Transformers and the national grid extra: GCSE Physics
Transformer equation, power, step-up/step-down and why the grid uses high voltage
The short version
Vp/Vs = np/ns. For a 100% efficient transformer VpIp = VsIs. High voltage means low current and less heating in cables.
Key idea
no electrical contact between the coils - energy transfers via the magnetic field
Grade 2–3: A transformer makes the electricity "push" bigger or smaller. It needs the kind of electricity that keeps swapping direction.
Grade 5–6: I can label primary, secondary and iron core and say step-up vs step-down.
Grade 9: I name induction of a pd by a changing field and I never say the coils are wired together as one loop.
Go a bit deeper
p = primary (input), s = secondary (output). Rearrange with care
Grade 2–3: Lots of extra turns on the output side means a bigger output voltage.
Grade 5–6: I can substitute into Vp/Vs = np/ns.
Grade 9: I rearrange without inverting the ratio by accident, and I state step-up or step-down.
Worked example
Worked example
Grade 2–3: A transformer makes the electricity "push" bigger or smaller. It needs the kind of electricity that keeps swapping direction. Grade 5–6: I can label primary, secondary and iron core and say step-up vs step-down. Grade 9: I name induction of a pd by a changing field and I never say the coils are wired together as one loop.
Lessons in this topic
- What a transformer does
- The transformer equation
- Power in a transformer
- The national grid
- Grid calculations and evaluation
In the app each lesson is video (when we have a real YouTube id) then intro, learn and summary steps.
What the examiner wants
Use both transformer equations, explain step-up at power stations, and calculate power loss ideas using I²R qualitatively or quantitatively.
Try these questions
Cover the answers and have a go first. Worked solutions are underneath.
A transformer needs
Vp / Vs =
A step-up transformer has
If Np = 200, Ns = 800, Vp = 12 V, Vs =
Common mistakes
- Transformers need a.c. (or at least a changing current). Do not write that they "slow down the electrons to reduce current" as the operating principle.
- Do not swap p and s. Do not use F = BIL here. Turns ratio is not the same as current ratio until you use power.
- Current is not "conserved". Voltage ratio is the inverse of current ratio for an ideal transformer.
- High voltage is not chosen "to make electricity faster". Current is not zero in the cables. 230 V is not safe to touch.
Answers and working
Frequently asked questions
Is GCSE Transformers and the national grid extra on Combined Science and Triple?
Transformers and the national grid extra is Triple-only Physics. Combined Science students can skip it.
Which exam board is this Transformers and the national grid extra page for?
The ideas are board-agnostic for AQA, Edexcel, OCR, WJEC and CCEA. Paper lengths and question numbers differ — check your specification.
Is Transformers and the national grid extra Foundation or Higher?
The core ideas appear on both tiers. Higher papers add extra application and some HT-only detail, marked in lessons with a Higher badge.
Learn, try, get feedback, fix the mistake
Open the stepped lesson, then the quiz with hints. Required-practical and calculation questions use the same method you will need in the paper.
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