From a magnetic field to a moving motor
In the previous guide, the magnetic field around a current-carrying wire was the subject. We learned to distinguish the direction of current from the direction of the magnetic field, to read a compass observation without claiming it measured field strength, and to distinguish a straight wire from a solenoid. This guide changes the question. What happens when a wire carrying current is placed inside an external magnetic field? Under suitable conditions, the wire experiences a force. Arrange two active sides of a coil in the field and those forces can turn the coil. Understanding that change is the beginning of understanding a simple electric motor.
This guide teaches the G3 Physics motor-effect route. Its particular purpose is to help you interpret force direction, apply Fleming’s left-hand rule, explain the turning effect on a coil, and understand what a split-ring commutator does. It does not attempt to teach every part of electromagnetism or to repeat the preceding compass investigation. You will work from observations toward diagrams, then from diagrams toward a careful written explanation. There are original practice tasks and worked answers throughout. The examples are learning exercises, not authentic examination questions or guarantees about an examination paper.
The quick route before any calculation
First locate the external magnetic field, normally shown from north to south between pole pieces. Then locate the conventional current through the active conductor. Ask whether the conductor is perpendicular to the field and whether current is actually flowing. Only after identifying those two directions should you use Fleming’s left-hand rule to deduce the direction of force. On a rectangular coil, examine each active side separately. Find the direction of the two forces and ask whether they produce a turning effect. Then explain how a commutator maintains rotation after each half-turn by reversing current through the coil at the appropriate point. Finally, check whether your conclusion is about force direction, greater turning effect, continued rotation, or experimental evidence. Those are different examination jobs.
The official 2027 G3 Physics syllabus is K323. Its electromagnetism outcomes include force on a current-carrying conductor, reversal of current or field, Fleming’s left-hand rule, the turning effect on a current-carrying coil, the effects of current and coil turns, and the split-ring commutator. The syllabus also includes the effect of a soft-iron cylinder inside the coil. Physics K323 has Paper 1 multiple choice (one hour, 40 marks, 30%), Paper 2 structured and free response (one hour 45 minutes, 80 marks, 50%) and Paper 3 practical (one hour 50 minutes, 40 marks, 20%). These are published specifications, not predictions about what questions will appear. See the 2027 official K323 Physics syllabus and SEAB’s 2027 G3 syllabus index.
The definition you need to hold steady
The motor effect is the force experienced by a current-carrying conductor when it is placed in a magnetic field under suitable conditions. A force is not the same thing as the magnetic field itself. A field describes where and how magnetic interactions can occur; the force is an effect on the conductor. The conductor must form part of a circuit with current. A diagram may show a battery, a wire and a pair of magnets, yet an open switch means there is no sustained current through the wire in that idealised experiment. A magnetic field can exist without an operating motor. Keep the cause and observed outcome separate.
The result is directional. If the field and conventional current are at right angles, the force is perpendicular to both. This gives a useful three-direction relationship rather than a single memorised arrow. If you reverse only the current, the force reverses. If you reverse only the external magnetic field, the force reverses. If you reverse both current and field, the two reversals cancel, so the force has its original direction for the same geometry. If either ingredient is absent, you cannot infer the usual motor force. The effect is not a claim that all moving conductors must accelerate in every practical arrangement; supports, resistance, friction and the coil geometry also matter.
Conventional current is a deliberate choice
In exam diagrams, current direction normally means conventional current: from the positive terminal through the external circuit toward the negative terminal. Electrons in metal conductors move on average in the opposite direction, but that fact is not permission to reverse a conventional-current arrow when applying Fleming’s left-hand rule. Read the labels before reasoning. A circuit question might explicitly give a current arrow without showing a battery. Use that arrow. Another might show the cell symbol and wires, requiring you to trace the circuit first.
Start by following the entire path, not only the short section inside the magnetic field. If a side of a rectangular coil carries current upward, the opposite active side carries current downward. This opposite direction matters because the sides then experience forces in opposite directions. Those forces can cooperate to turn the coil, rather than simply pulling the whole coil in one direction. A carefully marked pair of current arrows is often worth more than a long verbal guess.
