The National Grid

This lesson covers the National Grid — the system that transmits electrical energy from power stations to consumers across the country — as required by AQA GCSE Combined Science Trilogy (8464, section 6.2.2).

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What Is the National Grid?

The National Grid is a network of cables, pylons and transformers that connects power stations (and other electricity generators) to homes, businesses and factories across the UK.

Its purpose is to transfer electrical energy efficiently from where it is generated to where it is needed.

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How the National Grid Works

flowchart LR
    A["Power Station<br/>~25 000 V"] --> B["Step-Up Transformer<br/>Increases voltage to ~400 000 V"]
    B --> C["High-Voltage Transmission Lines<br/>(pylons and cables)<br/>~275 000 – 400 000 V"]
    C --> D["Step-Down Transformer<br/>Decreases voltage to ~230 V"]
    D --> E["Homes, Schools,<br/>Businesses<br/>~230 V"]

1. Electricity is generated at a power station at about 25 000 V.
2. A step-up transformer increases the voltage to about 400 000 V for transmission.
3. Electricity is transmitted at high voltage and low current through overhead cables on pylons (or underground cables).
4. A step-down transformer reduces the voltage to 230 V for safe use in homes and businesses.

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Why Use High Voltage for Transmission?

The key physics behind the National Grid is about reducing energy losses during transmission.

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

$P_{\text{lost}} = I^2 \times R$Plost​=I2×R

Where:

• $P_{\text{lost}}$Plost​ = power lost as heat (W)
• $I$I = current in the cables (A)
• $R$R = resistance of the cables (Ω)

Since $P = IV$P=IV, for a fixed amount of power being transmitted:

$I = \frac{P}{V}$I=VP​

If the voltage is increased, the current decreases. Since $P_{\text{lost}} = I^2R$Plost​=I2R, a lower current means much less energy is wasted as heat (because current is squared).

Numerical Example

A power station transmits 100 MW of power through cables with a total resistance of 10 Ω. Compare the power lost at 25 000 V and 400 000 V.

At 25 000 V:

$I = \frac{P}{V} = \frac{100 \times 10^6}{25{,}000} = 4000 \text{ A}$I=VP​=25,000100×106​=4000 A

$P_{\text{lost}} = I^2R = 4000^2 \times 10 = 160{,}000{,}000 \text{ W} = 160 \text{ MW}$Plost​=I2R=40002×10=160,000,000 W=160 MW

This is more power than is being transmitted — clearly impractical!

At 400 000 V:

$I = \frac{P}{V} = \frac{100 \times 10^6}{400{,}000} = 250 \text{ A}$I=VP​=400,000100×106​=250 A

$P_{\text{lost}} = I^2R = 250^2 \times 10 = 625{,}000 \text{ W} = 0.625 \text{ MW}$Plost​=I2R=2502×10=625,000 W=0.625 MW

The power loss is reduced by a factor of more than 250 by using higher voltage.

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Transformers

Step-Up Transformer

Property	Detail
Function	Increases the voltage
Where used	Between the power station and the transmission lines
Effect on current	Current decreases (to keep power approximately constant)
Coil turns	More turns on the secondary coil than the primary coil

Step-Down Transformer

Property	Detail
Function	Decreases the voltage
Where used	Between the transmission lines and consumers (homes, businesses)
Effect on current	Current increases
Coil turns	Fewer turns on the secondary coil than the primary coil

flowchart LR
    subgraph "Step-Up Transformer"
        direction LR
        A1["Primary Coil<br/>(fewer turns)<br/>Low V, High I"] --> B1["Iron Core"] --> C1["Secondary Coil<br/>(more turns)<br/>High V, Low I"]
    end

flowchart LR
    subgraph "Step-Down Transformer"
        direction LR
        A2["Primary Coil<br/>(more turns)<br/>High V, Low I"] --> B2["Iron Core"] --> C2["Secondary Coil<br/>(fewer turns)<br/>Low V, High I"]
    end

Exam Tip (AQA 8464): Transformers only work with AC — this is one of the main reasons the mains supply uses AC rather than DC. A changing (alternating) current in the primary coil creates a changing magnetic field, which induces a voltage in the secondary coil.

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Why Not Use DC for Transmission?

• Transformers only work with AC.
• Without transformers, you cannot step the voltage up for efficient long-distance transmission.
• DC transmission would require extremely low resistance cables (much thicker and more expensive) to reduce energy losses.

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Underground vs Overhead Cables

Feature	Overhead (Pylons)	Underground
Cost	Cheaper to install	More expensive to install and maintain
Visual impact	Visible — considered unsightly	Hidden from view
Safety	Risk from fallen cables; keep clear	Safer from accidental contact
Maintenance	Easier to access for repairs	Harder and more expensive to repair

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Common Mistakes

Mistake	Correction
Saying "high voltage reduces resistance"	High voltage reduces current, which reduces energy lost as heat; resistance of the cables stays the same
Saying transformers "create" energy	Transformers transfer energy — they change voltage but do not create energy
Confusing step-up and step-down	Step-up = increases voltage (more secondary turns); step-down = decreases voltage (fewer secondary turns)
Forgetting that transformers only work with AC	This is a key AQA exam point — transformers require a changing current

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Summary

• The National Grid transmits electricity from power stations to consumers using high-voltage cables and transformers.
• A step-up transformer increases voltage and decreases current for efficient transmission.
• A step-down transformer decreases voltage to 230 V for safe domestic use.
• High voltage is used because it reduces the current, and since $P_{\text{lost}} = I^2R$Plost​=I2R, a lower current means much less energy is wasted as heat.
• Transformers only work with AC — this is why the UK mains supply is AC.
• The key equation is $P_{\text{lost}} = I^2R$Plost​=I2R: halving the current reduces power loss by a factor of four.

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Extended Worked Examples

Worked Example 1 — Power Lost at High vs Low Voltage