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

(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

When filament breaks up, the temperature of filament will be higher so according to wein’s law

\((\lambda _m ∝\frac{1}{T},v_ m∝T)\) , the filament emits more light at higher band of frequencies.

As voltage is constant, so consumed electrical power is P = v² / R

As R increases with increase in temperature so the filament consumes less electrical power towards the end of the life of the bulb.