The National Grid

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CognitoThe National Grid and transformers

Key facts & equations

How the National Grid works

Power stations generate electricity at ~25,000 V.
Step-up transformer: increases voltage to ~400,000 V for transmission.
High voltage, low current in transmission cables → less power wasted as heat (P = I²R).
Step-down transformer: reduces voltage for consumers. 132 kV → 33 kV → 11 kV → 230 V homes.
The grid is an interconnected network — electricity can be routed from any power station to any consumer.

Transformers

Transformer: two coils of wire (primary and secondary) wound around an iron core.
Works on AC only — changing magnetic field induces changing voltage.
Turns ratio: Vs/Vp = Ns/Np.
Step-up: Ns > Np → Vs > Vp. (More turns on secondary = higher voltage out.)
Step-down: Ns < Np → Vs < Vp.
Ideal transformer: power in = power out. VpIp = VsIs.

Transformer calculations

Turns ratio equation: Vp/Vs = Np/Ns (or Vs/Vp = Ns/Np).
Power conservation (ideal): Vp × Ip = Vs × Is.
If Vs > Vp (step-up): then Is < Ip (current decreases as voltage increases — power conserved).
E.g. generator at 25 kV, 100 A. Step up 1:16 ratio → 400 kV. New current = 100 × (25/400) = 6.25 A.
Efficiency: in real transformers, energy lost as heat in core (eddy currents) and coil resistance.

Exam questions — 4 questions · 10 marks · AQA 8463 style

3 marksTransformer calculationAQA 8463 P2 style 🔢 Calculator

A step-up transformer has 500 turns on the primary and 10,000 turns on the secondary. The input voltage is 230 V. (a) Calculate the output voltage. (b) If the primary current is 8 A, calculate the secondary current (assuming ideal transformer). (3 marks)

Hint: (a) Turns ratio: Vs = Vp × (Ns/Np). (b) Power in = Power out: VpIp = VsIs. Solve for Is.

+30 XP

1 markWhy high voltage transmissionAQA 8463 style

Transmitting electricity at high voltage reduces energy losses. Why?

A Higher voltage means lower resistance in the cables

B Higher voltage allows higher current, which is more efficient

C For a given power, higher voltage means lower current, and power loss in cables = I²R — lower current means much less power wasted

D High-voltage cables have thicker insulation which prevents heat loss

3 marksWhy AC for transformersAQA 8463 style

Explain why transformers only work with alternating current (AC) and not direct current (DC). (3 marks)

Hint: Key concept: electromagnetic induction requires a CHANGING magnetic field. AC gives changing current → changing field. DC gives constant current → constant field → no induction.

+30 XP

1 mark Transformer direction — inversion trap AQA 8463 Paper 1 style ⚡ Common error

A student writes: "A step-up transformer increases the current in the transmission cables." Which of the following correctly identifies the error and gives the right answer?

A The student is correct — step-up transformers increase current.

B The student meant voltage, not current — step-up increases voltage.

C The student is wrong. Step-up transformers increase voltage and decrease current — the opposite of what was stated.

D The student is wrong, but step-up transformers also decrease resistance, not current.

2 marks Transformer gap-fill — paper format AQA 8463 style — same format as exam

Complete the sentences. A step-up transformer increases the ___ and decreases the ___. A step-down transformer decreases the ___. Using transformers decreases the ___ transfer from the overhead cables. (2 marks)

Words to choose from: charge, current, energy, potential difference, resistance. Each may be used once or not at all.

+20 XP

The National Grid

How the National Grid works

Transformers

Transformer calculations

Module complete! 🎉