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State/National, Divisions B and C: Radiant Exitance, Blackbody Radiation, Stefan-Boltzmann Law, Third Law

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Terms you will need

  • absolute zeroAbsolute zero is the lowest possible temperature, at which a system has the minimum possible energy.
  • thermal radiationThermal radiation is the electromagnetic radiation emitted by a body due to its temperature.
  • heat capacityHeat capacity is the amount of heat required to raise the temperature of a substance by one degree Celsius.
  • specific heatSpecific heat is the amount of heat required to raise the temperature of one kilogram of a substance by one degree Celsius.
  • adiabatic processAn adiabatic process is a process in which no heat is transferred into or out of the system.

How this topic runs

  1. 1Start here
  2. 2How it works

Radiant exitance, blackbody radiation, the Stefan-Boltzmann law, and the third law of thermodynamics are essential concepts in understanding how energy is emitted and transferred by objects, especially in relation to temperature. Radiant exitance refers to the total power emitted per unit area by a surface, and it is a key factor in quantifying how much energy an object radiates. Blackbody radiation describes the idealized emission of electromagnetic radiation by an object that absorbs all incident radiation. The Stefan-Boltzmann law provides a mathematical relationship between the radiant exitance of a blackbody and its temperature, showing that the power emitted increases with the fourth power of the temperature. The third law of thermodynamics addresses the behavior of systems as they approach absolute zero, stating that the entropy of a perfect crystal at absolute zero is exactly zero. Together, these concepts form the foundation for analyzing thermal radiation and its applications in engineering and physics.

Figure (1)

The Earth only has an absorbing area equal to a two dimensional disk, rather than the surface of a sphere.

Radiant exitance is the total energy an object emits as light or heat per second, per square meter of its surface. Blackbody radiation is the idealized form of this emission, where an object absorbs all light that hits it and emits light based only on its temperature. This means that as an object gets hotter, it glows much more brightly.

The third law of thermodynamics says that as a system gets closer to absolute zero, its disorder, or entropy, approaches zero. This law helps explain why it’s impossible to reach absolute zero in practice. Together, these ideas help scientists and engineers understand how heat and light behave in everything from stars to engines.

Key Points

  • Thermal radiation is the process in which a body emits electromagnetic waves due to its temperature, transferring energy through the emission of photons.

Terms

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