An electric heating coil rated at 1Kw is used to heat 2kg of water for 2 minutes. The initial water…

Analyze the question: A 1 kW heater runs for 2 minutes and all its energy goes into 2 kg of water starting at 30°C. We need the final temperature.

Concept: Electrical energy supplied = heat gained by the water.

• Electrical energy: E = P × t
• Heat gained: Q = mcΔθ

Solution:

1. Energy supplied: E = P × t = 1000 W × (2 × 60) s = 1000 × 120 = 120,000 J

2. Heat gained by water: Q = mcΔθ = 2 × 4000 × Δθ = 8000 Δθ

3. Equate the two: 8000 Δθ = 120,000 Δθ = 120,000 ÷ 8000 = 15°C

4. Final temperature: θ = 30 + 15 = 45°C

Answer: The final temperature is 45.0°C, which is option B.

Common trap: Option A (15.0°C) is only the rise in temperature, not the final temperature. Always add it to the initial temperature.

Tip: Convert units first (kW to W, minutes to seconds). Then use the chain: P × t = m × c × Δθ. Remember to ask yourself, "Is the question asking for the change or the final value?"