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Thermal Properties of Matter — Physics STD 11 Science — Question

Rajasthan BoardEnglish MediumSTD 11 SciencePhysicsThermal Properties of Matter3 Marks
Question
Explain why:
An optical pyrometer (for measuring high temperatures) calibrated for an ideal black body radiation gives too low a value for the temperature of a red hot iron piece in the open, but gives a correct value for the temperature when the same piece is in the furnace.
✓

Answer

An optical pyrometer calibrated for an ideal black body radiation gives too low a value for temperature of a red hot iron piece kept in the open.
Black body radiation equation is given by:
$\text{E}=\sigma(\text{T}^4-\text{T}_0^4)$
Where,
E = Energy radiation.
T = Temperature of optical pyrometer.
T0 = Temperature of open space.
$\sigma$ = Constant.
Hence, an increase in the temperature of open space reduces the radiation energy.
When the same piece of iron is placed in a furnace, the radiation energy,  $\text{E}=\sigma\text{T}^4$

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