JEE Advanced 2016 · Paper 1 · Physics Q8

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PhysicsMultiple correct+4 / -2Question 8 of 54

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?

The temperature distribution over the filament is uniform

The resistance over small sections of the filament decreases with time

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

Answer and worked solution

Official answer: C, D

(A) Evaporation is non-uniform, so thin spots have higher resistance. With the same current through them, they dissipate more power ( $I^2R$ ) and run hotter. The temperature is not uniform. False.
(B) As the filament thins, the resistance of small sections increases. False.
(C) The hot spots just before break-up are at higher temperature. By Wien's law they emit relatively more at higher frequencies. True.
(D) The total resistance rises, so at constant voltage $P=V^2/R$ falls. True.

(C), (D)

Topics:Radiation and Stefan's law Electrical power and heating Wien's displacement law

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JEE Advanced 2016 · Paper 1, Physics question 8