Distinguish motor commutators from generator slip rings
Compare segment switching with continuous rotating contacts and trace a complete motor current path without shorting the coil ends.
Exam scope and assessment notes
Extended only (Supplement) · 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
- Trace series current paths.
- Recognise coil ends and source contacts.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which component stays fixed in the simple motor contact arrangement?
What you will learn
- Compare segment switching with continuous rotating contacts and trace a complete motor current path without shorting the coil ends.
- Trace the insulated segment–coil loop and explain why it differs from continuous generator contacts.
Trace two insulated commutator segments
Each coil end connects to a different conducting segment of the split-ring commutator. The segments are electrically separated. Current follows source terminal → brush → segment → coil → other segment → other brush → source return while the contacts are engaged. Joining the segments by a conductor would defeat the intended path and may short the source.
The stationary brushes provide sliding contact to the rotating segments. Near the switching point each brush changes which coil end it reaches. The direction through fixed coil-side labels consequently reverses each half-turn. Do not describe the brushes themselves as rotating switches.
Compare with an alternating-current generator
Two separate continuous slip rings each remain connected to the same generator coil end through a stationary brush. They do not exchange output terminals each half-turn; the generator’s induced e.m.f. alternates because of rotation. The motor split ring instead changes the coil-end connection to maintain the desired driving turn with a d.c. source.
These descriptions compare the specified simple designs, not every real machine. A broken brush contact can interrupt coil current and driving action. Proper continuity, contact geometry and insulation matter; a decorative circle labelled ring does not establish the switching topology.
Pause and explain
What is the key difference from continuous generator slip rings?
Worked example
A diagram uses two continuous slip rings in place of a simple d.c. motor’s split ring. What intended switching action is missing?
Show the worked solution
- Continuous rings retain the same coil-end connections.
- The simple d.c. motor needs coil-current reversal after each half-turn.
- Use separated commutator segments with changing brush contact.
Answer The half-turn coil-current reversal is missing from that continuous-contact design.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
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.
Trace the complete current path through an engaged simple motor.
Give me a hint
Follow one source brush through the coil and back to the other source terminal.
Compare my reasoning
- Enter through one stationary brush and its contacted segment.
- Pass through the coil to the other segment.
- Return through the other brush to the source.
Source → brush → segment → coil → other segment → other brush → source.
Look for these in your work
- Close the intended loop.
- Keep segments electrically separate.
Compare motor split-ring and generator slip-ring contact functions.
Give me a hint
Ask whether brush-to-coil-end identity switches.
Compare my reasoning
- Split-ring contact changes coil-end identity at the brushes.
- Continuous slip rings retain separate fixed-end identities.
- The first provides the simple motor’s half-turn coil-current reversal.
Split ring switches the coil connection; continuous slip rings do not.
Look for these in your work
- Explain the topology.
- Relate it to the specified machine function.
A diagram connects the two split-ring segments with a wire and omits the coil from the source loop. Correct it.
Give me a hint
Trace the unintended bypass before redrawing.
Compare my reasoning
- The joining wire can bypass the intended coil path.
- Retain insulated segments.
- Route current through the coil between its two ends.
Remove the segment bypass in the conceptual diagram and restore the intended coil path.
Look for these in your work
- Identify the bypass error.
- State the required insulated topology.
Common mistakes
- Keep the two segments insulated.
- Trace source and coil paths.
- Differentiate continuous rings from switched segments.
What can you explain now?
What prevents the two commutator segments from directly joining the source paths?
Compare with the explanation
An insulating separation
The separated conducting segments must not provide an unintended direct connection between the source contacts.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Trace the insulated segment–coil loop and explain why it differs from continuous generator contacts.
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.