Questions & Answers

Which is or are the correct for filament of bulb?
A. Low melting point
B. High melting point
C. Made of tungsten
D. Made of nichrome

Answer Verified Verified
Hint: The material requirements for use in filament of bulb are high melting point and high resistivity material. Because, in the filament of the bulb the temperature produced is very high. To withstand that temperature, the material must have a high melting point. The material having a high melting point and high resistivity is tungsten.

Complete step by step solution:
In the bulb, main to requirements of the material are high resistivity and high melting point. Due to the high resistivity, when the current flows through the filament, it generates more heat energy. To withstand that amount of heat, the material should have a high melting point.

Resistivity of material:
Resistivity is the resistance per unit length, and the resistivity is the reciprocal of conductivity. It is the property of the material which opposes the flow of electric current. Metals which are good conductors of electricity, which have low resistivity. The insulating materials are having very high resistivity. Example: rubber, plastics, wood, etc.

The metal tungsten has the resistivity of $4.8 \times {10^{ - 8}}\;\Omega m$ and the melting point is $3422{\;^ \circ }C$. Hence, it is the suitable material for filament of bulb.

Hence, the options (B) and (C) are correct.

Note: The tungsten filament lamp temperature is about $2800 - 3300\;K$ and consists of a linear double-ended quartz tube containing a tungsten filament, which is resistively heated. The quartz tube heats up to $400{\;^ \circ }C$ or more and the upper permissible temperature limit is $900{\;^ \circ }C$. Halogen gas is used instead of inert gas because it increases the filament colour temperature by about $300\;K$. Also, it prevents tungsten deposition on the quartz wall, due to a regenerative tungsten halide chemical transport cycle above $250{\;^ \circ }C$ quartz wall temperature.