An electric heater is rated 220 V; 11 A. Calculate the power consumed if the heater is operated at 200 V.
Generated by claude-sonnet-4-6 · 2026-06-15 06:41 · grounding rag
Model Answer
Step 1: Find resistance of heater from rated values.
$$R = \frac{V}{I} = \frac{220}{11} = 20 \ \Omega$$
Step 2: Calculate power at 200 V using $P = V^2/R$.
$$P = \frac{V^2}{R} = \frac{(200)^2}{20} = \frac{40000}{20} = 2000 \ \text{W}$$
Power consumed = 2000 W
Source: Chapter 11, Section 11.8 Electric Power
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Explanation
- The heater's resistance is fixed (it's a resistive device), so first find R from rated voltage and current using Ohm's law.
- Since operating voltage changes, use $P = V^2/R$ (not $P = VI$ with rated current, because current will also change at the new voltage).
- Examiners award 1 mark for finding R correctly and 1 mark for the final power calculation.