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: V_p / V_s = N_p / N_s
• Power (ideal): V_p × I_p = V_s × I_s
• Power loss in cables: P = I²R
• Power = V × I

National Grid Sequence

1. Power station generates at 25,000 V
2. Step-up transformer → 275,000–400,000 V
3. Transmission via pylons (low current)
4. Step-down at substation → 33,000 V
5. Further step-down → 230 V for homes

How a Transformer Works

1. AC flows in primary coil
2. Creates changing magnetic field
3. Iron core transfers field to secondary
4. Changing field induces voltage in secondary
5. 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