Magnetic-field arrows mean north to south outside the magnet
A straight magnetic-field arrow between labelled pole pieces is interpreted as pointing from the north pole to the south pole, outside the magnet. Do not use the direction in which the conductor might move to invent the field direction. The printed N and S labels, or an explicit arrow, establish the field. If one pole label is omitted and the arrow remains, the arrow still tells you the field direction. If both are absent, the question must supply another way to identify the direction or you must acknowledge that it cannot be determined from the supplied information alone.
The field produced by the current in the conductor is not the same as the external field drawn between magnets. Last volume’s compass investigation was about identifying current-produced fields. This volume’s force question involves a current-carrying conductor placed in a magnetic field. Both ideas belong to electromagnetism, but mixing their diagrams causes avoidable mistakes. For the earlier field direction, revisit Vol 0492: current-produced magnetic fields.
Fleming’s left-hand rule in a calm three-step sequence
Hold the left hand so the thumb, first finger and second finger are mutually perpendicular. The first finger represents the external magnetic field, the second finger conventional current, and the thumb the force or motion tendency on the conductor. A useful memory aid is field, current, force, but understanding the physical arrows matters more than reciting the order. Fix the field first. Rotate the hand without changing which finger represents which quantity. Align the second finger with the current and then read the thumb.
You cannot responsibly determine a force by positioning a finger almost parallel to another and hoping the answer looks plausible. These are perpendicular directions in the standard right-angle application. If a problem uses a perspective drawing, identify which arrow points into or out of the page. A dot commonly means a vector toward the reader; a cross commonly means a vector away, like the tail of an arrow. These symbols are conventions: they are not positive and negative charges. When you have decided, write one short justification: ‘With the field to the right and conventional current into the page, the force is downward’ only when your own left-hand alignment supports it.
Work one conductor before attempting a complete coil
Imagine a straight horizontal conductor running across a gap between magnetic pole pieces. The field points from left to right and the current is directed out of the page. Align the first finger right and the second finger toward yourself; the thumb indicates the force direction. To avoid mental rotation errors, construct the three mutually perpendicular arrows explicitly, or use a labelled sketch. The essential method is reproducible by any student and does not depend on where a diagram happens to sit on the page.
Now deliberately alter one fact at a time. Reverse the current but leave the magnets where they are; the force points the opposite way. Restore the original current and exchange the north and south poles; the force again reverses. Reverse both in the original set-up; the force returns to its original direction. This four-case family is a better test of understanding than memorising one familiar diagram. It also teaches an examination habit: before you answer ‘reverse’, identify exactly which physical direction was changed.
Why a rectangular coil turns
A simple d.c. motor has a coil with two opposite active sides located in a magnetic field. Conventional current travels in opposite directions along those sides. The magnetic forces on the sides are correspondingly opposite and may form a couple, producing a turning effect about the axle. ‘Opposite forces’ does not mean ‘no movement’. If the forces act along different lines, they can turn an object. A door provides a familiar analogy: where a force is applied and which way it acts influence the turning effect.
Use a labelled coil diagram with left side AB and right side CD. Write the current direction on AB, then trace the circuit to CD; do not infer the second current arrow from visual symmetry. Apply the left-hand rule to each side. If AB is forced up and CD down, describe the resulting rotation from the specified viewing direction. Without a viewing direction, terms like clockwise may be ambiguous. A strong answer gives a viewpoint, such as ‘clockwise as seen from the axle’s labelled end’, rather than pretending that clockwise is universal.
The split-ring commutator is a timing mechanism
A coil that turns through a half-revolution changes the physical positions of its two active sides. Without appropriate current reversal, the turning effect would not continue in the same rotational sense throughout successive half-turns. The split-ring commutator reverses the current through the coil every half-turn as the rotating contacts exchange their connection with the brushes. This is a functional description; it does not mean the battery itself reverses polarity.
