In the Stefan-Boltzmann law, what does the symbol σ represent?

• A. The Stefan-Boltzmann constant
• B. The Boltzmann constant
• C. Emissivity
• D. Temperature

Answer: (A)

The symbol denotes a universal constant that sets the scale of radiant power emitted by a body as its temperature rises to the fourth power. This is the Stefan-Boltzmann constant, with a value of about 5.67×10^-8 W m^-2 K^-4. For a perfect blackbody, the emitted power per unit area is P/A = σ T^4. Real surfaces emit less, so emissivity ε (0 to 1) is included: P/A = ε σ T^4. The Boltzmann constant is a different constant, relating microscopic energy to temperature, and the temperature variable is T, not σ. Emissivity is denoted by ε, not σ.