SPM Physics: AHA! Transformer Question – Find the Primary Current (3 Marks)

 

Transformers are a classic SPM exam topic. Students often lose marks not because they don’t know the formula, but because they skip unit conversion or forget to show working.

Example Question: An iron ring is wound with copper wire to form two coils. The primary coil has 11 turns, the secondary coil has 7 turns, and the LED connected to the secondary coil requires 3.2 V and 20 mA.

Step-by-step solution: 1️⃣ Voltage at primary coil: $\frac{V_p}{V_s}=\frac{N_p}{N_s}$ $V_p=\frac{11}{7}\times 3.2=5.03V$

2️⃣ Current at primary coil: Convert $20mA=20\times {10}^{-3}A$ Use $V_p{I}_p=V_s{I}_s$ $(5.03)I_p=(3.2)(20\times {10}^{-3})$ $I_p=0.013A$

Key Takeaways:

• Always convert mA to A.

• Show working clearly for method marks.

• Use correct transformer formula.

• State final answer with unit.

AHA! Convert units, secure marks. Not by sight, but by INSIGHT!

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