A common weak answer says, ‘the split ring makes electricity flow’. Brushes and the circuit provide electrical contact; the commutator’s distinctive role is to reverse current in the rotating coil so its turning effect can continue in the same sense. Another weak answer says, ‘the magnetic field reverses every half-turn’. In the simple permanent-magnet model, the external field remains fixed; the commutator changes the current connections. To explain the motor, name the quantity that actually changes. Better language also improves the physics.
What more turns and more current can do
The K323 learning outcomes specify that the turning effect is increased by increasing the current through the coil or by increasing the number of turns on the coil. A current increase is not the same as reversing its direction. An increase in turns is a change in the winding, not an increase in revolutions per second. Those words look similar but identify different quantities. The coil turns are loops of wire; the motor’s rotations are mechanical motion over time.
Do not make an unsupported promise that doubling current always doubles motor speed. The syllabus relationship concerns turning effect; actual rotational speed depends on the motor, supply, load, friction and operating conditions. Likewise, a coil with twice as many turns does not necessarily lift exactly twice as many washers, travel twice as far or draw half the energy. Use the official trend for the turning effect and use experimental measurements for any numerical claim about a particular prototype.
The soft-iron cylinder is not decoration
A simple motor coil may be wound onto a soft-iron cylinder. That core strengthens the magnetic effect in the motor arrangement and can improve the turning effect. Soft iron is chosen because it can magnetise readily in the field and does not behave like a permanently fixed magnet in the same way as hard magnetic materials. In a school description, focus first on what the core changes and why the motor responds more strongly; avoid claiming a precise numerical gain unless measurements have been supplied.
If a diagram shows a permanent magnet, a coil and a soft-iron core, label all three components. The permanent magnet provides the external magnetic field. The coil carries current. The core modifies the magnetic response of the assembly. These roles are related but not interchangeable. A student who can name components but cannot say what changes when one is removed has not yet reached explanation. Try to answer a comparison question: with the rest held fixed, what effect is expected when the specified soft-iron core is inserted?
A note about practical evidence
A classroom motor test is affected by more than the magnetic force. A coil can fail to start because of poor contact, an unsuitable resting position, friction at the axle, low available current, or a mechanical obstruction. One unsuccessful trial does not disprove the motor effect. Equally, a spinning coil does not prove a precise equation for its rotational speed. Record whether the circuit was connected, what was changed and what was actually observed. Repeat trials under comparable conditions. Keep power supplies appropriate to school supervision and do not improvise with household mains electricity.
For K323 Physics practical work, SEAB assesses planning, manipulation and measurement, presentation of data, and analysis and evaluation. Treat this guide as reasoning preparation, not an instruction to recreate a specific examination set-up. Apparatus and examination requirements may vary. If you are preparing for Paper 3, practise with your teacher’s safe equipment and learn to keep records that another reader can audit.
Practice clinic: distinguish the exact decision
The exercises below use invented situations. For each one, say the first fact that matters, give a concise answer and name a tempting but wrong inference. They become harder gradually: direction first, then coil mechanics, then limits of evidence. Try the question without looking at the solution, then use the reasoning to mark the earliest error in your own response.
Clinic 01: Current is present, but no external field is shown
A drawing contains a cell, switch and straight wire. The caption asks for the direction of the motor-effect force, but it gives no external magnetic field. You cannot manufacture the required direction from the current arrow alone. The correct response is that the drawing does not give enough information to determine a motor-effect force direction. The current generates a magnetic field of its own, which was the focus of Vol 0492; that observation does not supply a separate external field vector for this left-hand-rule task. Before using the hand rule, check that you actually have both necessary directions.
Clinic 02: The switch has been opened
A student correctly identifies the north and south magnetic poles but forgets that the switch is open. In the ideal school circuit, no steady current passes through the conductor, so the usual magnetic force on a current-carrying wire is absent. Do not reply that the permanent magnets have disappeared or that the field has been ‘switched off’. The field remains in the gap. The missing ingredient is current through the wire. This distinction is a useful test of whether your explanation names a physical cause or merely repeats the desired outcome.
