Fast answer: A current-carrying conductor in a magnetic field may experience a force. Fleming’s left-hand rule links the direction of that force to conventional current and the magnetic field. Opposite forces on the sides of a coil produce a turning effect in a simple d.c. motor; a split-ring commutator reverses current every half-turn so the turning effect continues in the same rotational sense.
The quickest way to solve G3 questions is to label current, field and force separately, then compare one change at a time. This guide continues the field and electromagnet sequence without repeating the earlier electricity or energy chapters. The exercises are original teaching examples, not actual SEC examination items.
- Label N→S magnetic field, conventional current and force before choosing a direction.
- Reverse current alone or field alone to reverse the force; reverse both to keep its direction.
- Explain paired forces on the coil, brushes, split-ring commutator and soft-iron core.
- Avoid confusing force direction, turning effect, speed and energy efficiency.
- Use supervised low-voltage school demonstrations; no household electrical experiments.
2027 syllabus check: SEAB lists G3 Physics K323 (older reference 6091), official Physics syllabus, Mathematics K310 and English K300. The SEC begins in 2027 with G1/G2/G3 subject levels; check the official subject file for the subject you are actually taking.
Earlier routes: Physics Vol 0713: current-produced fields · EMS Vol 0716: electromagnet evidence · Physics Vol 0685: motor energy · post-PSLE transition · Science revision planning.
Contents
- The next step after an electromagnet
- What the examination actually asks
- Separate field, current and force
- Use Fleming’s left-hand rule correctly
- Reverse one variable at a time
- Recognise parallel and perpendicular cases
- Move from one wire to a coil
- Understand the split-ring commutator
- Why a soft-iron core helps
- Increase the turning effect without overclaiming
- A safe classroom thought experiment
- Draw a useful motor-effect diagram
- How the answer changes with command words
- Link the motor concept back to primary-school science
- Treat data as evidence rather than decoration
- Design revision around one weak link
- Twenty-four worked decisions
- Four-week revision plan
- Frequently asked questions
The next step after an electromagnet
An electromagnet can attract a piece of iron, but a current-carrying conductor placed in an external magnetic field can also experience a force. This distinction matters. Attraction of an iron armature in an electromagnet demonstration is not the same explanation as the motor effect on a conducting wire. In the previous guide, the circuit-breaker example taught you to connect cause and evidence. Now you must distinguish the current that creates a field from the interaction between a current and an existing magnetic field. The two ideas are related, but questions can ask about them separately.
Start with one straight wire between opposite magnetic poles. Establish the field direction from north to south, then establish the conventional current direction. If the current is across the field, the wire may be pushed. Only after these facts are clear should you determine the force direction. Do not begin with a memorised hand picture. In a diagram, an incorrectly positioned hand can lead to a confident but wrong answer. The concept must control the mnemonic, not the other way around.
What the examination actually asks
The official 2027 G3 Physics K323 syllabus includes the magnetic effect of current, forces on current-carrying conductors and the d.c. motor. It specifically expects the relative directions of current, magnetic field and force, including the effects of reversing current or field. The syllabus also includes the turning effect on a coil, the influence of current and number of turns, and the split-ring commutator in a simple two-pole, single-coil motor. These are defensible topics for a learner’s guide; detailed professional motor engineering is not a hidden syllabus requirement.
K323 Physics is assessed through Paper 1 multiple choice (40 marks, 1 hour, 30%), Paper 2 structured and free response (80 marks, 1 hour 45 minutes, 50%) and Paper 3 practical (40 marks, 1 hour 50 minutes, 20%). These weights are not a reason to neglect practical skill. Clear diagrams, safe experimental thinking and sensible treatment of evidence support theory questions too. An answer about a school motor model should be a scientific explanation rather than a promise about the safety or performance of real electrical equipment.
Separate field, current and force
Imagine three arrows. The first is the magnetic field, drawn outside a bar magnet from its north pole towards its south pole. The second is conventional current through a conductor, following the indicated circuit direction. The third is the force on that current-carrying conductor. Those arrows do not represent the same thing. A conductor can carry current without being forced sideways if the geometry does not produce a transverse magnetic force. Similarly, a magnetic field can exist when no current is flowing through the wire.
