Explain the turning effect on a current-carrying coil
Recognise a force pair on opposite coil sides and distinguish a turning effect from a resultant pushing force.
Section P4.4 · Cells, batteries, generators and motors
Cross-referenced to the 2028–2029 0653 section using original teaching from the related single-science courses. A 0653-specific objective-by-objective and Core/Supplement review is in progress. Written practice is self-checked and does not demonstrate supervised laboratory competence or award an official Cambridge grade.
Before you begin
- Use force and moment ideas.
- Read a supplied side-view force diagram.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which geometric fact matters for a turning pair?
What you will learn
- Recognise a force pair on opposite coil sides and distinguish a turning effect from a resultant pushing force.
- Interpret a coil force pair and distinguish zero resultant, non-zero moment and dead-centre limits.
Opposite sides can give a turning pair
A current-carrying coil placed in a magnetic field may experience a turning effect. Opposite active sides carry current in opposite directions, so they can experience opposite magnetic forces. If those force lines are separated about the rotation axis, the pair can turn the coil even when their resultant force is zero.
A supplied side view may mark the left side pushed upward and right side downward. Those opposite forces give clockwise turning in that stated view. Reversing only the force arrows gives anticlockwise turning. These are observed or supplied force arrows in Core work; the absolute motor-direction rule belongs to Supplement.
Keep the word may meaningful
Coil orientation matters. At an ideal dead-centre orientation the force lines can pass through the axis and give zero turning effect even if the active sides still experience magnetic forces. A current and field therefore do not guarantee a non-zero moment at every angle. Mechanical friction or a load can also prevent visible rotation.
A fixed-polarity coil may turn toward a position where the turning effect is zero and need not continue making full rotations. Continuous simple d.c. motor operation needs a suitable switching mechanism, explained in the Supplement lessons. Do not claim that one force pair alone proves a self-starting motor at every orientation.
Sources for this lesson
Pause and explain
Equal opposite forces on separated sides have zero resultant. Must their turning effect be zero?
Worked example
In a supplied side view, equal forces push the left coil side upward and the right side downward. What happens to resultant force and turning sense?
Show the worked solution
- Equal opposite forces sum to zero resultant.
- Their lines act on separated coil sides.
- The pair gives clockwise turning in the supplied view.
Answer Zero resultant force; clockwise turning effect.
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.
Interpret the supplied left-up, right-down coil-force pair.
Give me a hint
Separate translation balance from rotation balance.
Compare my reasoning
- The equal opposite forces have zero resultant.
- Their separated lines give moments in the same sense.
- The supplied view gives clockwise turning.
Zero resultant and clockwise turning.
Look for these in your work
- Discuss both balances.
- Retain the viewed force arrows.
Reverse the supplied coil-force pair to left down and right up. State the turning sense.
Give me a hint
Follow the coil edge movements in the same view.
Compare my reasoning
- The left side tends downward.
- The right side tends upward.
- This pair tends anticlockwise.
Anticlockwise.
Look for these in your work
- Use the same view.
- Interpret the supplied force pair.
A report says any coil carrying current in a field must rotate continuously. Correct it.
Give me a hint
Consider orientation and mechanical constraints.
Compare my reasoning
- Some orientations have zero turning moment.
- Friction and load can prevent motion.
- Continuous operation needs suitable switching in the simple motor.
A coil may have a turning effect; continuous rotation is not guaranteed by current alone.
Look for these in your work
- State orientation limits.
- Separate a turning pair from a full motor.
Common mistakes
- Zero resultant does not imply zero moment.
- Keep the supplied viewing frame.
- Allow zero turning effect at dead centre.
What can you explain now?
Can opposite equal forces turn a coil with zero resultant force?
Compare with the explanation
Yes
Separated force lines can form a turning pair even though their vector sum is zero.
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Make it stick
Interpret a coil force pair and distinguish zero resultant, non-zero moment and dead-centre limits.
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.