# The Stefan-Boltzmann Law

The total energy radiated by an object per unit time is proportional to the fourth power of
its absolute temperature *T* (in Kelvin, 0°C≈273K):

P=εσAT^{4}

where *A* is the surface area of the object, *σ* is the Stefan-Boltzmann constant, and *ε* is the
emissivity of the object's material. This is known as the **Stefan-Boltzmann Law**.

The emissivities of materials vary. If the object is a perfect black body, *ε*=1.
Metals tend to have a very low emissivity. Water and paper, on the other hand, have high emissivity (0.95).