An electrical device has 50 turns in its primary coil  and 20 turns in the secondary coil. The device can be a\an?

Analyze the question: The device has a primary coil (50 turns) and a secondary coil (20 turns). Having a primary and a secondary coil is the signature of a transformer, so we need to decide whether it steps voltage up or down.

Concept: For an ideal transformer:

Vs / Vp = Ns / Np

• If Ns > Np, the output voltage is larger than the input, so it is a step-up transformer.
• If Ns < Np, the output voltage is smaller than the input, so it is a step-down transformer.

Solution:

• Np = 50 turns
• Ns = 20 turns

Ns / Np = 20 / 50 = 0.4

Since the ratio is less than 1 (Ns < Np), the secondary voltage is only 0.4 times the primary voltage. For example, a 250 V input would give 100 V at the output. The voltage is reduced, so the device is a step-down transformer.

Why not the others?

• A (step-up): this would need more turns in the secondary than in the primary.
• C and D (d.c. and a.c. generators): generators have a single armature coil rotating in a magnetic field. They do not have "primary" and "secondary" coils.

Answer: B, step-down transformer.

Exam tip: Compare the turns. Secondary fewer than primary means step-down, and secondary more than primary means step-up. A memory trick is "Secondary Smaller = Step-down."