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InterviewSolution
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An incandescent bulb has a thin filament of tungsten that is heated to high temperature by passing an electric current. The hot filament emits black - body radiation. The filament is observed to break up at random to break up at random locations after a sufficiently long time of operation due to non-uniform evaporation of tungsten from the fialment. If the bulb is powered at constant voltage, which of the following statement (s) is (are) true?A. The temperature distribution over the filament is uniformB. The resistance over small sections of the filament decrease with timeC. The filament emits more light at higher band of frequencies before at breaks upD. The filament consumes less electrical power towards the end of the lift of the bulb `2` |
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Answer» Correct Answer - c,d Since evaporation is given be non-uniform, hence temperature most be non-uniform Then option (a) is wrong Due to evaporasion, the cross sectional area of the filament at section decrease Hence resistance of the given section increases as `R prop 1//A`. Thus option (b) is wrong. The portion of the filament of the bulb having highest evaporation rate would have rapidly reduced area and would becomes break up area of cross - section At break up junction temperature would be highest Due, to it the light of highest head frequency would be emitted at those cross section ,Thus potion ( c) is true Due to continuous heating of filament the cross-sectional area of a section of filament decrease Hence the resistance of the As a result of it , the power decrease because `P = V^(2) or P prop 1//R` as `V` is constant This potion (d) is true. |
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