A-Level · Physics · AQA · Mark scheme decoded
AQA A-Level Physics: Transformers and Power Transmission — mark scheme explained
The short answer
Transformers are essential devices in electrical engineering, used to step up or step down voltages in alternating current (AC) circuits. They play a crucial role in the efficient transmission of electrical power over long distances.
The question
A transformer has a primary coil with 100 turns and a secondary coil with 250 turns. If the input voltage is 120 V, what is the output voltage? [Paraphrased for study — not reproduced from any exam paper.]
Mark scheme, decoded
What each mark is really for — in plain English — and the wording trap that loses it.
- S1
Identify the given values: N p = 100, N s = 250, V p = 120 V.
- S2
Use the transformer equation: V s / V p = N s / N p .
- S3
Substitute the values into the equation: V s / 120 = 250 / 100.
- S4
Solve for V s : V s = (250 / 100) × 120 = 300 V.
Model answer
Worked through, with each step tagged to the mark it earns.
- S1
Identify the given values: N p = 100, N s = 250, V p = 120 V.
- S2
Use the transformer equation: V s / V p = N s / N p .
- S3
Substitute the values into the equation: V s / 120 = 250 / 100.
- S4
Solve for V s : V s = (250 / 100) × 120 = 300 V.
Final answer: The output voltage is 300 V.
Common mistakes
- Confusing the transformer equation with Ohm's law — Review the transformer equation and understand that it relates voltages and turns, not resistance and current.
- Forgetting to use the power conservation equation — Always remember to use the power conservation equation when dealing with transformers: V p × I p = V s × I s .
- Misinterpreting high-voltage transmission — Understand that high voltage reduces the current in the transmission line, which in turn reduces power loss (P loss = I 2 R).
- Confusing eddy currents with hysteresis losses — Eddy currents are induced currents that cause heating in the core, while hysteresis losses are due to the magnetic properties of the core material. Review the definitions and mechanisms of both.
- Incorrectly calculating transformer efficiency — Use the correct formula for efficiency: Efficiency = (P s / P p ) × 100%. Ensure you multiply by 100 to get the percentage.
- Forgetting to use laminations in the core — Always mention that the core is made from thin laminated sheets of iron to reduce eddy current losses when discussing transformer efficiency.
Where the marks go
- Full worked solution (all marking points)4 marks