AC and DC, Mains Electricity

This lesson covers the difference between alternating current (AC) and direct current (DC) and the key features of the UK mains electricity supply, as required by the Edexcel GCSE Combined Science specification (1SC0).

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Direct Current (DC)

Direct current is an electric current that flows in one direction only.

Feature	Detail
Direction	Always the same (one direction)
Provided by	Cells, batteries, solar cells, DC power supplies
Polarity	Fixed positive and negative terminals

DC on an Oscilloscope

On an oscilloscope (or voltage-time graph), a steady DC supply appears as a horizontal straight line above or below the zero line.

graph LR
    subgraph "DC Trace"
        A["Time -->"] --- B["Constant voltage line"]
    end

The height of the line above the zero line shows the voltage. The line is flat because the voltage does not change with time.

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Alternating Current (AC)

Alternating current is an electric current that repeatedly reverses direction.