Q1. [3] § Chapter Introduction § 11.7.1 Practical Applications of Heating Effect of Electric Current
The electrical resistivity of three materials A, B and C at 20°C is given below :
- (i) Classify these materials as conductor, alloy and insulator.
- (ii) Give one example of each of these materials and state one use of each material in the design of an electrical appliance say an electric stove or an electric iron.
Previously asked in CBSE board exam
2025 31/4/1 Q32
Generated by claude-sonnet-4-6 · 2026-06-15 06:45 · grounding rag
Model Answer
(i) Classification:
- A (resistivity = $10^{17}$ Ω m) → Insulator
- B (resistivity = $44 × 10^{-6}$ Ω m) → Alloy
- C (resistivity = $1.62 × 10^{-8}$ Ω m) → Conductor
(ii) Examples and uses:
| Material | Example | Use in Electric Stove/Iron |
|----------|---------|---------------------------|
| Insulator (A) | Rubber / Ebonite | Used as outer covering/handle of electric iron to prevent electric shock. |
| Alloy (B) | Manganin / Nichrome | Used as heating element (coil) in electric iron/stove because alloys have high resistivity and don't oxidise easily at high temperatures. |
| Conductor (C) | Copper | Used as connecting wires to carry current with minimum energy loss due to very low resistivity. |
Source: Chapter 11 – Electricity, Section 11.5 (Table 11.2)
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Explanation
- Resistivity range tells you the category: ~$10^{12}$–$10^{17}$ Ω m = insulator; ~$10^{-6}$ Ω m = alloy; ~$10^{-8}$ Ω m = conductor (metal).
- Examiners expect you to match the given values to Table 11.2 — C ($1.62 × 10^{-8}$) matches Copper exactly.
- For uses, always link the property (high resistivity → heating element; low resistivity → connecting wire; insulator → protective cover) to the function. This fetches full marks.
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