Transformers (HT only) — AQA GCSE Physics
Test yourself on Transformers (HT only) with AQA GCSE practice questions.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge
Transformers (HT only) explained
A transformer is two separate coils of insulated wire wound on the same iron core.
Read the full explanation
The coil connected to the supply is the primary; the coil connected to the output is the secondary. They are not joined electrically, which is why the labels on a diagram matter: the primary is simply whichever coil the input reaches. The core links them magnetically. An alternating current in the primary produces a magnetic field that grows, collapses and reverses; the iron carries that changing field round to the secondary; and the changing field through the secondary induces an alternating potential difference across it. Because every step depends on the field changing, a transformer does nothing at all with a direct current supply, however large that supply is.
Iron is used as it is easily magnetised.
Iron is chosen for transformer cores because it magnetises and demagnetises easily, allowing its magnetism to follow an alternating magnetic field that changes direction rapidly. Each time the primary alternating current changes, the magnetic field in the core changes with it. It is this changing magnetic field in the core that induces an alternating potential difference in the secondary coil. A magnetically hard material like steel would not be suitable because, once magnetised, it tends to stay magnetised. The magnetic field in the core would not continuously change with the primary current, preventing the induction of the output.
power input and output, in watts, W Students should be able to: explain how the effect of an alternating current in one coil in inducing a current in another is used in transformers explain how the ratio of the potential differences across the two coils depends on the ratio of the number of turns on each calculate the current drawn from the input supply to provide a particular power output apply the equation linking the p.d.s and number of turns in the two coils of a transformer to the currents and the power transfer involved, and relate these to the advantages of power transmission at high potential differences.
A transformer works because an alternating current in the primary coil produces a continuously changing magnetic field. 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
- Identify the primary coil, the secondary coil and the core on a diagram of a transformer.
- Explain how the two coils are linked when no wire joins them.
- Explain why a transformer connected to a battery gives no output.
Show all 10 objectives
- State that iron is used for transformer cores because it is easily magnetised.
- Explain why a changing magnetic field in the core is necessary for a transformer to work.
- Describe why magnetically hard materials are unsuitable for use in a transformer core.
- 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.
- Explain how step-up transformers reduce energy wasted in transmission cables.
Transformers (HT only) exam tips
Marking Points
- one mark for naming the primary coil as the one connected to the input supply
- one mark for naming the secondary coil as the one connected to the output
- one mark for the iron core linking the two coils magnetically
- one mark for stating that the two coils are not connected to each other electrically
- Iron is used for the core because it is easily magnetised and demagnetised.
- The primary coil produces an alternating magnetic field.
- The iron core carries this changing magnetic field to the secondary coil.
- A changing magnetic field is required to induce a potential difference in the secondary coil.
- 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
- 💡Trace the input wires on the diagram to find the primary before you answer anything else.
- 💡Learn the three labels, primary coil, secondary coil and iron core; they appear as a quick two mark question.
- 💡Write alternating every time you mention the supply to a transformer.
- 💡Always link the choice of iron to its ability to be easily magnetised and demagnetised.
- 💡Avoid using the word 'steel' when describing transformer cores, as its magnetic properties are unsuitable.
- 💡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
- labelling the coils by size rather than by which one the supply is connected to
- drawing a wire joining the primary circuit to the secondary circuit
- saying a transformer works on direct current, when a steady current gives an unchanging field and no output
- calling the core a magnet, rather than a material that carries the changing magnetic field
- Stating that iron is a good electrical conductor, which is relevant for the coils but not the core.
- Suggesting that steel is used, forgetting that steel is magnetically hard and stays magnetised.
- Saying iron is used because it is cheap or strong, rather than focusing on its magnetic properties.
- 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.