The National Grid

This lesson covers the National Grid — the system that transfers electrical energy from power stations to consumers across the UK. This is part of the AQA GCSE Physics specification (4.2.4) and includes understanding the roles of step-up and step-down transformers and why electricity is transmitted at high voltages.

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What Is the National Grid?

The National Grid is a system of cables, pylons and transformers that connects power stations to homes, businesses, schools, hospitals and factories across the UK. It allows electricity generated in power stations to be distributed efficiently to consumers wherever they are located.

graph LR
    A["Power Station<br>25,000 V"] --> B[Step-Up Transformer]
    B --> |400,000 V| C["Transmission Lines<br>National Grid"]
    C --> D[Step-Down Transformer]
    D --> |230 V| E[Homes and Businesses]

Key Components

Component	Function
Power stations	Generate electricity (typically at about 25,000 V)
Step-up transformers	Increase the voltage from 25,000 V to 400,000 V (or 275,000 V) for transmission
High-voltage transmission lines	Carry electricity across long distances on pylons
Step-down transformers	Decrease the voltage back to 230 V for domestic use
Local distribution network	Carries electricity from substations to homes and businesses

Exam Tip: The National Grid is one of the most commonly examined topics in the electricity unit. You must understand the complete chain: power station -> step-up transformer -> high-voltage transmission -> step-down transformer -> consumer.

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Why Is Electricity Transmitted at High Voltage?

This is one of the most important concepts in the electricity topic and is frequently asked in exam questions.

The Problem: Energy Loss in Cables

When current flows through the transmission cables, some energy is wasted as heat due to the resistance of the cables. The power lost in the cables is given by:

P(lost) = I^2 x R

Where:

• P(lost) = power lost as heat in watts (W)
• I = current in the cables in amperes (A)
• R = resistance of the cables in ohms

Since the resistance of the cables is fixed (determined by the material and dimensions of the cables), the only way to reduce power loss is to reduce the current.

The Solution: High Voltage, Low Current

The power transmitted is given by:

P = I x V

For a given amount of power to be transmitted, if the voltage is increased, the current can be decreased. Since power loss depends on I^2, even a small reduction in current gives a large reduction in energy wasted as heat.

Example:

A power station needs to transmit 100 MW of power.

Scenario	Voltage	Current	Power Loss (for R = 10 ohms)
Low voltage	25,000 V	4,000 A	I^2 x R = 4000^2 x 10 = 160,000,000 W = 160 MW
High voltage	400,000 V	250 A	I^2 x R = 250^2 x 10 = 625,000 W = 0.625 MW

At 25,000 V, the power loss (160 MW) actually exceeds the power being transmitted — this would be completely impractical.

At 400,000 V, the power loss is only 0.625 MW out of 100 MW — a loss of only 0.625%. This is much more efficient.

Exam Tip: This is a classic 6-mark question. The key logical chain is: (1) power is transmitted at high voltage; (2) from P = IV, for a given power, high voltage means low current; (3) from P = I^2R, low current means less power is wasted as heat in the cables; (4) therefore the system is more efficient and less energy is wasted.

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Transformers

A transformer is a device that changes the voltage of an alternating current (a.c.) supply. Transformers only work with a.c. — they do NOT work with d.c.

How a Transformer Works (Simplified for GCSE)

A transformer consists of:

• A primary coil — connected to the input (a.c.) supply
• A secondary coil — connected to the output
• An iron core — links the two coils magnetically

The process:

1. An alternating current flows through the primary coil.
2. This creates a changing magnetic field in the iron core.
3. The changing magnetic field passes through (links with) the secondary coil.
4. This induces an alternating potential difference across the secondary coil.
5. If there is a complete circuit connected to the secondary coil, an alternating current flows in the secondary circuit.

graph LR
    A["Primary Coil<br>(input a.c.)"] --> B["Iron Core"]
    B --> C["Secondary Coil<br>(output a.c.)"]

Step-Up Transformers

A step-up transformer increases the voltage. It has more turns on the secondary coil than on the primary coil.

• Used between the power station and the transmission lines.
• Increases voltage from ~25,000 V to ~400,000 V.

Step-Down Transformers

A step-down transformer decreases the voltage. It has fewer turns on the secondary coil than on the primary coil.

• Used between the transmission lines and consumers.
• Decreases voltage from ~400,000 V to ~230 V.

The Transformer Equation

V(p) / V(s) = n(p) / n(s)

Where:

• V(p) = potential difference across the primary coil (V)
• V(s) = potential difference across the secondary coil (V)
• n(p) = number of turns on the primary coil
• n(s) = number of turns on the secondary coil

For a step-up transformer: n(s) > n(p), so V(s) > V(p).

For a step-down transformer: n(s) < n(p), so V(s) < V(p).

Exam Tip: When using the transformer equation, always check whether the answer makes physical sense. If you are told it is a step-up transformer, the secondary voltage must be HIGHER than the primary voltage. If your answer gives a lower voltage, you have made an error — check which values you have put on which side of the equation.

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Worked Examples

Example 1: Step-Up Transformer

A step-up transformer has 500 turns on the primary coil and 20,000 turns on the secondary coil. The input voltage is 25,000 V. Calculate the output voltage.