Question

Alloys are commonly used in electrical heating devices like toaster, electric iron because:

• A. they oxidise readily at high temperature.
• B. they do not oxidise readily at high temperature.
• C. they are poor insulator of electricity.
• D. they have low resistivity.

Hint:

An ideal heating element requires:
1. High resistivity (to produce more heat per unit length).
2. High melting point.
3. High resistance to oxidation at high temperatures.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The question asks for the primary physical reason why alloys (such as Nichrome) are preferred over pure metals as heating elements in household electrical appliances.

Step 2: Key Formula or Approach:
Heating devices operate on the principle of Joule heating (\(H = I^2 R t\)). A good heating element must have high resistivity and a high tolerance to high temperatures without undergoing chemical degradation (oxidation/burning).

Step 3: Detailed Explanation:

• Alloys like Nichrome (nickel, chromium, and iron) possess significantly higher electrical resistivity compared to their constituent pure metals. High resistivity allows them to generate substantial heat.

• Crucially, when these appliances operate, they reach high temperatures (often glowing red hot).

• Pure metals tend to oxidize (react with oxygen in the air and burn) rapidly at elevated temperatures, leading to quick wear and failure of the element.

• Alloys do not oxidize or burn readily at these high temperatures. They form a protective surface layer of oxide that prevents further reaction.

• They also have high melting points, ensuring structural stability during operation.


Step 4: Final Answer:
Thus, alloys are used because they do not oxidize readily at high temperatures.