JEE Previous Year Question
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 locations after a sufficiently long time of operation due to non-uniform evaporation of tungsten from the filament. If the bulb is powered at constant voltage, which of the following statement(s) is (are)true?
Topic: JEE PYQ
Answer: C, D. Full step-by-step solution below.
- AThe temperature distribution over the filament is uniform
- BThe resistance over small sections of the filament decreases with time
- CThe filament emits more light at higher band of frequencies before it breaks up
- DThe filament consumes less electrical power towards the end of the life of the bulb
Correct answer: C, D
Solution
(c, d)Towards the end of the life filament will become thinner. Resistance will increase and so consumed power will be less, so it will emit less light. Temperature distribution will be non uniform. At the position where temperature is maximum, filament will break. Black body radiation curve will become flat so the filament consumes less electrical power towards the end of the life of the bulb.
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