The filament of electric bulb is generally made of tungsten because

1. Electric Current and Resistance Basics (basic)

To understand why certain materials are chosen for household appliances, we must first master the fundamentals of electric current and resistance. Think of electric current as a flow of charge (electrons) through a conductor, driven by a 'push' known as potential difference (voltage). However, no material is a perfect highway; as electrons move, they collide with the atoms of the conductor. This internal friction or opposition to the flow of charge is what we call Resistance (R), measured in Ohms (Ω) Science, Class X (NCERT 2025 ed.), Chapter 11, p.176.

The relationship between these factors is governed by Ohm’s Law, which states that the current (I) flowing through a conductor is directly proportional to the potential difference (V) across its ends, provided temperature remains constant (V = IR). This means if you keep the voltage the same but increase the resistance, the current will drop. Resistance isn't just a random value; it depends on the physical dimensions of the conductor and the nature of the material used Science, Class X (NCERT 2025 ed.), Chapter 11, p.178.

Mathematically, the resistance of a uniform conductor is directly proportional to its length (l) and inversely proportional to its area of cross-section (A). This gives us the fundamental formula: R = ρ (l/A), where ρ (rho) represents electrical resistivity. Resistivity is an intrinsic property of the material itself—metals like silver and copper have very low resistivity (making them great conductors), while insulators like rubber have incredibly high resistivity.

Factor	Change in Factor	Effect on Resistance
Length (l)	Increases	Increases (More collisions)
Thickness (Area)	Increases	Decreases (More space to flow)
Resistivity (ρ)	Higher for alloys/insulators	Increases

Sources: Science, Class X (NCERT 2025 ed.), Chapter 11: Electricity, p.176; Science, Class X (NCERT 2025 ed.), Chapter 11: Electricity, p.178; Science, Class X (NCERT 2025 ed.), Chapter 11: Electricity, p.181

2. Joule’s Law of Heating (basic)

At its heart, the heating effect of electric current is a transformation of energy. When an electric current passes through a conductor, the conductor offers resistance to the flow of electrons. As these electrons collide with the atoms of the conductor, they transfer a portion of their kinetic energy, which manifests as thermal energy, making the conductor hot Science, Class VIII, Electricity: Magnetic and Heating Effects, p.53. While this heating is often an 'inevitable consequence' that leads to energy waste in devices like electric fans, we purposefully harness it in many household appliances Science, Class X, Electricity, p.190. To understand this quantitatively, we look at Joule’s Law of Heating. This law states that the heat (H) produced in a resistor is directly proportional to three key factors:

• The square of the current (I²) for a given resistance.
• The resistance (R) for a given current.
• The time (t) for which the current flows.

This is mathematically expressed as H = I²Rt Science, Class X, Electricity, p.189. This formula explains why even a small increase in current leads to a significantly larger amount of heat, as the current term is squared. In everyday life, this principle is the 'engine' behind your electric iron, toaster, and room heater. It is also the reason an electric bulb glows; the filament is heated to such an extreme temperature that it begins to emit visible light Science, Class X, Electricity, p.190. To prevent the filament from melting under this intense heat, we use materials like tungsten, which has an incredibly high melting point of approximately 3380°C Science, Class X, Electricity, p.190. Additionally, this effect provides a safety mechanism in the form of an electric fuse, which melts and breaks the circuit if the current becomes dangerously high, protecting our homes from electrical fires.

Sources: Science, Class VIII, Electricity: Magnetic and Heating Effects, p.53; Science, Class X, Electricity, p.188-190

3. Alloys vs. Pure Metals in Heating Elements (intermediate)

To understand why we don't use simple copper wires in our electric heaters or toasters, we must look at the Joule heating effect. A heating element must convert electrical energy into heat efficiently, which requires high electrical resistance. While pure metals like copper or aluminum are excellent conductors, they are actually too good at conducting electricity for this purpose; they don't offer enough resistance to generate significant heat, and more importantly, they tend to oxidize (react with oxygen) and degrade quickly when they get hot Science, Class X, Metals and Non-metals, p.42. This is where alloys come in. An alloy is a metallic substance composed of two or more elements. For most household appliances like electric irons, room heaters, and water heaters, we use an alloy called Nichrome (a mix of nickel and chromium) Science, Class VIII, Electricity: Magnetic and Heating Effects, p.52. Alloys are preferred over pure metals for two primary reasons: first, they have much higher resistivity, meaning they produce more heat for the same amount of current; and second, they do not oxidize (burn) easily even when they are red-hot at high temperatures Science, Class VIII, Electricity: Magnetic and Heating Effects, p.59. However, there is a famous exception to the "alloy-only" rule in the world of lighting. For the filaments of incandescent light bulbs, we use the pure metal Tungsten. This is because a bulb filament needs to reach extreme temperatures (over 2,500°C) to glow and emit visible light. Tungsten is chosen because it has the highest melting point of all metals (approx. 3,422°C), allowing it to remain structurally sound while white-hot Science, Class X, Electricity, p.179.

