Mains Electricity (AC and DC)
Learn Mains Electricity (AC and DC) for GCSE Physics with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. UK mains electricity is an alternating current supply at about 230 V and 50 Hz, unlike the direct current supplied by a battery.
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
Topic overview
Mains electricity in the UK is an alternating current supply at 230 volts and a frequency of 50 hertz. These three figures must be learned precisely.
Alternating current, or ac, means the current repeatedly changes direction, because the potential difference across the supply alternates between positive and negative. Direct current, or dc, flows in one direction only, and is produced by cells and batteries.
On an oscilloscope trace, ac appears as a wave crossing the zero line, whereas dc appears as a flat horizontal line. Recognising the two traces is a common exam question, as is stating which sources give which type.
Revision notes
The UK mains supply
Alternating current, 230 volts, 50 hertz.
The frequency of 50 Hz means the current changes direction 50 times per second. All three figures are examinable and should be given together.
Alternating and direct current
Alternating current repeatedly changes direction, because the supply potential difference alternates between positive and negative.
Direct current flows in one direction only, produced by a source with a constant potential difference such as a cell or battery.
Recognising the traces
On an oscilloscope, ac appears as a wave that crosses the zero line, going alternately positive and negative.
Direct current appears as a flat horizontal line above the zero line. Being able to identify each from a diagram is frequently tested.
Key points
- UK mains is alternating current.
- The supply is 230 volts.
- The frequency is 50 hertz.
- Alternating current changes direction repeatedly.
- Direct current flows in one direction only.
- Cells and batteries produce direct current.
Worked examples
Example 1
State the potential difference and frequency of the UK mains supply. [2 marks]
Model answer
230 volts (1 mark) and 50 hertz (1 mark).
Example 2
Explain the difference between alternating and direct current. [2 marks]
Model answer
Alternating current repeatedly changes direction because the supply potential difference alternates between positive and negative (1 mark). Direct current flows in one direction only, produced by a constant potential difference (1 mark).
Example 3
Describe how alternating and direct current appear differently on an oscilloscope. [2 marks]
Model answer
Alternating current appears as a wave crossing the zero line, going alternately positive and negative (1 mark). Direct current appears as a flat horizontal line (1 mark).
Common mistakes
Giving 240 V for the mains supply.
The UK mains supply is 230 V.
Saying batteries produce alternating current.
Cells and batteries produce direct current.
Confusing frequency with potential difference.
50 Hz is the frequency; 230 V is the potential difference.
Saying dc has no potential difference.
It has a constant potential difference in one direction.
Exam tips
- Learn 230 V and 50 Hz together as a pair.
- Say alternates between positive and negative when defining ac.
- Practise identifying oscilloscope traces.
- Name cells and batteries as dc sources.
Key terms
- Alternating current
- Current that repeatedly changes direction.
- Direct current
- Current flowing in one direction only.
- Frequency
- The number of cycles per second, in hertz.
- Oscilloscope
- An instrument displaying how potential difference varies with time.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.