Write three labels before using any rule: B for magnetic field, I for conventional current and F for force. A useful classroom shorthand is to circle the arrow that the question has not given. You are solving for that missing direction. If the source is alternating rather than a constant direct-current supply, directions may vary with time. Do not import those complications into a question that clearly uses steady d.c. without being asked to do so.
Use Fleming’s left-hand rule correctly
For the motor effect, Fleming’s left-hand rule relates force, magnetic field and conventional current when the directions are mutually perpendicular. Point the first finger in the magnetic-field direction, the second or middle finger in the conventional-current direction and the thumb in the force direction. Keep those three fingers at right angles. The mnemonic alone earns little value unless the written answer matches the diagram. The direction is not decided by which pole looks nearer or which way an arrow happens to point on the page.
If two directions are given, check both before deciding the third. If the direction points into the page, a cross symbol can denote a direction away from you; a dot can denote a direction towards you. Confirm the diagram’s convention rather than guessing. Hand positions are difficult to describe in text, so in this text-only guide, practise the logical reversal relationships as well as drawing the rule on paper. No special physical motor is needed to learn the core directional reasoning.
Reverse one variable at a time
Reversing the current while keeping the external field unchanged reverses the force. Reversing the magnetic field while the current stays unchanged also reverses the force. Reversing both the current and field leaves the force direction unchanged. These conclusions are easier to trust if you treat a change as a comparison between two clearly defined experimental states. It is not enough to say ‘the magnet has been reversed’ if the poles and field direction in the new diagram are unspecified.
Keep a small comparison table with columns for current direction, field direction and force direction. Use a baseline row and make only the changes requested. In a fair practical comparison, also keep the wire position and other relevant conditions stable. A question about force magnitude may depend on details not supplied; never invent a numerical force simply because a diagram contains arrows. Direction changes can be determined qualitatively even when magnitude cannot be calculated.
Recognise parallel and perpendicular cases
A current-carrying wire in a field is not automatically pushed equally in every orientation. The turning and sideways effects depend on the relationship between current direction and field direction. The familiar simple motor-effect diagram is drawn so that the directions form right angles. If the wire lies parallel to the field, the force from this interaction is zero in the idealised treatment. This is different from a field-free situation: the field exists, but the geometry does not give the transverse force.
Students sometimes apply a left-hand rule even after the condition for it has disappeared. In a response, name the limitation: ‘The wire is parallel to the field, so the motor-effect force is zero in this idealised case.’ In a more demanding explanation, point out that real apparatus can include other forces such as gravity, supports and friction. The question normally isolates one interaction, so keep your response within that model unless the problem includes additional forces.
Move from one wire to a coil
A simple d.c. motor uses a coil placed in a magnetic field. With current flowing, opposite sides of the coil can experience forces in opposite directions. These forces form a turning effect about the axis. The key idea is not merely that magnets attract. It is that forces on separated sides act to rotate the coil. If you draw the two forces on the same side or in the same direction without checking the current, the turning explanation becomes inconsistent.
Label the coil sides A and B, then trace the current through each active side. Use the external field direction and Fleming’s left-hand rule for both sides separately. After that, look at the turning effect of the force pair. A force diagram can answer a motor question more clearly than a long paragraph because it makes incorrect directions visible. The prose should explain why the current-carrying coil rotates, not simply repeat that a motor ‘turns electricity into movement’.
Understand the split-ring commutator
A split-ring commutator in a simple two-pole, single-coil d.c. motor reverses the connection to the coil after each half-turn so that the current in each coil side reverses at the appropriate point. The purpose is to keep the turning effect acting in a consistent rotational direction through successive half-turns. Brushes provide sliding electrical contact with the rotating split-ring. These functions are different: the commutator switches which half-ring contacts which brush; the brushes provide the path between the stationary supply and rotating coil.