Feature	Pure Metals (e.g., Copper)	Alloys (e.g., Nichrome)
Resistivity	Low (Good conductors)	High (Good for heating)
Oxidation at high heat	Oxidize rapidly	Highly resistant to oxidation
Primary Use	Transmission wires	Heating elements (Irons, Heaters)

Sources: Science, Class VIII (NCERT 2025), Electricity: Magnetic and Heating Effects, p.52, 53, 59; Science, Class X (NCERT 2025), Metals and Non-metals, p.42; Science, Class X (NCERT 2025), Electricity, p.179

4. Chemical Stability and Inert Gases in Bulbs (intermediate)

To understand why we fill light bulbs with specific gases, we first have to look at the extreme conditions inside an incandescent bulb. To produce visible light, the filament—usually made of Tungsten—must be heated to temperatures so high that most other materials would simply melt or vaporize. Tungsten is chosen because it has an incredibly high melting point of approximately 3380°C to 3422°C, allowing it to remain solid while glowing white-hot Science, Class X (NCERT 2025 ed.), Electricity, p.190. However, this high temperature creates a major chemical problem: oxidation.

At such intense heat, Tungsten becomes highly reactive. If any oxygen were present inside the bulb, the filament would react with it almost instantly, burning up and snapping in a flash of combustion. To prevent this, the air is removed and replaced with chemically inactive gases like Argon or Nitrogen. These gases are "inert," meaning they do not easily react with other substances. By surrounding the hot filament with these gases, we create a protective environment that prevents oxygen from reaching the metal, thereby prolonging the life of the bulb Physical Geography by PMF IAS, Earths Atmosphere, p.272.

This principle of using inert gases to block oxidation is a common theme in chemistry. For example, the same logic applies to the food industry. Nitrogen is used to flush bags of potato chips because it is relatively unreactive and displaces the oxygen that would otherwise cause the fats and oils in the chips to turn rancid (a process of oxidation that ruins taste and smell) Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.13. Whether it is protecting a metal filament at 3000°C or preserving the crunch of a snack, the goal is the same: excluding oxygen to maintain chemical stability.

Application	Gas Used	Primary Purpose
Electric Bulbs	Argon / Nitrogen	Prevents Tungsten filament from burning (oxidation).
Chip Packets	Nitrogen	Prevents fats/oils from turning rancid (oxidation).
Atmosphere	Nitrogen	Dilutes Oxygen to prevent spontaneous combustion.

Sources: Science, Class X (NCERT 2025 ed.), Chapter 11: Electricity, p.190; Physical Geography by PMF IAS, Earth's Atmosphere, p.272; Science, Class X (NCERT 2025 ed.), Chapter 1: Chemical Reactions and Equations, p.13

5. The Physics of Incandescence (exam-level)

Incandescence is a physical process where light is emitted by a solid body as a result of being heated to a very high temperature. At the atomic level, when a material is heated, its atoms vibrate vigorously, and the thermal energy is converted into electromagnetic radiation. In a common light bulb, this happens via Joule heating: as an electric current passes through a thin wire (the filament), the resistance of the wire generates intense heat. This thin wire is the heart of the lamp and is the specific part that glows to provide illumination Science-Class VII, Electricity: Circuits and their Components, p.26.

For a material to serve as an effective filament, it must survive temperatures high enough to emit visible light (typically above 2,500°C) without melting or evaporating. Tungsten is the gold standard for this application because it possesses the highest melting point of all metals, approximately 3,422°C. Beyond its thermal resilience, tungsten has an exceptionally low vapor pressure, which prevents the filament from thinning out and breaking too quickly through evaporation at extreme temperatures. Furthermore, its high tensile strength allows it to be drawn into the incredibly fine, coiled wires required to fit inside a small glass bulb Science-Class X, Electricity, p.179.