Clinic 03: A current arrow points into the page
The diagram uses a cross in a small circle. The question defines that symbol as current directed into the page. Translate the symbol into a verbal direction before arranging the left hand. It does not represent the north pole and it does not mean negative current. Place the first finger along the given field and the second finger into the page. Only then identify the thumb. If you find yourself changing the sign of current because the symbol looks like a multiplication sign, revisit the legend.
Clinic 04: Only the battery terminals are exchanged
The external magnets stay fixed, and the circuit is reconnected so the conventional current in the active wire reverses. The motor-effect force reverses direction. The field remains in the original direction. In a two-sided coil, the reversal changes the force on both active sides and therefore the initial turning sense, provided the arrangement allows the motion. Do not say the current has become stronger merely because its arrow changed. Reversal is a directional operation; magnitude requires separate information.
Clinic 05: Only the permanent magnet is turned around
The battery terminals and wire connections remain unchanged while the external pole orientation is reversed. The magnetic-field direction through the active conductor reverses. Therefore the force reverses. A good answer states both the changed quantity and consequence. ‘The conductor moves differently because north becomes south’ is too imprecise. The action that matters is the reversal of field direction relative to the unchanged current.
Clinic 06: Both current and field reverse
A technician swaps the battery connections and reverses the permanent magnet. For the same conductor geometry, the motor-effect force returns to its original direction, because reversing either factor alone would reverse the force, while reversing both restores its direction. Students sometimes count two changes and predict double force. That is not justified. Force magnitude cannot be inferred from two direction reversals without other facts. The examination skill is to track direction and magnitude separately.
Clinic 07: A student uses the right hand by habit
The pupil remembers a rule for another electromagnetic relationship and applies the right hand to a motor-force question. Stop before moving any fingers. For the force on a current-carrying conductor in an external magnetic field, K323 expects Fleming’s left-hand rule. Name the physical target—force—before choosing the mnemonic. Later study of induction uses different directional reasoning; do not transfer a method merely because both contexts mention magnets and current.
Clinic 08: Clockwise from whose view?
A diagram labels the left end of an axle as P and the right end as Q. A learner writes ‘clockwise’ without indicating the viewing end. Rotation seen from opposite ends can be described with opposite clockwise labels. State the perspective supplied in the question, or add it if an explanatory answer needs clarity: ‘clockwise when viewed from P’. Your answer should be checkable by someone who did not sit beside you while you imagined the coil.
Clinic 09: Two forces have equal magnitude
A coil diagram shows an upward force on the left active side and a downward force on the right active side. A student concludes the coil cannot turn because the forces cancel. The correction is that equal and opposite forces acting along different lines can create a turning effect. The lines of action and the axle matter. A balanced net force can coexist with a nonzero turning effect. Sketch the coil and its axis instead of relying on the sentence ‘equal forces cancel’.
Clinic 10: The coil reaches a half-turn
A simple motor has rotated so the active sides exchange positions. Ask what must happen to the coil current for rotation to continue in the same rotational sense. The split-ring commutator reverses the current through the coil at each half-turn. It does not reverse the permanent magnet’s field. Explain the relationship in cause-and-effect order: positions exchange, contacts change, coil current reverses, and the forces keep providing the desired turning effect.
Clinic 11: The split ring is replaced by two uninterrupted rings
A student claims that any two metal rings will do exactly the same job as the split-ring commutator. A continuous slip-ring arrangement does not perform the same half-turn reversal of coil connections. In the standard simple d.c. motor explanation, that reversal is the crucial function of the split ring. Name the difference in electrical contact as the axle turns. Do not assume an alternative arrangement behaves identically merely because both have rotating metal contacts.
Clinic 12: The supply is a higher current, not a reversed current
The teacher increases the current through the same motor coil while holding other relevant conditions comparable. According to the syllabus trend, the turning effect increases. This does not by itself establish a numerical speed ratio. Separate ‘stronger turning effect’ from ‘exactly twice the rotations per second’. If the question asks only for an expected qualitative effect, answer it directly. If it asks for a numerical speed, use supplied measurements or an explicitly stated model.
