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Which of the following statements are correct for a blackbody? 1. A blackbody continues to emit radiation even when it is in thermal equilibrium with its surroundings. 2. A blackbody is a perfect emitter. 3. Absorptivity, α = 1 represents a black surface.

Which of the following statements are correct for a blackbody? 1. A blackbody continues to emit radiation even when it is in thermal equilibrium with its surroundings. 2. A blackbody is a perfect emitter. 3. Absorptivity, α = 1 represents a black surface. Correct Answer 1, 2 and 3 

Concept:

Blackbody

  • A blackbody allows all incident radiation to pass into it (no reflected energy) and internally absorbs all the incident radiation (no energy transmitted through the body).
  • This is true for radiation of all wavelengths and for all angles of incidence. Hence the blackbody is a perfect absorber for all incident radiation.
  • we have α + ρ + τ = 1
  • For black body, ρ = 0, τ = 0 ⇒ α = 1;
  • For a black body from Kirchoff's law, emissivity = absorptivity = 1 ⇒ Perfect emitter too.
  • As long as there is temperature, there is heat and the emissivity of a blackbody does exist.
  • But after equilibrium, the objects still exchange radiation with each other, but now the objects are absorbing and emitting energy in the form of radiation at exactly the same rate, so no net heat exchange takes place.

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