Distinguish power supplies and generators in circuit diagrams
Recognize source representations and separate electrical energy delivery from the mechanism supplying it.
Section 4.3.1 · Circuit diagrams and circuit components
Exam scope and assessment notes
Core and Extended · Physics 0972, 2026–2028. Use the stated source, operating conditions and meter ranges. Written practice is self-checked. Human subject-teacher review, supervised practical competence and cumulative assessment remain pending. Cambridge states that IGCSE 0972 has the same subject content as IGCSE 0625, with 9–1 grading. This StudyForA pathway maps the 2026–2028 syllabus only. This 9–1 graded qualification is available only in a limited number of administrative zones. Confirm that your centre can enter you for this code and exam series with Cambridge International. Cambridge has published a separate syllabus for 2029. StudyForA has not reconciled that version yet; this page supports exams in 2026, 2027 and 2028 only.
Before you begin
- Describe d.c. and a.c. direction.
- Recall an electrical transfer pathway.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What distinguishes a.c.?
What you will learn
- Interpret power-supply and generator symbols.
- Use labelled d.c. or a.c. output without assuming every generator gives one type.
Read the supply label
A power supply provides an electrical output to a circuit. In a diagram, labelled output terminals or a labelled source symbol identify the connection points. A +/− reference indicates d.c. polarity; a tilde or a sinusoidal mark indicates a.c. A supply may convert energy from another source rather than storing it like a cell.
A generator transfers mechanical input into electrical output. A common symbol is a circle labelled G with terminal wires. The G label alone does not specify the output waveform, voltage or maximum current. Use the supplied a.c./d.c. designation and rating when interpreting a particular generator.
Explain the circuit behaviour
A conducting load needs a complete path between the source terminals. A generator can drive a lamp while mechanical input maintains suitable electrical output; stopping that input can remove the sustained output. A power supply likewise needs its energy input and suitable settings. Neither is a device that creates energy from nothing.
For school construction, use a teacher-approved isolated low-voltage source and its stated current limit. These diagrams are connection models, not mains wiring instructions. Draw two source terminals clearly and keep source polarity or a.c. labels visible whenever they matter. Later electromagnetic-induction lessons explain generator operation in more detail.
Pause and explain
A diagram labels a source G but gives no waveform. What is established?
Worked example
A circle labelled G supplies an a.c. lamp circuit. Describe the source and its energy input.
Show the worked solution
- G identifies a generator.
- The a.c. label indicates repeated output-direction reversal.
- Mechanical input is converted to electrical transfer to the load.
Answer An a.c. generator supplies the circuit using mechanical input.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Draw a generator symbol feeding a lamp through a closed return path, and label its output a.c.
Give me a hint
Use a G-labelled circle with two terminal wires.
Compare my reasoning
- Draw a circle containing G and show its terminals.
- Connect a lamp symbol in a closed loop.
- Add an a.c. label so the waveform is not inferred from G alone.
A G-labelled source and lamp in a closed loop, with explicit a.c. output label.
Look for these in your work
- Use electrical symbols.
- Close the return path.
Sketch an isolated d.c. supply with labelled positive and negative outputs and explain its role.
Give me a hint
A source representation must identify the actual terminal connections.
Compare my reasoning
- Draw a supply block or labelled source terminals.
- Mark the outputs + and − and connect them to a complete load path.
- Explain that the supply delivers electrical energy using its energy input.
A labelled d.c. supply maintains an electrical output across its terminals.
Look for these in your work
- Label polarity.
- Do not treat a supply as energy creation.
A generator-powered lamp goes out when mechanical input stops. A learner says the lamp consumed the generator charge. Explain a better account.
Give me a hint
Distinguish carriers in the circuit from sustained source energy input.
Compare my reasoning
- Charge is not used up as lamp fuel.
- The generator requires mechanical input to sustain electrical transfer.
- Losing suitable output can stop current and light production.
Loss of sustained mechanical input can stop electrical delivery; charge consumption is not the explanation.
Look for these in your work
- Trace the energy input.
- Keep charge conservation explicit.
Common mistakes
- G alone does not specify a.c. or d.c.
- A supply delivers energy rather than creating it.
- Read source ratings instead of inferring them from artwork.
What can you explain now?
Which input is associated with a generator?
Compare with the explanation
Mechanical input.
The generator transfers mechanical energy into electrical output.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Recognize supply terminals and G-labelled generators while reading the stated output type and energy input.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.