The National Grid

Section 1 of 8Journey of electricity

1Power station

• Power station Generators produce electricity at about 25 000 V.
• The electricity is generated as alternating current, which is needed so that transformers can be used.

2Step-up transformer

• Step-up transformer Raises the potential difference to as much as 400 000 V for transmission.

3Transmission lines

• Transmission lines High-voltage cables carry the electricity across the country.
• Cables are held high on pylons or buried so that people cannot touch them, and they are made of low-resistance metal such as aluminium.

4Step-down transformer

• Step-down transformer Lowers the potential difference to a safe value, 230 V for homes, near where it is used.
• Individual homes and businesses are connected to the local network, so each user receives the same 230 V.

Section 2 of 8Why use high voltage

1

Power transmitted is P = I × V, so the same power can be sent with a high voltage and a low current.

2

A lower current means less heating in the cables, because power lost is P = I² × R.

3

Doubling the voltage halves the current and cuts the heat loss to a quarter.

4

Therefore a very high potential difference makes the transmission efficient and saves energy.

5

Step-down transformers then make the supply safe for homes and businesses.

Section 3 of 8Transformers

Structure

• Structure Two coils of wire, the primary and secondary, wound around an iron core.
• Transformers work only with alternating current, since a changing field is needed.
• Because the iron core is magnetised and demagnetised again and again, it carries the changing magnetic field from the primary to the secondary coil.

Step-up and step-down

• Step-up and step-down A step-up transformer has more turns on the secondary coil than the primary;
  • a step-down transformer has fewer.
• Transformer equation (HT only)V_p × I_p = V_s × I_s for a 100% efficient transformer.

Section 4 of 8Overhead and underground cables

Feature	Overhead	Underground
Cost	Cheaper to build and repair	More expensive to install and fix
Visual impact	Can spoil the landscape	Hidden from view
Hazards	Danger to aircraft, and to people nearby	Safer from weather and accidental contact

Section 5 of 8Worked example: why voltage matters

Quantity	Low voltage (230 V)	High voltage (230 000 V)
Power to transmit	23 000 W	23 000 W
Current (I = P ÷ V)	100 A	0.1 A
Power lost in a 10 Ω cable (I2 × R)	1002 × 10 = 100 000 W	0.12 × 10 = 0.1 W
Result	More power lost than supplied	Almost no loss

Section 6 of 8Wider issues of the grid

Running the National Grid

Meeting demand

• Meeting demand Demand changes through the day, so power stations are switched on and off to match it.

Connected network

• Connected network Many power stations feed one network, so if one fails others can supply the electricity.

Cost of cables

• Cost of cables Thicker cables lose less energy as heat but cost more to make, so a balance is needed.

Section 7 of 8Key terms

Term	Meaning
National Grid	The network of cables and transformers that carries electricity from power stations to consumers.
Step-up transformer	A transformer that increases the potential difference.
Step-down transformer	A transformer that decreases the potential difference.
Transmission	Carrying electrical energy over long distances.
Efficiency	The proportion of energy transferred usefully rather than wasted.
Primary coil	The input coil of a transformer, connected to the alternating supply.
Secondary coil	The output coil of a transformer, where a changing potential difference is induced.

Section 8 of 8Exam technique

1

Saying that high voltage reduces energy loss because it is safer; it is because the current is smaller.

2

Forgetting that transformers need alternating current to work.

3

Mixing up step-up and step-downstep-up has more turns on the secondary coil.

4

Stating that transformers create energy; they only change the potential difference.

5

Saying a step-up transformer increases power; the output power cannot exceed the input.

6

Forgetting that the current falls by the same factor that the voltage rises in an ideal transformer.