Clinic 13: The winding is changed from 20 to 40 turns
Here ‘turns’ means loops of wire on the coil, not mechanical revolutions. With other relevant conditions held suitable, increasing the number of coil turns increases the turning effect. You cannot conclude the time for one revolution becomes exactly half as long unless a particular model or data justifies that relation. A quick annotation such as ‘winding turns N; rotations per second r’ prevents a common vocabulary collision.
Clinic 14: A soft-iron core is inserted
The original model contains a current-carrying coil in an external field. The teacher winds it onto a soft-iron cylinder and repeats the investigation. The qualitative syllabus expectation is an enhanced motor action or turning effect, all else suitably controlled. A claim such as ‘soft iron doubles the motor speed’ needs actual evidence and is not established by the material label. A scientific explanation should say what the added material does, not attach an invented percentage.
Clinic 15: The motor does not start on one attempt
A coil remains still after a student closes the switch. Another student immediately concludes that the magnetic force is absent. The first response should examine contact, friction, starting orientation, connections and whether the intended current can flow. If the coil is stuck against a support, the presence of a force does not ensure visible rotation. Evidence that nothing moved is narrower than evidence that no magnetic interaction exists. Design a fair retest before drawing a universal conclusion.
Clinic 16: A prototype runs for only three seconds
Three seconds of observed movement establish that the prototype rotated during that observed interval. They do not establish continuous safe operation for an hour, long-term reliability or a particular efficiency. Report the duration accurately. If the design question asks whether it can run for ten minutes, the observed trial is insufficient for that stronger claim. A disciplined learner writes what the apparatus showed, then proposes what must be tested next.
Clinic 17: The compass is used to measure turning force
A compass beside the coil responds to a magnetic field, but its direction reading is not a direct numerical measurement of torque. The previous magnetic-field lesson already separated compass direction from field-strength claims. Here the intended variable is the coil’s turning behaviour. A credible investigation should use an appropriate observable or calibrated measure and state its limitations. Do not relabel the angle of a nearby compass as the motor’s rotational force.
Clinic 18: A motor is said to spin because north attracts south
Attraction between magnetic poles can be part of a broader description of magnetic interactions, but that single phrase does not correctly analyse the force on a current-carrying side of a coil. Name the current direction, the external field and the resulting force. Then show how the forces on the two active sides create rotation. A mechanism-based explanation survives changes to the diagram; a slogan about opposite poles often does not.
Clinic 19: Force and movement are treated as synonyms
A diagram shows a conductor held rigidly between supports in a magnetic field. A magnetic force may act without visible displacement because the supports exert balancing forces. If the question asks for force direction, give that direction; do not insist the bar must fly free. If the question asks about actual motion, examine the constraints. The distinction transfers into mechanics: an effect on a body and the eventual motion of that body are connected but not identical.
Clinic 20: A school poster calls the set-up a generator
A generator and a motor are different devices in the basic school comparison. A motor uses electrical input to produce mechanical rotation; a generator converts mechanical input into electrical output through electromagnetic induction. Both can contain coils and magnets, so the visible components alone may not be enough. Ask what supplies the energy and what output is intended. For the separate induction topic, see Vol 0271: induction, generators and transformers.
Clinic 21: A diagram omits the commutator
A simple coil between magnetic poles may experience a turning effect for part of its motion, yet a claim that it continues rotating steadily in the same direction needs an account of current reversal or an appropriate alternative design. Ask how the electrical connections behave as the coil turns. Do not treat the existence of a force as a complete account of sustained motor operation. Good explanations include the part of the mechanism that makes the observation possible over time.
