matter for cooking?
Cookware with high diffusivity (like aluminum or copper) spreads heat evenly, reducing hot spots. Cast iron has lower diffusivity, so it heats unevenly at first but retains heat longer once hot. Food engineers also use diffusivity to model how quickly the center of a roast or frozen item reaches a safe temperature.
How is thermal diffusivity measured in the lab?
The most common method is the laser flash technique (ASTM E1461), where a short laser pulse heats one face of a sample and an infrared detector records how quickly the temperature rises on the opposite face. The diffusivity is calculated from the sample thickness and the time to reach half the maximum temperature rise.
Related Calculators
- Thermal Conductivity Calculator — calculate steady-state heat transfer through a material.
- Thermal Expansion Calculator — find how much a material grows or shrinks with temperature.
- Density Calculator — calculate density, mass, or volume for any material.
- Fourier Number Calculator — dimensionless time ratio that uses thermal diffusivity.
- Energy Unit Converter — convert heat energy between joules, BTU, and calories.
References:
Incropera, Frank P. et al. 2006. Fundamentals of Heat and Mass Transfer. John Wiley & Sons. 6th ed.
Cengel, Yunus A. 2007. Heat and Mass Transfer: A Practical Approach. McGraw-Hill. 3rd ed.