A common mistaken answer says that the commutator reverses the magnetic field. In this model, it reverses the current through the rotating coil rather than changing the external field supplied by the magnets. Another answer says it makes current ‘stronger’. That is not its principal purpose. In an examination, one precise sentence explaining the reversal timing is stronger than three vague sentences about the motor turning continuously.
Why a soft-iron core helps
A soft-iron cylinder within a motor coil strengthens the magnetic field associated with the coil and can increase the motor’s turning effect. It should not be confused with the split-ring commutator. The core is a magnetic material placed in or around the coil region; the commutator is a segmented electrical contact arrangement. Both may appear in the same labelled diagram but perform different jobs. A good answer matches each component to its correct function.
In a labelled question, read the leader line carefully. If it touches the central core, naming a brush is wrong even if both are present in the apparatus. If the item asks for the role of the core, answer in terms of strengthened magnetism and turning effect, not circuit reversal. You can learn this distinction by making a ‘part, position, job, consequence’ table. Such a table also provides a reliable revision checkpoint before moving into electromagnetic induction.
Increase the turning effect without overclaiming
The official syllabus expects learners to know that increasing current or the number of turns on the coil can increase the turning effect of the motor. Those are qualitative relationships under suitable conditions. They are not an unrestricted engineering law saying that doubling current always doubles the rotation speed. Speed depends on the load, friction, electrical characteristics and operating conditions. A trial may show a larger movement, but the wording of the conclusion must match the measured quantity.
If a question asks how to increase the turning effect, answer with a syllabus-supported variable and keep the rest of the explanation short. If it asks you to evaluate experimental data, do not convert ‘a larger current’ directly into ‘twice as fast’ unless the observations justify it. Distinguish applied current, measured displacement, rotational speed and motor torque. Each is a different variable, even if they may change together in a particular demonstration.
A safe classroom thought experiment
Suppose a teacher uses a low-voltage classroom demonstration in which a straight wire is supported between two magnet poles. A small sideways deflection is observed while current passes. The class reverses the current and observes a deflection in the opposite direction. This supports a claim about force direction under the conditions described. It does not by itself establish the exact force magnitude, the performance of a production motor, or the safety of any mains-voltage apparatus.
Your scientific record should state the independent change, the observation and the limited conclusion. A sentence such as ‘When the current was reversed while the field remained unchanged, the direction of deflection reversed’ is stronger than ‘the magnet caused the wire to work better’. Keep practical work under school supervision. Do not dismantle powered devices or wire up mains supplies for a home demonstration. Paper diagrams and teacher-approved equipment are enough for this revision route.
Draw a useful motor-effect diagram
A diagram must contain only the features needed to justify the answer. Show north and south magnetic poles or a clearly labelled field arrow; a conductor or two active coil sides; the current direction; and the resulting force arrows. If explaining a motor, indicate the turning direction and relevant moving components. The reader should be able to follow your reasoning without mentally rotating an unlabeled picture. A neat diagram that contradicts the prose does not rescue the answer.
Test each arrow by asking which fact produced it. The field arrow comes from the poles; the current arrow comes from the circuit; the force arrow follows from their relationship. Avoid drawing the force before deciding current direction. In structured-response questions, a simple annotated sketch can save time, but do not spend minutes decorating it. Keep the same labels and units through the question so that later references remain unambiguous.
How the answer changes with command words
‘State’ usually asks for a concise fact, such as the purpose of a split-ring commutator. ‘Describe’ asks what happens or what is observed, such as the direction of a wire’s deflection when current is reversed. ‘Explain’ requires a mechanism: current in an external magnetic field produces forces on opposite sides of a coil, giving a turning effect. ‘Deduce’ asks you to use given information to reach a justified new conclusion. ‘Compare’ asks for a direct statement about two conditions rather than two unrelated descriptions.
A good response is proportional to the task. If the question is worth a small number of marks, make your causal chain visible without writing a page. Use accurate terms in place of emphatic adjectives. The phrase ‘the force changes direction’ is testable; ‘the motor becomes much more powerful’ may be unjustified. Build the explanation from physics facts, then stop when you have answered the question.
