Correct Option
The short operational life of an ordinary incandescent light bulb is attributable to a combination of practical limitations and physical phenomena:
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Statement 1: Filament wire is not uniform.
Manufacturing processes can result in non-uniformity in the thickness of the tungsten filament wire. Thinner sections possess higher electrical resistance, leading to localized overheating (hot spots). These hot spots accelerate the evaporation of tungsten, causing the filament to thin further and eventually break, leading to bulb failure.
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Statement 2: Bulb cannot be evacuated completely.
Achieving a perfect vacuum within a commercially produced light bulb is technically difficult and economically impractical. Residual gas molecules, even in trace amounts, can interact with the hot tungsten filament. They can chemically react with the tungsten, causing degradation, or facilitate heat transfer away from the filament by convection, increasing energy loss and accelerating tungsten evaporation, thereby shortening the bulb's lifespan.
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Statement 3: Wires supporting the filament melt at high temperatures.
The lead-in and support wires, typically made of materials like molybdenum or nickel, are designed to withstand high temperatures. However, prolonged exposure to the intense heat radiated by the filament, especially under conditions of poor heat dissipation or operation at voltages exceeding the rated specification, can cause these wires to weaken, deform, or even melt. This compromises the structural support for the filament, leading to its premature failure.
All three statements accurately describe factors contributing to the relatively short life of an ordinary incandescent bulb.
Incorrect Options
Options (a), (b), and (c) are incorrect because they do not encompass all the valid reasons contributing to the limited lifespan of an ordinary light bulb. Each of these options omits at least one correct contributing factor.