The primary of a transformer has 100 turns and its secondary has 200 turns. If the voltage at the primary is 100V, determine the voltage at the secondary.

Analyze the question We are given an ideal transformer with:

• Primary turns, Np = 100
• Secondary turns, Ns = 200
• Primary voltage, Vp = 100 V
• Secondary voltage, Vs = ?

Concept A transformer works by electromagnetic induction. The ratio of voltages equals the ratio of turns:

Vs / Vp = Ns / Np

If Ns > Np, it is a step-up transformer (voltage increases). If Ns < Np, it is a step-down transformer (voltage decreases).

Solution

Vs = Vp × (Ns / Np)

Vs = 100 × (200 / 100)

Vs = 100 × 2

Vs = 200 V

Since the secondary has more turns than the primary, this is a step-up transformer, so the voltage doubles.

Correct option: C (200 V)

Why the other options are wrong

• A (50 V): this results from inverting the ratio (100 × 100/200), which would be a step-down.
• B (100 V): this would need equal turns on both coils.
• D (400 V): this would need a turns ratio of 4.

Exam tip Remember: "Secondary over Primary" for both voltage and turns: Vs/Vp = Ns/Np. Quickly check your answer: more secondary turns means higher voltage, fewer means lower. Don't confuse this with current, which goes the opposite way (Is/Ip = Np/Ns).