Sham Wow Of Physics 

Black Body Radiation

 "Blackbody radiation" or "cavity radiation" refers to an object or system which absorbs all radiation incident upon it and re-radiates energy which is characteristic of this radiating system only, not dependent upon the type of radiation which is incident upon it. The radiated energy can be considered to be produced by standing wave or resonant modes of the cavity which is radiating.

 The amount of radiation emitted in a given frequency range should be proportional to the number of modes in that range. The best of classical physics suggested that all modes had an equal chance of being produced, and that the number of modes went up proportional to the square of the frequency


 But the predicted continual increase in radiated energy with frequency (dubbed the "ultraviolet catastrophe") did not happen. Nature knew better.

 Definition Provided by Hyperphysics

 Stefan Boltzmann Law

For Black Bodies 

     The Stefan–Boltzmann law, also known as Stefan's law, states that the total energy radiated per unit surface area of a black body per unit time (known variously as the black-body irradiance, energy flux density, radiant flux, or the emissive power), P, is directly proportional to the fourth power of the black body's thermodynamic temperature T (also called absolute temperature):

p = σ A T4

For Grey Bodies 

    A more general case is of a grey body, the one that doesn't absorb or emit the full amount of radiative flux. Instead, it radiates a portion of it, characterized by its emissivity, e:

                                              p = e σ A T4

The irradiance p has dimensions of energy flux (energy per time), and the SI units of measure are joules per second per square metre, or equivalently, watts per square metre. The SI unit for absolute temperature T is the kelvin. e is the emissivity of the grey body; if it is a perfect blackbody,  e = 1. Still in more general (and realistic) case, the emissivity depends on the wavelength, e = e(λ).


     The emissivity of a material (usually written ε or e) is the relative ability of its surface to emit energy by radiation. It is the ratio of energy radiated by a particular material to energy radiated by a black body at the same temperature. A true black body would have an ε = 1 while any real object would have ε < 1. Emissivity is a dimensionless quantity.

In general, the duller and blacker a material is, the closer its emissivity is to 1. The more reflective a material is, the lower its emissivity. Highly polished silver has an emissivity of about 0.02.

 Definition provided by Wikipedia

 Example - Radiation from the surface of the Sun

If the surface temperature of the sun is 5800 K and if we assume that the sun can be regarded as a black body the radiation energy per unit time can be expressed by modifying (1) like

 p / A = σ T4

    = (5.6703 10-8 W/m2K4) (5800 K)4

    = 6.42 107 W/m2 

Example Provided by Engineering Toolbox 

   Example - Radiation from a Sphere

A solid sphere (emissivity = 0.65) has a radius of 5cm is heated to 873 K and suspended from a thin wire in the center of the room whose temperature is 295 K.  At what rate is heat released to the room

   p = e σ A T4

    p = (0.65)(6.57x10^-8)(4 x 3.14 x 0.05^2)(873)^4

    p = 672 watts

Example Provided by 

 Example - Radiation transfer to a loaf of bread from an oven

Calculate the net heat transfer by radiation to a loaf of bread in an oven at a uniform temperature of 177°C, if the emissivity of the surface of the loaf is 0.85. The total surface area and temperature of the loaf are respectively 0.0645 m2 and 100°C.

 p = e σ A (T4 - T4)                    

         p   = 0.0645 x 0.85 x 5.73 x 10-8 (4504- 3734)

 p    = 68.0 J s-1.


Alloy 24ST Polished0.09
Alumina, Flame sprayed0.8
Aluminum Commercial sheet0.09
Aluminum Foil0.04
Aluminum Commercial Sheet0.09
Aluminum Heavily Oxidized0.2 - 0.31
Aluminum Highly Polished0.039 - 0.057
Aluminum Anodized0.77
Aluminum Rough0.07
Aluminum paint 0.27 - 0.67
Antimony, polished0.28 - 0.31
Asbestos board 0.96
Asbestos paper0.93 - 0.945
Beryllium, Anodized0.9
Bismuth, bright0.34
Black Body Matt1.00
Black lacquer on iron0.875
Black Parson Optical0.95
Black Silicone Paint0.93
Black Epoxy Paint0.89
Black Enamel Paint0.80
Brass Dull Plate0.22
Brass Rolled Plate Natural Surface0.06
Brass Polished0.03
Brass Oxidized 600oC0.6
Brick, red rough0.93
Brick, fireclay0.75
Carbon, not oxidized0.81
Carbon filament0.77
Carbon pressed filled surface0.98
Cast Iron, newly turned0.44
Cast Iron, turned and heated0.60 - 0.70
Chromium polished0.058
Concrete, rough0.94
Concrete tiles0.63
Cotton Cloth0.77
Copper electroplated0.03
Copper heated and covered with thick oxide layer0.78
Copper Polished0.023 - 0.052
Copper Nickel Alloy, polished0.059
Glass smooth0.92 - 0.94
Glass, pyrex0.85 - 0.95
Gold not polished0.47
Gold polished0.025
Ice smooth0.966
Ice rough0.985
Inconel X Oxidized0.71
Iron polished0.14 - 0.38
Iron, plate rusted red0.61
Iron, dark gray surface0.31
Iron, rough ingot0.87 - 0.95
Lampblack paint0.96
Lead pure unoxidized0.057 - 0.075
Lead Oxidized0.43
Lime wash0.91
Magnesium Oxide0.20 - 0.55
Magnesium Polished0.07 - 0.13
Marble White0.95
Masonry Plastered0.93
Mercury liquid0.1
Mild Steel0.20 - 0.32
Molybdenum polished0.05 - 0.18
Nickel, elctroplated0.03
Nickel, polished0.072
Nickel, oxidized0.59 - 0.86
Nichrome wire, bright0.65 - 0.79
Oak, planed0.89
Oil paints, all colors0.92 - 0.96
Paper offset0.55
Platinum, polished plate0.054 - 0.104
Porcelain, glazed0.92
Plaster, rough0.91
Porcelain glazed0.93
Quartz glass0.93
Roofing paper0.91
Rubber, hard glossy plate0.94
Rubber Nat Hard0.91
Rubber Nat Soft0.86
Silicon Carbide0.83 - 0.96
Silver Polished0.02 - 0.03
Steel Oxidized0.79
Steel Polished0.07
Stainless Steel, weathered0.85
Stainless Steel, polished0.075
Stainless Steel, type 3010.54 - 0.63
Steel Galvanized Old0.88
Steel Galvanized New0.23
Tin unoxidized0.04
Titanium polished0.19
Tungsten polished0.04
Tungsten aged filament0.032 - 0.35
Water0.95 - 0.963
Wood Beech, planned0.935
Wood Oak, planned0.885
Wrought Iron0.94
Zink Tarnished0.25
Zink polished0.045