Generated by claude-sonnet-4-6 · 2026-06-15 06:42 · grounding rag
Model Answer
(i)
Given: V = 220 V, R = 55 Ω
Current, $I = \dfrac{V}{R} = \dfrac{220}{55} = \mathbf{4 \text{ A}}$
Power consumed, $P = VI = 220 \times 4 = \mathbf{880 \text{ W}}$
(ii)
Energy consumed by 3 bulbs in 30 days = 3 × 100 W × 5 h × 30 = 45,000 Wh = 45 kWh
Energy consumed by heater in 30 days = 1.0 kW × 0.5 h × 30 = 15 kWh
Total energy = 45 + 15 = 60 kWh
Cost = 60 × ₹3.60 = ₹ 216
(iii)
Alloys are used in heating devices because:
- The resistivity of alloys is higher than that of pure metals, so they produce more heat.
- Alloys do not oxidise (burn) readily at high temperatures, making them durable.
(Example: Nichrome, an alloy of Ni, Cr, Mn and Fe, is widely used.)
Source: Chapter 11 – Electricity, Sections 11.5 and 11.7
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Explanation
- (i) Direct application of Ohm's law ($I = V/R$) and power formula ($P = VI$). Both steps must be shown.
- (ii) Convert all power to kW and time to hours before multiplying. Add both energies, then multiply by rate. Show each step clearly.
- (iii) Examiners expect two reasons: high resistivity AND resistance to oxidation at high temperatures. These exact points are stated in Table 11.2's accompanying text in the textbook. Mentioning an example (Nichrome) adds value but is optional.