While incandescence was the primary source of artificial light for over a century, it is inherently inefficient because the vast majority of the energy consumed is lost as heat rather than visible light. This is why modern lighting has shifted toward Light Emitting Diodes (LEDs) Science-Class VII, Electricity: Circuits and their Components, p.27. From an environmental perspective, the use of energy-efficient bulbs with appropriate spectral power distributions is critical to reducing both energy waste and light pollution in urban environments Environment, Shankar IAS Academy, Environmental Pollution, p.82.

Property	Importance for Incandescence
High Melting Point	Allows the filament to reach the "white-hot" state without liquefying.
Low Vapor Pressure	Reduces the rate of evaporation, extending the life of the bulb.
High Resistivity	Enables the conversion of electrical energy into thermal energy efficiently.

Sources: Science-Class VII, NCERT, Electricity: Circuits and their Components, p.26-27; Science-Class X, NCERT, Electricity, p.179; Environment, Shankar IAS Academy, Environmental Pollution, p.82

6. Physical Properties of Tungsten (exam-level)

In the world of materials science, Tungsten (W) stands out as a metal of extremes. When we look at an incandescent bulb, we are witnessing a material being pushed to its thermal limits. For a substance to emit visible light through incandescence, it must be heated to thousands of degrees Celsius. While most metals would simply liquefy or vaporize under such stress, Tungsten remains structurally sound due to its unique physical profile.

The most defining characteristic of Tungsten is its extraordinarily high melting point. Reaching approximately 3380°C to 3422°C, it possesses the highest melting point of all pure metals. To put this in perspective, consider the following comparison of common materials:

Material	Melting Point (°C)
Gallium / Caesium	~28°C - 30°C (Melts in your palm)
Iron (Fe)	1538°C
Tungsten (W)	3380°C

As noted in Science, Class X (NCERT 2025 ed.), Chapter 11: Electricity, p.190, this high melting point ensures the filament does not melt even as it glows white-hot. This stability is further supported by Tungsten's low vapor pressure, which prevents the metal atoms from evaporating rapidly and coating the inside of the glass bulb with a dark soot.

Beyond its thermal endurance, Tungsten possesses high tensile strength and ductility. Ductility is the ability of a metal to be drawn into thin wires Science, Class X (NCERT 2025 ed.), Chapter 3: Metals and Non-metals, p.38. This allows manufacturers to create the incredibly fine, tightly coiled filaments required to fit within a small glass envelope. Furthermore, when used as an alloying agent, Tungsten significantly raises the melting point and toughness of other metals, such as iron Certificate Physical and Human Geography, GC Leong, Chapter 24, p.284.

Sources: Science, Class X (NCERT 2025 ed.), Chapter 11: Electricity, p.190; Science, Class X (NCERT 2025 ed.), Chapter 3: Metals and Non-metals, p.38; Science, Class VIII (NCERT 2025 ed.), Particulate Nature of Matter, p.103; Certificate Physical and Human Geography, GC Leong, Chapter 24: Manufacturing Industry, p.284

7. Solving the Original PYQ (exam-level)

Now that you have explored Joule’s Heating Effect and the principle of incandescence, you can see how those building blocks lead directly to this question. For an electric bulb to emit visible light, the filament must be heated to temperatures so extreme that it glows. As you learned in the context of electrical resistance, this process generates intense heat. If a material with a standard melting point were used, the filament would liquify or evaporate almost instantly. This is where the unique physical properties of tungsten become essential, as it can withstand the thermal stress required to produce light without breaking.

To arrive at the correct answer, (D) melting point of tungsten is high, think like a material scientist: the primary constraint of an incandescent bulb is thermal stability. With a melting point of approximately 3,422°C, as cited in Science, class X (NCERT 2025 ed.), tungsten remains solid and maintains its high tensile strength even when white-hot. This high melting point, combined with a low vapor pressure, prevents the filament from thinning out too quickly during operation, ensuring the bulb has a functional lifespan while operating at peak brightness.

UPSC often includes distractors that sound plausible but lack scientific precision. For instance, while tungsten is indeed durable (Option B), "durability" is a generic term that doesn't explain the specific physics of light production. Similarly, claiming tungsten is "cheap" (Option A) is factually incorrect in the context of industrial metals, and "light-emitting power" (Option C) is a vague distractor; the light is a byproduct of the temperature allowed by the high melting point, not an inherent "power" of the metal itself. Always look for the fundamental physical property that enables the technology to function.

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