Transformers (HT only) — AQA GCSE Physics
Test yourself on Transformers (HT only) with AQA GCSE practice questions.
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Transformers (HT only) explained
A transformer works because an alternating current in the primary coil produces a continuously changing magnetic field.
Read the full explanation
The iron core carries this changing magnetic field through the secondary coil, which induces an alternating potential difference across it, driving a current if the circuit is complete. Two equations govern this. The turns rule states Vₚ ÷ Vₛ = nₚ ÷ nₛ, so more secondary turns step the potential difference up. The power rule assumes 100% efficiency, meaning power in equals power out (Vₚ × Iₚ = Vₛ × Iₛ). The National Grid uses step-up transformers to increase potential difference and decrease current. Since energy wasted heating cables depends on the square of the current, a smaller current significantly reduces energy transferred to the surroundings.
Your focus
- Explain how an alternating current in the primary coil induces a current in the secondary coil.
- Calculate an unknown potential difference or number of turns using the transformer turns ratio.
- Apply the power equation to calculate the current drawn from a supply for a specific power output.
Show all 4 objectives
- Explain how step-up transformers reduce energy wasted in transmission cables.
Transformers (HT only) exam tips
Marking Points
- An alternating current in the primary coil creates a changing magnetic field.
- The iron core directs this changing magnetic field through the secondary coil, inducing a potential difference.
- A correct substitution into the turns ratio equation, keeping primary values on the same side.
- Using the principle that power input equals power output to calculate an unknown current.
- Transmitting at a high potential difference reduces the current, which decreases energy wasted heating the cables.
Examiner Tips
- 💡When explaining how a transformer works, explicitly state that the alternating current creates a changing magnetic field.
- 💡Write out the power equation (Vₚ × Iₚ = Vₛ × Iₛ) before substituting values to avoid calculation errors.
Common Mistakes
- Forgetting to mention that the magnetic field must be changing to induce a potential difference.
- Inverting the turns ratio, resulting in a step-up transformer calculating a smaller output potential difference.
- Stating that a step-up transformer increases both potential difference and current, violating the conservation of energy.