Electricity transmission and distribution - CORE Physics (9223) - Oxford AQA IGCSE

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⚡️ Study Notes: Electricity Transmission and Distribution ⚡️

Hello future physicist! This chapter explains the incredible journey electricity takes from the power station to your socket. Understanding this system, often called the National Grid, is essential because it shows how engineers maximize efficiency while keeping us safe. Don't worry if this seems tricky at first—we'll break down the high-voltage concepts into simple steps!

1. The National Grid: Electricity's Highway

The National Grid is a huge network of cables, pylons, and transformers that link power stations to consumers (homes, schools, factories). It's the infrastructure that makes modern life possible!

The Four Key Stages of Distribution

Generation: Electricity is created at the power station (usually at moderate voltages, e.g., 25,000 V).
Step-Up Transformation: The voltage is drastically increased (stepped up) to very high levels (e.g., 400,000 V) for efficient transmission across the country.
Transmission: The high-voltage electricity travels long distances using thick overhead cables (pylons) or underground wires.
Step-Down Transformation & Distribution: The voltage is reduced in stages (stepped down) until it reaches the safe household voltage (e.g., 230 V in many regions).

Quick Review: The main goal of the Grid is to move large amounts of power over vast distances with minimal energy waste.

2. The Problem of Power Loss

Imagine trying to move electricity from one side of a country to another. Just like water flowing through a pipe encounters friction, electrical current flowing through a wire encounters resistance (R).

What is Power Loss?

When current flows through a wire, the resistance causes energy to be converted into heat. This heat energy is wasted (lost) to the surroundings. This is why electrical appliances get warm!

The key equation for electrical power is:
$$P = I \times V$$
Where $P$ is total power delivered (Watts), $I$ is current (Amps), and $V$ is voltage (Volts).

The equation for power lost (wasted as heat) during transmission is:
$$P_{\text{loss}} = I^2 R$$
Where $P_{\text{loss}}$ is the wasted power, $I$ is the current flowing, and $R$ is the resistance of the cables.

Why High Voltage is Essential

The formula $P_{\text{loss}} = I^2 R$ tells us something crucial:

Power loss depends on the current squared ($I^2$).
If you double the current (I), the power loss goes up by four times ($2^2 = 4$)!

To minimize this devastating loss, engineers must keep the current ($I$) as low as possible.

Since the total power delivered ($P = I V$) needs to remain constant, if we decrease the current ($I$), we must increase the voltage ($V$) by the same factor.

Example Analogy: Imagine moving a delivery truck (Power, P) full of goods. To move it quickly and efficiently (low loss), you don't want to use a weak engine running at high speed (High Current, I). Instead, you use a massive, powerful engine (High Voltage, V) that lets you cruise with minimum effort (Low Current, I).

Key Takeaway: The Grid uses extremely high voltage (up to 400,000 V) to achieve low current, which minimizes energy waste through heating ($I^2 R$). This makes the transmission process highly efficient.

3. Transformers: The Voltage Changers

Since power stations generate electricity at moderate voltages, and homes use low voltages, we need devices that can efficiently change the voltage up and down. These devices are called transformers.

Important Rule: Transformers only work with Alternating Current (AC). This is why the National Grid uses AC rather than Direct Current (DC).

The Two Types of Transformers

Transformers work by having two coils of wire—a primary coil (input) and a secondary coil (output)—wrapped around an iron core. The ratio of turns in these coils determines the voltage change.

Step-Up Transformer:
- Function: Increases Voltage and Decreases Current.
- Location: Used right next to the power station before transmission begins.
- Structure: Has more turns on the secondary coil than on the primary coil.
Step-Down Transformer:
- Function: Decreases Voltage and Increases Current.
- Location: Used near consumers (cities, towns, and local substations) to reduce the voltage in safe steps.
- Structure: Has fewer turns on the secondary coil than on the primary coil.

Memory Aid: Think about the voltage: If the voltage steps up, the number of coils must also step up (more turns on the output).

Did you know? The efficiency of large transformers can be over 99%! They are one of the most efficient electrical devices ever invented.

The Relationship between Voltage and Current in Transformers

Since we assume an ideal transformer is 100% efficient, the power input must equal the power output:
$$P_{\text{input}} = P_{\text{output}}$$
$$I_p V_p = I_s V_s$$
(Where 'p' is primary and 's' is secondary).

This relationship confirms the conservation of energy:

If Voltage ($V$) goes up (Step-Up), Current ($I$) must go down.
If Voltage ($V$) goes down (Step-Down), Current ($I$) must go up.

Key Takeaway: Transformers use AC to efficiently trade voltage for current (and vice versa) to ensure electricity is transmitted efficiently at high voltage and used safely at low voltage.

4. Safety Concerns and Final Distribution

High voltages are incredibly efficient, but they are also incredibly dangerous. This is why the high-voltage transmission lines are held up high on massive pylons, away from public access.

The Final Step Down

Before electricity enters your home, the voltage must be reduced to the standard mains supply voltage (e.g., 230 V). If the voltage remained high, it would be instantly fatal and would destroy every appliance in your house.

Household Safety Connection (A Prerequisite)

Although transmission is about efficiency, household distribution focuses on safety. Once the electricity is at a low, usable voltage, safety devices protect users:

Fuses and Circuit Breakers: These stop the flow of current if it becomes too high (a surge or short circuit), preventing wires from overheating and causing fires.
Earth Wire: This provides a path for dangerous currents (due to faults) to flow safely into the ground, protecting the user from electric shock.

Quick Summary of Transmission