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Thermal Properties of Matter — Physics STD 11 Science — Question

Gujarat 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.
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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. $T_0$ = 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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