AC & DC

2.6

2 guided lessons · 18 checks · 2. Electricity → Electrical Power & Mains Electricity

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Lesson 1 of 2

Direct current from cells & batteries

A battery pushes charge the same way all the time: its positive and negative terminals never swap, so the current flows steadily one way around the circuit. That is direct current, and every cell or battery you have wired into a circuit produces it.

What you know already

Two kinds of current power the world: the steady one-way flow from a battery, and the back-and-forth flow from the mains. This note defines direct and alternating current, contrasts their terminals, and pins down the UK mains figures — alternating current at 50 Hz and about 230 V — alongside the oscilloscope traces that tell them apart.

Key points of understanding

Direct current flows steadily one way round the circuit, positive to negative.
A d.c. supply keeps fixed positive and negative terminals.
Cells and batteries are the everyday sources of direct current.

Voltmeter in a circuit, downloadable AS & A Level Physics revision notes — Circuits powered by cells or batteries use a d.c. supply

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1.Direct current from cells & batteries

2.6

Key points

Direct current flows steadily one way round the circuit, positive to negative.
A d.c. supply keeps fixed positive and negative terminals.
Cells and batteries are the everyday sources of direct current.

Guided content from the SME authoring corpus — note rn_NgxJhPpm5Zs7kFSz, pinned at 502107a3fcac · promoted SyllabAI/syllabai-resources b3caaaa90c5e

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2.Alternating current & the UK mains

2.6

The mains behaves differently: its two terminals keep trading polarity, so the current sweeps back and forth instead of holding one direction. That is alternating current. The frequency counts the sweeps each second — 50 of them in UK sockets — and the voltage sits near 230 V.

ac-&-dc, IGCSE & GCSE Physics revision notes — Two graphs showing the variation of current with time for alternating current and direct current

Key points

Alternating current changes direction continuously, moving back and forth.
An a.c. supply has two identical terminals whose polarity keeps swapping.
The frequency counts the back-and-forth direction changes each second.
UK mains delivers alternating current at 50 Hz and about 230 V.
Generators at power stations produce the mains alternating current.

First pass

Alternating current keeps reversing direction; UK mains does this 50 times a second at about 230 V.

Analogy — A pendulum versus a one-way slide: the pendulum sweeps both ways, the slide only down.

Exam pointer — Learn the two mains numbers as a pair — 50 Hz and 230 V — questions rarely ask for one without the other.

Going deeper

Because the terminals swap polarity continuously, neither is 'the positive one'; each is positive half the time and negative the other half.

Analogy — Two children on a see-saw taking turns being 'up' — neither holds the title for long.

Exam pointer — Sketch the two oscilloscope traces: a flat line for d.c., an oscillating wave for a.c.

Checks

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1.Alternating current is defined by:
2.UK mains potential difference is about:
3.The frequency of UK mains is:
4.An a.c. supply's two terminals are:
5.Mains alternating current is produced by:
6.Define alternating current.
Written response · recall
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7.State the frequency and approximate voltage of UK mains electricity.
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8.Explain why the two terminals of an a.c. supply are described as identical.
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9.An oscilloscope shows one trace flat and one trace oscillating. Identify each supply.
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