Link the motor concept back to primary-school science
A Primary 6 learner may already know that magnets have poles, similar poles repel and different poles attract. That knowledge is a starting point, not a complete motor explanation. In secondary Physics, we ask how electric current and a magnetic field interact, how a conductor experiences a force, and how a rotating coil uses that force. The sequence builds from observable effects to a more precise model. If basic pole directions are shaky, repairing those first is efficient preparation for G3 work.
The route from PSLE to SEC is therefore not about doing only harder questions. It is about adding missing mechanisms to familiar observations. A learner who says ‘the magnet pulls the wire’ should be asked whether a non-magnetic wire can experience a motor effect, which variable changes when the current is reversed, and whether the model is being described accurately. The goal is independent reasoning rather than a phrase remembered from a worked answer.
Treat data as evidence rather than decoration
Suppose one demonstration shows three observations: a wire deflects right with the baseline current, left with reversed current and right again when both current and field are reversed. These outcomes fit the expected reversal pattern. But observations are not a substitute for checking the diagram; a reversed supply could also change something unexpected if apparatus is moved. Ask whether only the intended variable changed and whether the direction was recorded consistently.
Do not average labels such as ‘left’ and ‘right’ as though they were numerical measurements. Directional evidence belongs in a categorical comparison. If you later record angles, distance or time, specify the unit and measuring method. The next Mathematics volume addresses a different question: how to interpret repeated trials and ratios without quietly changing their definitions. Keeping physical explanation separate from numerical processing makes both stronger.
Design revision around one weak link
A student who makes errors with force direction needs short directional drills, not more lengthy essays. A student who confuses a commutator and soft iron core needs component-function comparisons. A student who knows the physics but loses marks in responses needs command-word practice. Use the first mistake to select the next exercise. Work accurately for several short sessions, then return to mixed questions so the correct method must be chosen without a topic label.
For a forty-minute study session, spend ten minutes retrieving field and current directions, fifteen minutes on three motor diagrams, ten minutes explaining commutator function and five minutes marking errors with a reason. Repeat after a delay using different diagrams. Do not chase speed before the reasoning is reliable. Time pressure should be added only when explanations remain correct when the easy visual cues are removed.
Twenty-four worked decisions: motor-effect questions with explained answers
1. Which direction is the field?
Question: A wire sits between a pole labelled N on the left and S on the right.
Answer and reasoning: In the space between the poles, the field is directed from left to right. The secure approach is to name what is fixed, identify the one relationship the question tests and make the conclusion no broader than the conditions permit. In a timed answer, show the relevant arrow or label as well as the short explanation.
Common error and transfer: Do not point the field in the direction of the wire merely because the wire is vertical. Mark the field first in every subsequent force problem. Return to the original statement and ask whether the same reasoning would still hold after just one variable was changed. This extra check is especially useful when a diagram looks familiar but is rotated or labelled differently.
2. A single reversal
Question: The current is reversed while the magnetic field remains unchanged.
Answer and reasoning: The motor-effect force reverses direction relative to the starting arrangement. The secure approach is to name what is fixed, identify the one relationship the question tests and make the conclusion no broader than the conditions permit. In a timed answer, show the relevant arrow or label as well as the short explanation.
Common error and transfer: A reversal of current does not automatically mean the external magnet has changed poles. Write ‘relative to the original direction’ when no absolute arrow was supplied. Return to the original statement and ask whether the same reasoning would still hold after just one variable was changed. This extra check is especially useful when a diagram looks familiar but is rotated or labelled differently.
3. Reverse only the field
Question: The two magnet poles swap position; the wire’s conventional current is unchanged.
Answer and reasoning: The field reverses and the motor-effect force also reverses. The secure approach is to name what is fixed, identify the one relationship the question tests and make the conclusion no broader than the conditions permit. In a timed answer, show the relevant arrow or label as well as the short explanation.
