Heat Transfer - Engineering

Q1:

While the total emissivity of a perfect black body is unity, the same for a real body is

A 0

B 1

C > 1

D between 0 and 1

ANS:D - between 0 and 1

The correct answer is: d) between 0 and 1 Total emissivity (�ε) of a real body is a dimensionless quantity that represents the efficiency with which a surface emits thermal radiation compared to a perfect black body at the same temperature. A perfect black body has a total emissivity of 1, meaning it emits thermal radiation as efficiently as possible at a given temperature. For real bodies, total emissivity can range from 0 to 1, where:

  • �=0ε=0 implies that the surface does not emit any thermal radiation (perfectly reflective),
  • 0<�<10<ε<1 implies that the surface emits thermal radiation to some extent, but not as efficiently as a perfect black body,
  • �=1ε=1 implies that the surface emits thermal radiation as efficiently as a perfect black body.
Therefore, the total emissivity of a real body is indeed between 0 and 1.