Section 3 review
Thermal radiation comprises electromagnetic waves, including sunlight and ultraviolet and infrared light, that are emitted as a result of thermal motion of atoms and molecules. Thermal radiation is emitted by all objects with temperatures above absolute zero and thermal radiation is also absorbed by all objects. The power in thermal radiation over all wavelengths increases as the absolute temperature to the fourth power. A perfect blackbody is a surface that absorbs all incident radiation and emits only thermal radiation. The blackbody spectrum describes the relationship between power and wavelength for thermal radiation. An object in thermal equilibrium balances heat gained with heat lost. In space, conduction and convection cannot occur so radiation dominates heat transfer. Radiative heat balance determines the average temperature of Earth, the planets, and all other Solar System objects except the Sun itself. Read the text aloud
emissivity, blackbody

P=εσA T 4
P=εσA( T 4 T 4 )

Review problems and questions

  1. The Stefan–Boltzmann equation describes how much energy every object in the universe radiates away. But in the universe, some objects are warming up, while others are cooling down. How can this be? Read the text aloud Show
  1. If you heat steel from room temperature (20°C) to its melting point at 1,510°C, by what increased factor will it radiate thermal energy? Read the text aloud Show
  1. Telescope mirrors are coated with a thin layer of a shiny metal so that light striking them will mostly reflect, rather than be absorbed. For infrared astronomy, however, there is another important reason: As the mirror becomes less reflective, it radiates more energy at infrared wavelengths—which can overwhelm the images from distant stars or galaxies. Why does a less-reflective mirror radiate more infrared radiation? Read the text aloud Show
  1. The dwarf planet Eris has an average orbital distance from the Sun of 68 AU. Estimate the average temperature of Eris based on a radiation balance between the thermal energy it receives from the Sun and the energy Eris radiates as a blackbody. Express your answer in degrees Celsius. Read the text aloud Show
  1. While the Earth has a wide range of temperatures across its surface, its average temperature is approximately 15°C or so. How much power is radiated by the Earth at that temperature, assuming the Earth is 100% emissive? (The radius of the Earth is 6,370 km.) Compare this power to the Hoover Dam, which can generate up to 2,080 MW. Read the text aloud Show

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