Example 1 — Finding Output Voltage
Question: A step-up transformer has 500 turns on the primary coil and 10,000 turns on the secondary. The primary voltage is 25,000 V. Calculate the secondary voltage.
Using: Vp / Vs = Np / Ns
25,000 / Vs = 500 / 10,000
25,000 / Vs = 0.05
Vs = 25,000 / 0.05 = 500,000 V
Example 2 — Finding Output Current
Question: A transformer steps up voltage from 230 V to 23,000 V. The primary current is 10 A. Assuming 100% efficiency, find the secondary current.
Using: Vp × Ip = Vs × Is
230 × 10 = 23,000 × Is
2,300 = 23,000 × Is
Is = 2,300 / 23,000 = 0.1 A
Voltage increased by ×100, so current decreased by ×100 (from 10 A to 0.1 A).
This worked example covers Worked Examples within National Grid & Transformers for GCSE Physics. Revise National Grid & Transformers in Extra Topics for GCSE Physics with 14 exam-style questions and 12 flashcards. This topic appears regularly enough that it should still be part of a steady revision cycle. It is section 5 of 12 in this topic. Treat this as a marking guide for what examiners are looking for, not just a fact list.
Practice questions for National Grid & Transformers
What is the function of a step-up transformer in the National Grid?
Explain why electricity is transmitted at high voltage and low current through the National Grid power cables.