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A2 Daily A Level Physics question

2026-06-13 OCR A AC generators & transformers (Module 6.3): efficiency and current/voltage ratios OCR-A 6.3.1 Alternating current (rms values and power) OCR-A 6.3.2 Transformers (power transfer, efficiency; qualitative current/voltage relationships)

In a school lab, a 12 V (rms) AC supply feeds an 80% efficient step-up transformer that provides 120 V (rms) to a small neon sign. The primary current is 2.0 A (rms). What is the secondary current (rms)?

  1. A 0.20 A
  2. B 0.80 A
  3. C 0.16 A (correct)
  4. D 0.25 A

Answer

The correct answer is C.

Correct: C — 0.16 A. Power in equals 12 V × 2.0 A = 24 W (rms), so output power is 0.80 × 24 = 19.2 W; hence I_secondary = 19.2 W ÷ 120 V = 0.16 A. A ignores the 80% efficiency and assumes power is fully conserved, giving 24 W ÷ 120 V = 0.20 A. B assumes current increases with voltage (I ∝ V), which contradicts near power balance for a transformer. D divides by efficiency instead of multiplying, overestimating output power and current.

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