Question

The filament of a bulb is usually a:
A. Thin wire with many coils
B. Thick wire with many coils.
C. Thin straight wire
D. Thick straight wire

Answer 1

Hint: An electric bulb emits light due to being extremely hot. For a wire to be extremely hot it should have high resistance and the factors determining resistance can be altered to get the desired set of factors for a high resistance filament wire.

Formula used:
\[R=\rho \dfrac{l}{A}\]

Complete step-by-step answer:
An electric bulb emits light due to the glowing of the filament wire of the bulb. The filament of the bulb glows due to being extremely hot and emitting heat energy as light. For this purpose, the Filament of the bulb is designed to provide the maximum resistance and it is placed inside the glass filled with noble gas to prevent burnout.
Resistance (R) of a wire is given by:
\[R=\rho \dfrac{l}{A}\]
Where,
R stands for resistance
$\rho$ stands for resistivity. It is the inherent property of the substance.
A stand for the area of the cross-section of the wire
It can be observed that resistance of a wire is related to the factors as:
\[R\propto \dfrac{l}{A}\]
So, to have a wire of maximum resistance we should have a wire with: -
1. Maximum length, which can be achieved by coiling the wire to get more length of the wire into the same space.
2. Small area of cross Section, so a thin wire.
Putting these points together it can be easily seen that an ideal filament wire would be a thin wire with many coils.

So, the correct answer is “Option A”.

Note: The filament wire of the bulb should also have a high melting point as the massive heat energy that is generated due to high resistance could melt the filament. Also, to have high resistance high resistivity wire is used, an ideal wire should have high melting point and high resistivity. In commercially used bulbs. Tungsten alloy is used that has a good balance of all these elements.