A Levels Physics (9702)•9702/12/O/N/20

Explanation
Power proportional to current squared for fixed resistance
Steps:
- Original: P1 = I1²R = 2W, I1 = 3A, so R = 2 / 3² = 2/9 Ω.
- New power: P2 = 2W + 50% = 3W.
- I2²R = 3W, so I2² = 3 / (2/9) = 3 × 9/2 = 13.5.
- I2 = √13.5 ≈ 3.7A.
Why B is correct:
- P = I²R for fixed R means I ∝ √P; I2 = 3 × √(3/2) ≈ 3 × 1.2247 = 3.7A.
Why the others are wrong:
- A: 2.4A gives P = (2.4/3)² × 2 = 1.28W (decreases power).
- C: 4.5A gives P = (4.5/3)² × 2 = 4.5W (increases beyond 3W).
- D: 14A gives P = (14/3)² × 2 ≈ 43.5W (far too high).
Final answer: B
Topic: Potential difference and power
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