Clinic 22: A direction statement uses magnetic north incorrectly
A student labels a current arrow north merely because it points toward the magnet labelled N. Current is an electrical direction along the conductor, whereas north and south label magnet poles. The two types of arrow need separate legends. Reconstruct the field arrow from N to S in the gap, identify conventional current through the conductor, and only then infer force. Labelling an arrow precisely is part of physics, not merely neat presentation.
Clinic 23: A reversal changes two more things unnoticed
A learner swaps battery leads while also shifting the magnet closer to the wire. The direction inference may still be clear if the field direction is known, but any claim about the size of the turning effect is now less well controlled. Two variables changed. A fair comparison should restore the original geometry and vary only the intended factor. Examination answers about experimental design should name that confound rather than vaguely instructing the pupil to ‘do it carefully’.
Clinic 24: Repeated trials disagree
At one current setting, the coil rotates successfully in four out of five starts. At a different setting, it rotates in five out of five starts. Those observations suggest a difference in the measured starting reliability within these trials; they are not proof that the second setting will always work. The sample is small and the method must be comparable. Report the observations first, then a cautious conclusion. A count of successful starts is not itself a measurement of speed or turning force.
Clinic 25: The motor spins faster with a light load
A fan blade is removed and the motor’s observed rotation rate increases. This cannot be used as proof that the applied current or coil winding has changed; the mechanical load has changed. The general motor-effect explanation concerns the electromagnetic cause of turning, while actual speed depends on operating conditions. The right answer identifies the altered load and resists assigning the observation to a different quantity without evidence.
Clinic 26: Battery heating is overlooked
A coil connected to a cell becomes warm after repeated demonstrations. The student assumes all electrical input became useful mechanical output. The temperature change is a warning that energy is also being transferred to thermal stores. K323 includes energy ideas elsewhere; apply them without making an unmeasured efficiency claim. The proper response is to stop the unsupervised test, follow school safety instructions and avoid presenting the prototype as lossless.
Clinic 27: A question asks to suggest, not prove
An unfamiliar motor diagram shows a weak turning effect and asks how it might be improved. Increasing coil turns or current is an appropriate syllabus-based suggestion under suitable conditions. The question does not grant a precise target speed or efficiency. Phrase the answer as a justified predicted effect, not a certified performance result. The verb ‘suggest’ allows a reasoned proposal; it does not remove the responsibility to remain within the evidence.
Clinic 28: The claim includes an exact percentage
A school display announces that inserting a soft-iron core increased motor efficiency by 37%, but the experimental sheet gives only starting success and no energy input or useful output measurements. Reject the specific efficiency claim as unsupported. Starting success and efficiency are different quantities. The display may state the observed starts and describe the expected role of the core; it may not manufacture a percentage. This is the bridge to the later EMS audit.
Clinic 29: The pupil reports a rotation without a direction
A motor model rotates when the switch is closed. The paper asks how reversing the external magnetic field affects the initial direction of rotation. Simply answering ‘it rotates’ is incomplete. The field reversal changes the forces on the active sides and hence reverses the initial turning sense for the comparable arrangement. Return to the exact command at the end of your answer; the requested change matters more than repeating the original observation.
Clinic 30: A final answer repeats a memorised rule without applying it
The candidate writes, ‘Fleming’s left-hand rule says thumb, finger, finger,’ and stops. That recalls a procedure but has not resolved the case. A complete answer identifies the actual field and current directions and gives the resulting force or rotation as requested. For explanation questions, connect the forces on two sides to turning and the commutator to sustained direction. The examiner cannot award full credit for a named rule in place of the physical conclusion the question requests.
A longer worked example: the model that will not sustain rotation
A school team builds a rectangular coil that can turn between north and south pole pieces. They connect the coil to a small d.c. supply through two fixed brushes, but their rotating contact is not a split-ring commutator. The coil starts to turn from one position, slows after part of a turn, and sometimes oscillates. The team’s first claim is: ‘The motor effect is too weak to turn a coil.’ This conclusion exceeds the observation. The start of movement already suggests a turning effect was present; the failure to maintain rotation calls for a mechanism review.