Common error and transfer: If the wire was moved or the current changed as well, the comparison is no longer controlled. Keep a baseline drawing and draw the new field arrow beside it. Return to the original statement and ask whether the same reasoning would still hold after just one variable was changed. This extra check is especially useful when a diagram looks familiar but is rotated or labelled differently.
4. Reverse both variables
Question: The current and external magnetic field are each reversed.
Answer and reasoning: The two reversals cancel for force direction; the force points as in the baseline case. The secure approach is to name what is fixed, identify the one relationship the question tests and make the conclusion no broader than the conditions permit. In a timed answer, show the relevant arrow or label as well as the short explanation.
Common error and transfer: Do not multiply changes to force magnitude without numerical information. Track each reversed variable in a separate column. Return to the original statement and ask whether the same reasoning would still hold after just one variable was changed. This extra check is especially useful when a diagram looks familiar but is rotated or labelled differently.
5. Parallel wire
Question: A current travels in the same direction as a uniform external field.
Answer and reasoning: The ideal motor-effect force is zero because current and field are parallel. The secure approach is to name what is fixed, identify the one relationship the question tests and make the conclusion no broader than the conditions permit. In a timed answer, show the relevant arrow or label as well as the short explanation.
Common error and transfer: The existence of current does not ensure a sideways force in every geometry. Compare with a perpendicular wire using one carefully labelled sketch. Return to the original statement and ask whether the same reasoning would still hold after just one variable was changed. This extra check is especially useful when a diagram looks familiar but is rotated or labelled differently.
6. Coil-side forces
Question: Two opposite active sides of a rectangular coil carry currents in opposing directions within the same transverse field.
Answer and reasoning: The force directions on those sides are opposite; when separated they can produce a turning effect. The secure approach is to name what is fixed, identify the one relationship the question tests and make the conclusion no broader than the conditions permit. In a timed answer, show the relevant arrow or label as well as the short explanation.
Common error and transfer: Avoid drawing both force arrows in the same direction simply because both sides belong to one coil. Trace each side’s current before deciding the resulting rotation. Return to the original statement and ask whether the same reasoning would still hold after just one variable was changed. This extra check is especially useful when a diagram looks familiar but is rotated or labelled differently.
7. Part identification
Question: A labelled component is a split conducting ring attached to the rotating coil.
Answer and reasoning: It is the split-ring commutator, which reverses coil connections every half-turn. The secure approach is to name what is fixed, identify the one relationship the question tests and make the conclusion no broader than the conditions permit. In a timed answer, show the relevant arrow or label as well as the short explanation.
Common error and transfer: It is not the soft-iron core or a magnetic pole. Explain position and function together when naming parts. Return to the original statement and ask whether the same reasoning would still hold after just one variable was changed. This extra check is especially useful when a diagram looks familiar but is rotated or labelled differently.
8. Brush identification
Question: Two fixed contacts touch the rotating split-ring.
Answer and reasoning: These are brushes, supplying electrical contact to the rotating commutator. The secure approach is to name what is fixed, identify the one relationship the question tests and make the conclusion no broader than the conditions permit. In a timed answer, show the relevant arrow or label as well as the short explanation.
Common error and transfer: The brushes alone do not set the force direction without the field and coil current. Differentiate contact function from commutation timing. Return to the original statement and ask whether the same reasoning would still hold after just one variable was changed. This extra check is especially useful when a diagram looks familiar but is rotated or labelled differently.
9. Core identification
Question: A cylinder of magnetic material lies within the coil.
Answer and reasoning: A soft-iron core can strengthen the magnetic effect and increase turning effect. The secure approach is to name what is fixed, identify the one relationship the question tests and make the conclusion no broader than the conditions permit. In a timed answer, show the relevant arrow or label as well as the short explanation.
Common error and transfer: A metal core is not the same as the contact ring and cannot be described as the current-reversing switch. Name the property and the consequence in a single explanatory sentence. Return to the original statement and ask whether the same reasoning would still hold after just one variable was changed. This extra check is especially useful when a diagram looks familiar but is rotated or labelled differently.
