The National Grid | Physics

📖 In-Depth Theory

What Is the National Grid?

The NATIONAL GRID is the network of cables and transformers transmitting electrical energy from power stations to consumers.

Path:

POWER STATIONS → STEP-UP TRANSFORMERS → HIGH-VOLTAGE CABLES → STEP-DOWN TRANSFORMERS → HOMES (230 V)

Transmission uses very high voltages (132 kV to 400 kV).

Before homes receive it, voltage is stepped down to 230 V by local substations.

Why Transmit at High Voltage?

P = IV. For a given power:

High voltage → LOW current (for the same power)

Low current → less I²R heating in cables → less energy wasted.

Current is SQUARED in power loss (P = I²R) — halving current reduces cable losses by ¾.

EXAMPLE:

Transmit 1 MW through cables (R = 10 Ω):

At 1000 V: I = 1000 A → P_lost = 1000² × 10 = 10 MW (more than transmitted!)

At 100,000 V: I = 10 A → P_lost = 10² × 10 = 1000 W (tiny fraction)

High voltage = far more efficient.

Transformers

TRANSFORMERS change voltage of AC only (not DC).

STEP-UP: increases voltage (decreases current) — at power stations.

STEP-DOWN: decreases voltage (increases current) — at local substations.

Transformer equation:

Vp ÷ Vs = np ÷ ns

Vp = primary voltage; Vs = secondary voltage

np = primary turns; ns = secondary turns

EXAMPLE:

500 primary turns, 50 secondary turns, Vp = 10,000 V:

Vs = 10,000 × (50 ÷ 500) = 1000 V (step-down)

⚠️ Common Mistake

Transformers only work with AC — DC produces no output. High voltage reduces CURRENT (not power) — cable losses are proportional to I² so even a small current reduction gives large savings.

📐 Variables

VpPrimary voltage (Vp) is measured in volts (V)

VsSecondary voltage (Vs) is measured in volts (V)

npPrimary turns (np) is measured in ()

nsSecondary turns (ns) is measured in ()

ICurrent (I) is measured in amperes (A)

PPower (P) is measured in watts (W)

📐 Key Equations

Vp ÷ Vs = np ÷ ns

P_lost in cables = I² × R

📌 Key Note

Power stations → step-up → high-voltage cables → step-down → homes (230 V). High V = low I = low I²R losses. Step-up: more secondary turns. Step-down: fewer secondary turns. Transformers: AC only. Vp/Vs = np/ns.

🎯 Matching Activity — National Grid Components

Match each component to its role. — drag the symbols on the right to match the component names on the left.

Step-up transformer

Drop here

Transmission cables

Drop here

Step-down transformer

Drop here

High-voltage transmission

Drop here

At power stations — increases voltage to ~400 kV for efficient transmission

Carry high-voltage AC across the country on pylons or underground

Reduces current → reduces I²R losses — more efficient

At substations — reduces voltage to 230 V for homes

⚽ FIFA Worked Examples

Transformer Calculation

A step-down transformer: 2000 primary turns, 100 secondary turns, primary voltage 230,000 V. Find secondary voltage.

F

Vs = Vp × (ns ÷ np)

I

Vp = 230,000 V; np = 2000; ns = 100

F

Vs = 230,000 × (100 ÷ 2000) = 230,000 × 0.05

A

Vs = 11,500 V

🎯 Test Yourself

Question 1 of 2

1. Why is electricity transmitted at high voltage across the National Grid?

2. A transformer has 4000 primary turns, 200 secondary turns, primary voltage 20,000 V. What is the secondary voltage?

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⚡ The National Grid

What Is the National Grid?

Why Transmit at High Voltage?

Transformers

Mr. Badmus AI