AC, transformers and the grid · Grade 11
Mains electricity is alternating current, which can be stepped up or down by transformers. Power stations generate at about 25 kV; step-up transformers raise this to hundreds of kilovolts for the National Grid so the current – and the energy wasted heating the cables – is small. Step-down transformers then bring it safely to 230 V (or 120 V) for homes.
Key facts
- Transformer equation: Vs ÷ Vp = Ns ÷ Np. More secondary turns = step-up.
- Ideal (100% efficient) transformer: Vp Ip = Vs Is.
- Transformers only work with a.c.: a changing current makes a changing field in the core.
- Cable losses P = I²R: raising V by a factor k cuts the current by k and the losses by k².
- r.m.s. values: V_rms = V₀ ÷ √2; average power P = V_rms I_rms = V_rms² ÷ R.
- On an oscilloscope: peak = divisions × volts/div; period = divisions × time/div.