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The filament of an electric bulb is made of: A. Carbon B. Aluminium C. Tungsten D. Nickel

The filament of an electric bulb is made of:
A. Carbon
B. Aluminium
C. Tungsten
D. Nickel

Grade:12th pass

1 Answers

Pawan Prajapati
askIITians Faculty 9723 Points
14 days ago
Remember that electric bulbs convert electrical energy to heat and light energy and thus the filaments of the bulbs are expected to withstand high temperatures while at the same time ensure that their structure does not get affected via heating effects from the current passing through. Think of which of the above materials have the highest melting point and is able to conduct electricity. Complete answer: Let us first understand what a filament actually is. An incandescent lightbulb, which is a lightbulb that produces light from electricity, usually has a small thin wire that is held up in the glass casing by two bigger/thicker wires coming from the base of the bulb. This thin wire is called the filament of the bulb. Whenever electric current flows through the filament, it glows. To ensure an optimal production of light, the filament is usually made up of coils of fine wire, or coiled wire. These wires are usually many feet long but are most often coiled in such a way that they are one-hundredth of an inch thick and less than an inch long. Now, filaments of electric bulbs are made up of tungsten. To understand why, let us look at some properties of tungsten that contribute to its versatility. As a metal, Tungsten is a good electrical conductor. It is also a good thermal conductor. In pure form, tungsten has the highest known melting point among all metals. This means that tungsten is able to maintain its form irrespective of the large heating effects from electrical current. Tungsten has the lowest coefficient of thermal expansion of any pure metal. It is also very ductile and can be readily drawn into a wire, and possesses the highest tensile strength than any pure metal. This means that it can withstand a lot of stress before breaking while being stretched or pulled. Now, incandescent bulbs use the heat generated by the resistance of the filament to create light. In order to get light in the white-yellow range the filament will need to reach temperatures of 2700K-3000K. The higher the temperature, whiter the light. High temperatures mean increased resistance which implies increased heating and then irradiation. The photons emitted at these temperatures fall under the wavelength of yellowish-white light. Tungsten has a melting point at around 3400K, which is higher than any metal in pure form. And owing to its low thermal expansion, tungsten is able to maintain its form even when temperatures change drastically. This is why tungsten is used in the filaments of light bulbs. So, the correct answer is “Option C”. Note: It is important to understand why filaments are usually coiled in the first place. We know that resistance increases with decrease in area and increase in length. Since the heat and subsequent light produced by a resistor is directly proportional to its resistance, we are using longer tungsten wires which are coiled to ensure they occupy a small area, both of which contribute to a desirable amount of resistance. Also note that Tungsten always needs to be placed in an inert atmosphere or vacuum, just like in the glass casing of a bulb, since it is easily oxidizable when exposed to air, which may then chip away at the usefulness of tungsten. This is also the reason why tungsten is not used in heaters.

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