39
Experiment 6
Radiation from a Black Body: StefanBoltzmann Law
Apparatus
Blackened hemisphere (metal), heating coil, blackened silver disk, thermocouple, power
supply, stop clock etc.
Objective
To determine the Stefan–Boltzmann constant
°
by studying the radiation received from a
black body radiator.
Theory
The total radiation from black body is given by StefanBoltzmann relation
4
T
R
σ
=
(per cm
2
per sec)
……………….
(1)
where R is the radiant energy per unit area per unit time and T is the absolute temperature
of the body. This relation was empirically deduced by Josef Stefan from the experimental
results of John Tyndall and was derived by Ludwig Boltzmann from theoretical
considerations based on thermodynamics.
Consider a black body radiator at temperature T
1
and a metal disc at temperature T
2
as
the receiver of the radiation (fig 1.). If R
1
and R
2
be the radiation absorbed and emitted by
the receiving metal disc respectively, then the energy gained per second by the disc is (R
1

R
2
)A ; where A is the area of crosssection of the disc. If
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 Fall '09
 LIND
 Thermodynamics, Energy, Heat, Black body, Josef Stefan

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