CHEE 2695 – Lecture 17
Radiation:
- Conduction and convection require a medium, whereas radiation does not.
Electromagnetic spectrum:
- Thermal radiation has a wavelength of 10-7 – 10-4 m.
- This includes part of the infrared and UV region, and all the visible region.
- Radiation does not need a medium to propagate but can be blocked through
reflection and absorption.
- Any material above 0 K emits radiation.
Black-body radiation:
- A black body is an idealised body with perfect emissivity (zero reflection).
- It represents ‘perfect’ or ‘ideal’ radiation and is never quite achieved in the real
world.
Radiation reference points:
, Emissive power:
- This is the energy due to original emission only, and does not include
reflected energy which originates elsewhere.
- The total blackbody emissive power is:
- Emissive power is given by:
- Emissive power at a particular wavelength is given by:
- Wien’s law is:
- The fraction of radiation emitted to a particular wavelength is:
Real surfaces:
- A real surface has an emissive power less than a blackbody.
- The ratio of total emissive power of a body, to that of a blackbody is total
emissivity, or total hemispherical emissive power.
Radiation:
- Conduction and convection require a medium, whereas radiation does not.
Electromagnetic spectrum:
- Thermal radiation has a wavelength of 10-7 – 10-4 m.
- This includes part of the infrared and UV region, and all the visible region.
- Radiation does not need a medium to propagate but can be blocked through
reflection and absorption.
- Any material above 0 K emits radiation.
Black-body radiation:
- A black body is an idealised body with perfect emissivity (zero reflection).
- It represents ‘perfect’ or ‘ideal’ radiation and is never quite achieved in the real
world.
Radiation reference points:
, Emissive power:
- This is the energy due to original emission only, and does not include
reflected energy which originates elsewhere.
- The total blackbody emissive power is:
- Emissive power is given by:
- Emissive power at a particular wavelength is given by:
- Wien’s law is:
- The fraction of radiation emitted to a particular wavelength is:
Real surfaces:
- A real surface has an emissive power less than a blackbody.
- The ratio of total emissive power of a body, to that of a blackbody is total
emissivity, or total hemispherical emissive power.
