Knowledge Organiser: National Grid and Transformers
Key Terms
- National Grid — network distributing electricity nationally
- Transformer — changes AC voltage using electromagnetic induction
- Step-up — increases voltage, decreases current
- Step-down — decreases voltage, increases current
- Turns ratio — ratio of primary to secondary turns = ratio of voltages
Key Equations
- Turns ratio: Vp / Vs = Np / Ns
- Power (ideal): Vp × Ip = Vs × Is
- Power loss in cables: P = I²R
- Power = V × I
National Grid Sequence
- Power station generates at 25,000 V
- Step-up transformer → 275,000–400,000 V
- Transmission via pylons (low current)
- Step-down at substation → 33,000 V
- Further step-down → 230 V for homes
How a Transformer Works
- AC flows in primary coil
- Creates changing magnetic field
- Iron core transfers field to secondary
- Changing field induces voltage in secondary
- Ratio of voltages = ratio of turns
Common Mistakes
- Thinking transformers work with DC: Transformers require AC — a direct current produces a constant magnetic field that does not induce a voltage in the secondary coil
- Confusing step-up and step-down transformers: Step-up has more turns on the secondary (increases voltage, decreases current); step-down has fewer secondary turns (decreases voltage, increases current)
- Saying high voltage is used for safety in transmission: High voltage is used to reduce current, which reduces power loss (P = I²R) — safety is actually a challenge of high voltage, not a benefit
- Forgetting that power is conserved in an ideal transformer: If voltage doubles, current halves — power (P = IV) stays the same in a 100% efficient transformer
- Confusing the role of the iron core: The iron core transfers the changing magnetic field from primary to secondary coil — it is not part of the electrical circuit; the two coils are not electrically connected
Memory Aid
The Turns Ratio Rule — "UP means MORE" Step-UP transformer = MORE turns on secondary side. Fewer → More = voltage goes Up. The pattern: Up = More turns secondary, More voltage out. Remembering the Equations The voltage/turns equation: think of it as two fractions that must be equal: Vp / Vs = Np / Ns → "Primary over Secondary = Primary over Secondary" The power equation: think of both sides of the transformer having the same "power budget": Vp × Ip = Vs × Is → "Power in = Power out" Why High Voltage for Transmission — the I² reminder P(loss) = I²R. The current is squared, so halving current quarters the loss. High voltage means low current. Low current means tiny losses. Remember: high V → low I → tiny I² loss.