10. Increase current
Question: The current through a classroom motor coil is increased while other relevant conditions are held stable.
Answer and reasoning: The motor’s turning effect can increase, as described in the syllabus. The secure approach is to name what is fixed, identify the one relationship the question tests and make the conclusion no broader than the conditions permit. In a timed answer, show the relevant arrow or label as well as the short explanation.
Common error and transfer: Do not infer an exact proportional increase in revolutions per minute from this fact alone. Separate turning effect from observed speed in your working. Return to the original statement and ask whether the same reasoning would still hold after just one variable was changed. This extra check is especially useful when a diagram looks familiar but is rotated or labelled differently.
11. Increase turns
Question: A coil is rewound with more turns and assessed under comparable conditions.
Answer and reasoning: More turns can increase the turning effect of the motor. The secure approach is to name what is fixed, identify the one relationship the question tests and make the conclusion no broader than the conditions permit. In a timed answer, show the relevant arrow or label as well as the short explanation.
Common error and transfer: A different wire resistance or supply condition can complicate an uncontrolled real test. Identify what must stay constant before comparing outcomes. Return to the original statement and ask whether the same reasoning would still hold after just one variable was changed. This extra check is especially useful when a diagram looks familiar but is rotated or labelled differently.
12. Describe a practical result
Question: With current one way, a supported wire deflects towards one side; with current reversed, it deflects oppositely.
Answer and reasoning: State the observed reversal and the controlled field direction. The secure approach is to name what is fixed, identify the one relationship the question tests and make the conclusion no broader than the conditions permit. In a timed answer, show the relevant arrow or label as well as the short explanation.
Common error and transfer: Do not claim that every wire in every apparatus moves the same distance. Distinguish observation from general explanation. Return to the original statement and ask whether the same reasoning would still hold after just one variable was changed. This extra check is especially useful when a diagram looks familiar but is rotated or labelled differently.
13. Name a force condition
Question: A student argues a wire will be pushed even if there is no external field.
Answer and reasoning: The stated motor-effect mechanism requires an interaction between the current and a magnetic field. The secure approach is to name what is fixed, identify the one relationship the question tests and make the conclusion no broader than the conditions permit. In a timed answer, show the relevant arrow or label as well as the short explanation.
Common error and transfer: A conductor may experience other forces, which does not prove this magnetic interaction. Test whether the cause needed by the explanation is actually present. Return to the original statement and ask whether the same reasoning would still hold after just one variable was changed. This extra check is especially useful when a diagram looks familiar but is rotated or labelled differently.
14. Explain continuous rotation
Question: The coil has reached approximately half a revolution in a simple two-pole d.c. motor.
Answer and reasoning: The split-ring changes the coil’s current connection so the turning effect continues in the same rotation sense. The secure approach is to name what is fixed, identify the one relationship the question tests and make the conclusion no broader than the conditions permit. In a timed answer, show the relevant arrow or label as well as the short explanation.
Common error and transfer: The external field is not normally switched each half-turn. Mention the commutator, current reversal and continued rotation. Return to the original statement and ask whether the same reasoning would still hold after just one variable was changed. This extra check is especially useful when a diagram looks familiar but is rotated or labelled differently.
15. Interpret a graph cautiously
Question: A classroom motor rotates faster at one of two tested current settings.
Answer and reasoning: The observation supports a speed comparison at those settings, not an exact general speed law. The secure approach is to name what is fixed, identify the one relationship the question tests and make the conclusion no broader than the conditions permit. In a timed answer, show the relevant arrow or label as well as the short explanation.
Common error and transfer: A test of turning effect and a measurement of speed are related but not identical. Report which quantity was actually measured. Return to the original statement and ask whether the same reasoning would still hold after just one variable was changed. This extra check is especially useful when a diagram looks familiar but is rotated or labelled differently.
16. Use precise terminology
Question: An answer says the north pole sends electricity through the moving wire.
