VCE Physics — Unit 3 AOS 2
Transformers — Flashcards & Quiz
Transformers use electromagnetic induction to change AC voltages up or down, and VCE Physics Unit 3 AOS 2 expects you to apply the ideal turns ratio, explain why transformers only work with AC, and justify why long-distance power transmission uses high voltages and low currents.
Key Points
- Turns ratio: V_s/V_p = N_s/N_p. Step-up has N_s > N_p; step-down has N_s < N_p.
- Ideal transformer: input power equals output power, so V_p I_p = V_s I_s. Stepping voltage up requires stepping current down.
- Only AC works — DC produces constant flux (dΦ/dt = 0) and therefore no induced EMF in the secondary.
- Real losses: resistance in the coils (I²R heating), eddy currents (reduced by lamination), hysteresis, and flux leakage.
- Power transmission uses step-up at the source and step-down near consumers. Lower I for the same P reduces I²R losses in the transmission lines.
- Efficiency η = P_out/P_in is typically >95% for modern transformers operating near rated load.
Common Mistakes to Avoid
- Claiming transformers work on DC — they need changing flux.
- Forgetting the inverse current-voltage relationship when power is conserved.
- Mixing up step-up (more secondary turns) and step-down (fewer secondary turns).
- Ignoring real losses in efficiency calculations.
- Applying the ideal transformer equation V_s/V_p = N_s/N_p in reverse without justification.
Exam Strategy
VCAA Unit 3 AOS 2 transformer questions give turns ratios or voltage/current values and ask you to calculate the unknown. Method: (1) write the turns ratio and power-conservation equations, (2) substitute known values, (3) for power transmission questions, link to I²R losses and explain the high-voltage choice, (4) account for real losses if efficiency is given.
Sample Flashcards
Q1: Explain the structure and operating principle of a transformer.
A transformer has two coils (primary and secondary) wound on a common iron core. Alternating current in the primary creates changing magnetic flux in the core. This flux links the secondary, inducing EMF by Faraday's law. The voltage ratio equals the turns ratio: Vs/Vp = Ns/Np. Iron core concentrates flux and links the coils efficiently.
Q2: What are step-up and step-down transformers, and where are they used?
Step-up transformers have more secondary turns than primary (Ns > Np), increasing voltage. Step-down transformers have fewer secondary turns (Ns < Np), decreasing voltage. Both conserve power (ideally): if voltage increases, current decreases proportionally. Used throughout power distribution from generator (step-up) to household (step-down).
Sample Quiz Questions
Q1: Transformers can operate with both AC and DC current equally effectively.
Answer: FALSE
FALSE. Transformers only work with AC (or any changing current). They require changing magnetic flux to induce EMF in the secondary coil (Faraday's law). DC produces constant flux with no change, so ΔΦ/Δt = 0 and no EMF is induced in the secondary.
Q2: In an ideal transformer, the voltage ratio equals the turns ratio: Vs/Vp = Ns/Np.
Answer: TRUE
TRUE. The transformer equation Vs/Vp = Ns/Np follows from Faraday's law applied to both coils. Both coils experience the same rate of flux change, so EMF per turn is identical. Total EMF is proportional to number of turns.
Q3: A step-up transformer increases both voltage and current in the secondary coil compared to the primary.
Answer: FALSE
FALSE. While step-up transformers increase voltage (Vs > Vp), they decrease current (Is < Ip) to conserve power. For ideal transformers: VpIp = VsIs. Increasing both voltage and current would create energy from nothing, violating energy conservation.
Revision Tip
Transformer calculations are procedural — drill a Revizi deck with step-up and step-down scenarios covering both directions of the calculation.
Related Concepts
Last updated: March 2026 · 2 flashcards · 3 quiz questions