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?

• A. The temperature distribution over the filament is uniform.
• B. The resistance over small sections of the filament decreases with time.
• C. The filament emits more light at higher band of frequencies before it breaks up.
• D. The filament consumes less electrical power towards the end of the life of the bulb.

Answer 1

Answer: (C), (D)

The filament emits more light at higher band of frequencies before it breaks up.
The filament consumes less electrical power towards the end of the life of the bulb.

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.

Answer is C and D.