Understanding Electric Bulb Filaments and High Temperatures

The question asks us to evaluate an assertion and a reason regarding the behavior of the filament in an electric bulb when it glows at a very high temperature.

Analyzing Assertion (A)

Assertion (A) states: The filament in an electric bulb does not burn up although its temperature is about 2700°C when it glows.

• An electric bulb filament works by getting extremely hot when electric current passes through it. This process is called Joule heating or resistive heating.
• At operating temperatures, typically around 2500°C to 2700°C, the filament emits light.
• "Burning up" implies combustion, which is a chemical reaction, usually with oxygen (oxidation).
• However, the filament, despite the high temperature, does not undergo combustion within the bulb. This is because the bulb is either evacuated (vacuum) or filled with an inert gas (like argon or nitrogen) to prevent oxidation.
• Operating at such high temperatures without burning or melting requires a special material.
• Therefore, Assertion (A) is correct. The filament does reach a very high temperature and does not burn up in the way combustion implies.

Analyzing Reason (R)

Reason (R) states: Bulb filament is made of tungsten.

• Tungsten (chemical symbol W) is a metal known for its extremely high melting point, which is approximately 3422°C (\( \approx 3695 \text{ K} \)).
• Tungsten is the material predominantly used for filaments in incandescent light bulbs due to this property.
• Other desirable properties of tungsten for this application include relatively low evaporation rate at high temperatures (though evaporation is still a limiting factor for bulb life, especially in vacuum bulbs, leading to the use of inert gas filling and sometimes halogen cycles), and sufficient ductility to be drawn into fine wires.
• Therefore, Reason (R) is correct. Bulb filaments are indeed made of tungsten.

Evaluating if (R) is the Correct Explanation for (A)

Now we need to determine if the fact that the filament is made of tungsten (R) explains why it does not burn up at 2700°C (A).

• As discussed, "burning up" primarily refers to oxidation (combustion). The absence of oxygen inside the bulb is the direct reason for preventing combustion.
• However, operating at 2700°C is also a major challenge. Most materials would melt, evaporate rapidly, or chemically degrade at this temperature.
• Tungsten's extraordinarily high melting point allows it to reach and sustain a temperature of around 2700°C without melting. This high temperature is necessary for the filament to glow brightly and efficiently emit light.
• If the filament were made of a material with a lower melting point, it would simply melt before reaching the required temperature for effective light emission, or it would evaporate excessively, leading to rapid failure.
• While the inert atmosphere prevents combustion, the choice of tungsten is what allows the filament to operate at the necessary high temperature without melting or significant degradation (other than slow evaporation over time). Thus, tungsten's properties are fundamental to the filament's ability to function at such high temperatures without being destroyed ("burning up" in a broader sense of high-temperature failure).
• Considering the context of a light bulb, the high temperature is key to its function, and tungsten is chosen because it can withstand this temperature. The ability to withstand this temperature without melting or excessive evaporation is what prevents the filament from "burning up" in the sense of rapid thermal destruction during operation.
• Therefore, the property of tungsten (high melting point) is a correct and crucial explanation for why the filament can operate at 2700°C without melting or failing rapidly, thus not "burning up" under operating conditions.

Both Assertion (A) and Reason (R) are correct, and Reason (R) provides a correct explanation for why the filament can operate at such a high temperature without failing, enabling Assertion (A) to be true.

Conclusion

Based on the analysis:

• Assertion (A) is correct.
• Reason (R) is correct.
• Reason (R) is the correct explanation for (A).

This matches Option 1.

Revision Table: Electric Bulb Filament

Additional Information: Incandescent Light Bulbs

• How they work: Electric current heats the tungsten filament to a very high temperature, causing it to become incandescent (glow and emit light).
• Temperature: The operating temperature of a typical incandescent filament is between 2500°C and 2700°C. Halogen bulbs can operate at slightly higher temperatures.
• Preventing Filament Failure:
  • The glass envelope creates a sealed environment.
  • This environment is either a vacuum or filled with inert gases like argon, nitrogen, or a mixture. This prevents the hot tungsten from reacting with oxygen (burning).
  • Tungsten's high melting point is crucial for withstanding the operating temperature without melting.
  • Inert gas filling also helps reduce the rate at which tungsten evaporates from the hot filament.
  • Halogen bulbs use a halogen gas (like iodine or bromine) that participates in a regenerative cycle, redepositing evaporated tungsten back onto the filament, extending bulb life and allowing higher operating temperatures.
• Why not other materials? While other materials can withstand high temperatures, tungsten offers the best combination of high melting point, low vapor pressure (rate of evaporation), and sufficient strength at high temperatures required for a durable filament. Carbon has a higher melting point but evaporates too quickly at high temperatures.