Begin with a labelled sketch. On the first active side mark conventional current, external field and inferred force. Trace the coil to determine current on the other active side. Show the turning effect. Next ask what happens to those forces as the coil rotates into the next half-turn. The decisive missing function is current reversal relative to the rotating coil. In the school model, a split-ring commutator supplies that reversal every half-turn. Adding more current may increase the turning effect, but it does not correct a missing commutation mechanism. This is why an engineering correction must follow the actual diagnosis.
The team also records that the coil starts in seven of ten trials. That count does not justify ‘70% efficient’, ‘70% reliable for a year’ or ‘safe at any current’. It only reports successful starts under those trial conditions. The fuller record should state coil geometry, connection, load, trial duration and whether every start used the same procedure. This is how scientific evidence remains useful when the immediate question changes from force to repeatability.
Examination performance without guesswork
For Paper 1, expect to make a decision efficiently after reading a diagram. Trace the arrows, use the correct hand rule, and reject options that confuse current, field and force. Do not spend more time reciting an explanation than the item requires. For Paper 2, build your response around the command word. ‘Describe’ often needs what happens; ‘explain’ needs a reasoned causal connection; ‘deduce’ needs application of given information. In longer questions, a small labelled sketch or sequence may make the reasoning easier to assess. For practical work, distinguish an observation, interpretation and evaluation.
A sound revision sequence has four passes. First, recall the definitions with the diagram hidden. Second, solve right-angle direction questions with different page orientations. Third, explain a complete simple d.c. motor from the active-side forces through the split-ring commutator. Fourth, inspect invented experiment records and decide which conclusions the evidence supports. If one pass fails, repair that specific skill rather than beginning the whole chapter again. A student who knows the rule but repeatedly confuses the viewing direction needs diagram practice, not another generic lecture about electromagnets.
A short self-test with answer boundaries
Question A: The conventional current in an active conductor is reversed while the external magnetic field remains unchanged. Predict the force-direction change. Answer: the force reverses. Question B: Both the current and external field reverse. Predict the direction compared with the original. Answer: it is unchanged for the same arrangement. Question C: Why can opposite forces on active coil sides cause rotation? Answer: they act along different lines and produce a turning effect. Question D: What distinctive job does the split ring perform? Answer: it reverses the coil current every half-turn to maintain rotation in the same sense. Question E: Which two changes increase turning effect in the simple syllabus model? Answer: increase current or the number of winding turns, under otherwise comparable suitable conditions.
Now ask one question of your own. Take an unfamiliar coil diagram, label its current and field, state a predicted force on each side, and specify the viewpoint for its turning sense. Then change only one fact and predict the new result. Finally, write a sentence stating a conclusion the diagram cannot support, such as an exact efficiency. That last step is not pessimism. It is the habit of keeping explanations accurate. The strongest learner knows both what follows and what does not.
Where this lesson belongs in the learning route
For the immediate prerequisite, use Vol 0492: current-produced magnetic fields. For the preceding cross-subject evidence assessment, read Vol 0495: the magnetic demonstration audit. For the broader subject structure, open the Physics, energy, forces, electricity and waves learning hub. If reading science questions remains the obstacle, return to the PSLE Learning Guide and practise identifying task, evidence and explanation before returning to upper-secondary problems.
The new skill is simple to state even if it takes practice to master: read the field, read the conventional current, infer the force, and explain what makes the coil keep turning. In the next volume, Mathematics will analyse an invented motor demonstration’s rotation counts, rates, repeated trials and operating constraints. It will not pretend that every motor has a fixed speed formula. Scientific reasoning and mathematical calculation should support each other without replacing each other.
Official sources and scope
This article follows SEAB’s 2027 Singapore–Cambridge SEC G3 subject specifications, particularly K323 Physics. SEAB explains that SEC begins in 2027 and that G3 subject standards and assessment modes continue from the corresponding O-Level standards. Consult the official SEC examination overview and the examination-year syllabuses for updates. This is an independent learning guide; it is not an SEAB paper, official mark scheme or promise about the exact content of a future examination.
