A 120 watt filament lamp loses all its energy by radiation from its surface. The emissivity of the surface is 0.6. The area of the surface is \(3.53\times10^{-3}\) m2. Calculate the temperature of the filament.

Q.25 A 120 watt filament lamp loses all its energy by radiation from its surface. The emissivity of the surface is 0.6. The area of the surface is \(3.53\times10^{-3}\) m². Calculate the temperature of the filament.

Doubtly Asked 28 Oct 2026 Board exam · July 2025 hsc 12th Save WhatsApp Report

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Answer: \(T \approx 1000\) K

By the Stefan–Boltzmann law, power radiated \(P = e\sigma AT^4\).

\[T^4 = \frac{P}{e\sigma A} = \frac{120}{0.6\times5.67\times10^{-8}\times3.53\times10^{-3}} = \frac{120}{1.2009\times10^{-10}} = 9.99\times10^{11}\]\[T = (9.99\times10^{11})^{1/4} \approx 1000\ \text{K}\]

(Using log tables: \(\log T = \frac{1}{4}\log(9.99\times10^{11}) = \frac{11.9996}{4} = 2.9999\), so \(T \approx 999.8\) K \(\approx 1000\) K.)

Doubtly AI · 28 Oct 2026 AI-verified · Oct 2026 Report

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A 120 watt filament lamp loses all its energy by radiation from its surface. The emissivity of the surface is 0.6. The area of the surface is \(3.53\times10^{-3}\) m2. Calculate the temperature of the filament.

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