Guided lesson

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.

First pass

Direct current goes one way only, from the positive terminal to the negative.

Analogy — A one-way escalator: everyone moves in the same direction, all the time.

Exam pointer — All the circuits you have built so far are d.c. — the calculations you know already apply unchanged.

Going deeper

The fixed polarity of the terminals is what fixes the direction: the current follows the potential difference, and that never reverses.

Analogy — A river bed that only ever slopes one way — the water has no choice of direction.

Exam pointer — If asked for a source, say cells or batteries, not 'the plug'.

Checks

0 of 9 attempted · 0 correct

1.In a d.c. circuit the charge flows:
2.Which supply has fixed terminals?
3.Cells and batteries provide:
4.The current in a d.c. circuit flows:
5.On an oscilloscope, a direct current trace appears as:
6.Define direct current.
Written response · recall
Your answer
7.Name two sources of direct current.
Written response · recall
Your answer
8.Explain why a battery cannot power a device that needs the current to reverse.
Written response · explain why
Your answer
9.A torch runs from two cells in a holder. State the kind of supply and the direction of the current through the lamp.
Written response · application
Your answer

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
Your answer
7.State the frequency and approximate voltage of UK mains electricity.
Written response · recall
Your answer
8.Explain why the two terminals of an a.c. supply are described as identical.
Written response · explain why
Your answer
9.An oscilloscope shows one trace flat and one trace oscillating. Identify each supply.
Written response · application
Your answer

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Back to the revision note All revision notes

1.Direct current from cells & batteries

2.6

2.Alternating current & the UK mains

2.6