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Electricity · Chapter 4: Transformers and the national grid

Year 11 Science · About 17 minutes with activities · Turn your sound on

Chapter 4 · Transformers and the grid
Up to 220 000 VNZ’s biggest power lines
High V = low currentso less energy is lost as heat
230 Vat your wall socket
Soft iron core
Primary coilinput
Secondary coiloutput
AC in
AC out
Step-upmore turns on the secondary
Step-downfewer turns on the secondary
V ↑ I ↓voltage up, current down
V ↓ I ↑voltage down, current up
Power intotal power input
Useful power outuseful power output
Wasted as heat
Power stationhigh voltage
Step-uptransformer
High-voltage linesup to 220 kV in NZ
High V = low Iless power lost as heat
Step-downtransformer
Homes: 230 Vsafe for domestic use
High currentlots of heat lost
Low currentlittle heat lost
½ current → ¼ heat

Chapter 4 recap

  • A transformer is two coils on a soft iron core. Step-up increases the voltage (more secondary turns); step-down decreases it.
  • Ideal transformer: VpIp = VsIs. Efficiency = Ps ÷ Pp × 100 (%); the rest is wasted as heat.
  • Grid: power station → step-up → high-voltage lines → step-down → home (230 V in NZ).
  • High voltage → low current → less energy lost as heat in the lines.
  • Next: Chapter 5 · Nuclear power

Why do power lines carry hundreds of thousands of volts?

Chapter 4 · About 17 minutes · sound on

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