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Electricity Transmission & DistributionOxford AQA IGCSE Physics: Revision notes

Section 1

How does electricity get from power stations to our homes?

Electricity generated at power stations is distributed to consumers via the National Grid: a network of transmission cables with transformers at both ends.

  • Step-up transformers increase the voltage for transmission
  • The electricity travels through cables at very high voltage across the country
  • Step-down transformers reduce the voltage back down before it reaches homes and businesses
Key termsNational Gridtransmission cables

Section 2

Why is electricity transmitted at high voltage?

Transmitting electricity at a high voltage reduces the current flowing through the cables for a given power.

  • Lower current means less energy is lost as heat due to resistance in the cables
  • This makes transmission much more efficient, since less useful energy is wasted before it reaches the consumer
  • This is why the National Grid uses very high voltages (hundreds of thousands of volts) for long-distance transmission
Key termscurrentefficiency
Exam tip

Link the chain clearly: higher voltage → lower current (for the same power) → less heating in the cables → less energy wasted.

Section 3

How does a transformer work?

A transformer consists of a primary coil and a secondary coil, both wound on a soft iron core.

  1. An alternating current in the primary coil creates a changing magnetic field in the iron core
  2. This changing magnetic field induces a potential difference (and alternating current) in the secondary coil

Transformers only work with alternating current (ac) — a steady direct current would not create a changing magnetic field, so no pd would be induced.

Key termstransformerprimary coilsecondary coil
Common mistake

Transformers only work with ac, not dc — a common exam error is forgetting to state this.

Section 4

Step-up vs step-down transformers and the transformer equations

  • A step-up transformer has a secondary pd greater than the primary pd (more turns on the secondary coil)
  • A step-down transformer has a secondary pd less than the primary pd (fewer turns on the secondary coil)

Transformer equation:

Vp ÷ Vs = np ÷ ns

where Vp and Vs are the primary and secondary potential differences, and np and ns are the number of turns on each coil.

Power equation for an ideal (100% efficient) transformer:

Vp × Ip = Vs × Is

Key termsstep-up transformerstep-down transformer
Example

A step-up transformer with 100 primary turns and 1000 secondary turns, supplied with 230 V, gives a secondary pd of 230 × (1000/100) = 2300 V.

Section 5

What are switch mode transformers?

Switch mode transformers operate at a much higher frequency than traditional transformers (50 kHz – 200 kHz).

  • They are much lighter and smaller than traditional transformers
  • They use very little power when switched on but with no load connected
  • They are useful in devices such as mobile phone chargers, where small size and low standby power use matter
Key termsswitch mode transformer

Must Know

  • Electricity is distributed via transmission cables with transformers at both ends
  • High transmission voltage reduces current, reducing heating losses and improving efficiency
  • A transformer has a primary coil and secondary coil wound on a soft iron core; it only works with ac
  • Step-up transformers increase pd (more secondary turns); step-down transformers decrease pd (fewer secondary turns)
  • Transformer equation: Vp/Vs = np/ns; ideal power equation: Vp×Ip = Vs×Is
  • Switch mode transformers are small, light, high-frequency, and use little power with no load (e.g. phone chargers)

That's the notes covered.

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