Compare factors that increase a coil turning effect
Compare turns, current and imposed magnetic-field strength one at a time with coil geometry and orientation fixed.
Section P4.5.5 · The d.c. motor
Exam scope and assessment notes
Mapped to the named 0973 section using teaching from the corresponding single-science course. A 0973-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade. This qualification reports two grades on the 9–1 scale; confirm the entry route for your zone and exam series.
Before you begin
- Understand a coil turning pair.
- Identify current and mechanical controls.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Does doubling turns at unchanged supply voltage necessarily keep current unchanged?
What you will learn
- Compare turns, current and imposed magnetic-field strength one at a time with coil geometry and orientation fixed.
- Compare turns, current and field under matched geometry while separating torque tendency from final speed.
Compare all three specified factors
For a matched current-carrying coil arrangement, increasing the number of turns, increasing current or increasing magnetic-field strength increases the turning effect. Keep coil area, shape, orientation and imposed-field geometry fixed when comparing these factors. More current-carrying turns contribute to the turning action in the same supplied field.
An ideal zero-moment orientation remains a special limit: multiplying a turning effect that is zero in that geometry does not make a non-zero moment. Compare a specified non-zero-turning orientation rather than silently changing the angle. The Core requirement is qualitative; exact numerical torque ratios need a separately justified model.
Do not replace turning effect with speed
A greater turning effect does not by itself specify final speed. Friction, mechanical load, source capability and operation can change the motion. More turns may alter coil resistance; the comparison must state measured or maintained current rather than assuming it stays equal from a fixed voltage setting.
A trial changing current and coil turns together can show a combined change but cannot isolate either factor. Compare one factor at a time, retain suitable apparatus limits and describe the resulting turning tendency. Written comparisons remain distinct from teacher-reviewed practical performance.
Sources for this lesson
Pause and explain
Which is a matched current-strength comparison?
Worked example
Coils with 20 and 40 turns have the same maintained current, area, orientation and imposed field, at a non-zero-turning position. Compare turning effects qualitatively.
Show the worked solution
- The stated geometry and current are matched.
- The second coil has more turns contributing to the force pair.
- Its turning effect is greater under the supplied conditions.
Answer Greater turning effect for 40 turns; no exact ratio is asserted.
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.
List three qualitative changes that can increase the specified non-zero coil turning effect.
Give me a hint
Keep the geometry and other factor values matched.
Compare my reasoning
- Increase the number of turns.
- Increase the maintained current.
- Increase the imposed field strength.
More turns, greater current, stronger field.
Look for these in your work
- Include all three factors.
- State matched conditions.
Why is a fixed-voltage 20-turn versus 40-turn test not automatically a turns-only comparison?
Give me a hint
Current may change with the winding.
Compare my reasoning
- The wire length and resistance can differ.
- The same voltage can give different current.
- Maintain or measure current for the turns comparison.
A changed current can confound the turns comparison.
Look for these in your work
- Name resistance and current.
- Explain the required control.
A report says a stronger field guarantees double final speed and removes every dead-centre problem. Correct it.
Give me a hint
Separate turning effect, motion and orientation.
Compare my reasoning
- Stronger field can increase a non-zero turning effect.
- Final speed depends on load and other conditions.
- An ideal zero-moment orientation can remain zero.
Greater turning effect does not establish an exact speed ratio or universal self-starting.
Look for these in your work
- Retain qualitative scope.
- State both motion and orientation limits.
Common mistakes
- Compare one factor at a time.
- Use maintained current, not only source voltage.
- Turning effect is not final speed.
What can you explain now?
Name the three listed ways to increase the matched coil turning effect.
Compare with the explanation
More turns, greater current and stronger field
Each can increase the turning effect when the other relevant conditions are held matched.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Compare turns, current and field under matched geometry while separating torque tendency from final speed.
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.