Given below are two statements: one is labelled as Assertion A and the other is labelled as Reason R.
Assertion A: The filament in an electric bulb does not melt although its temperature is about 2700°C when it glows.
Reason R: Bulb filament is made of tungsten.
In the light of the above statements, choose the most appropriate answer from the options given below.
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Both A and R are correct and R is the correct explanation of A
A solid stays solid as long as its temperature is below its melting point. A filament must glow white-hot without melting, so it is made of a metal whose melting point is far above its working temperature.
Assertion A: The filament in an electric bulb does not melt although its temperature is about 2700°C when it glows. Correct. The filament glows at about 2500–2700°C, hot enough to give out bright light, yet it stays solid for hundreds of hours.
Reason R: Bulb filament is made of tungsten. Correct. Filaments are thin coils of tungsten wire.
R explains A. Tungsten has the highest melting point of all metals, about 3400°C, well above 2700°C, so the filament does not melt. A copper wire (melting point about 1085°C) would melt long before it glowed this brightly.