Question

What are the essential requirements of a heater filament?
(A) high resistivity, low melting point
(B) low resistivity, low melting point
(C) high resistivity, high melting point
(D) low resistivity, high melting point

Answer 1

Hint : The requirements for a heater filament include having a high temperature of conversion of the material from solid to liquid. This is because the main purpose of a heater element is to generate heat from electricity according to Joule’s law of heating.

Complete step by step answer
The heater filament should have a high melting point so that it can be heated to higher temperatures below that specified melting point of the filament. It would thus be possible to produce high temperatures if required using the same filament. The high melting point of the filament is required and essential for it to withstand high temperatures so that any small increase in the temperature may not destroy the heating filament.
Heating would take place according to Joule’s law of heating which states that the heat produced in a resistor is directly proportional to the square of the current passing through it, the resistance of the resistor, and the time for which the current flows through the resistor.
The material should be of high resistance as well so that a small length of the wire is used to produce the given amount of heat. High resistivity enforces Joule’s law of heating. The higher resistance would also keep the current in the circuit controlled and thus prevent the filament from melting.
Thus the correct option is (C) high resistivity, high melting point.

Note
The other essentials of a good heating element include high mechanical strength, non-corrosive, economical, free from oxidation, low- temperature coefficient of resistance, high melting point, high specific resistance, etc. A low- temperature coefficient of resistance is required as the radiant heat is proportional to the fourth power of temperature and becomes tough for proper temperature control.