Answer and reasoning: The supply drives conventional current; the magnetic poles establish the external field. The secure approach is to name what is fixed, identify the one relationship the question tests and make the conclusion no broader than the conditions permit. In a timed answer, show the relevant arrow or label as well as the short explanation.
Common error and transfer: Confusing electric current with magnetic-field direction destroys the mechanism. Underline each physical quantity in the sentence and replace incorrect verbs. Return to the original statement and ask whether the same reasoning would still hold after just one variable was changed. This extra check is especially useful when a diagram looks familiar but is rotated or labelled differently.
17. Choose the right-hand or left-hand rule
Question: The question asks for the direction of force on a current-carrying conductor in an external field.
Answer and reasoning: Use Fleming’s left-hand rule for the motor effect. The secure approach is to name what is fixed, identify the one relationship the question tests and make the conclusion no broader than the conditions permit. In a timed answer, show the relevant arrow or label as well as the short explanation.
Common error and transfer: Do not import the generator mnemonic before identifying the physical process. First ask whether the diagram describes a force on a wire or an induced e.m.f. Return to the original statement and ask whether the same reasoning would still hold after just one variable was changed. This extra check is especially useful when a diagram looks familiar but is rotated or labelled differently.
18. Dots and crosses
Question: A question marks one arrow as a dot pointing out of a page and another as a cross pointing in.
Answer and reasoning: Translate the symbols into three-dimensional directions before applying the hand rule. The secure approach is to name what is fixed, identify the one relationship the question tests and make the conclusion no broader than the conditions permit. In a timed answer, show the relevant arrow or label as well as the short explanation.
Common error and transfer: Treating dot and cross as left-right symbols reverses several answers. Draw a small labelled legend next to the diagram. Return to the original statement and ask whether the same reasoning would still hold after just one variable was changed. This extra check is especially useful when a diagram looks familiar but is rotated or labelled differently.
19. A stronger electromagnet
Question: A device attracts an iron armature when current flows through its solenoid.
Answer and reasoning: This illustrates the magnetic effect of current and induced magnetism, not necessarily the force rule on a transverse conducting wire. The secure approach is to name what is fixed, identify the one relationship the question tests and make the conclusion no broader than the conditions permit. In a timed answer, show the relevant arrow or label as well as the short explanation.
Common error and transfer: The presence of a coil is not enough to identify a d.c. motor. Ask which material moves and what interaction moves it. Return to the original statement and ask whether the same reasoning would still hold after just one variable was changed. This extra check is especially useful when a diagram looks familiar but is rotated or labelled differently.
20. A false safety promise
Question: A display says that a student motor can safely run from any household outlet.
Answer and reasoning: The demonstration does not justify a claim about mains-powered operation, insulation or electrical protection. The secure approach is to name what is fixed, identify the one relationship the question tests and make the conclusion no broader than the conditions permit. In a timed answer, show the relevant arrow or label as well as the short explanation.
Common error and transfer: School revision should not turn into instructions for unsafe wiring. Use supervised low-voltage apparatus only and leave mains work to qualified personnel. Return to the original statement and ask whether the same reasoning would still hold after just one variable was changed. This extra check is especially useful when a diagram looks familiar but is rotated or labelled differently.
21. Two changes with different effects
Question: A trial increases the number of turns but also lowers supply voltage.
Answer and reasoning: A changed result cannot be attributed to turns alone unless the other change is accounted for. The secure approach is to name what is fixed, identify the one relationship the question tests and make the conclusion no broader than the conditions permit. In a timed answer, show the relevant arrow or label as well as the short explanation.
Common error and transfer: An uncontrolled comparison is weak evidence even if the final rotation is greater. List all changed variables before writing a causal conclusion. Return to the original statement and ask whether the same reasoning would still hold after just one variable was changed. This extra check is especially useful when a diagram looks familiar but is rotated or labelled differently.
22. Unclear field direction
Question: The diagram labels only the force and current arrows, leaving the field to be deduced.
Answer and reasoning: Use the three mutually perpendicular directions in Fleming’s left-hand rule to determine the missing field. The secure approach is to name what is fixed, identify the one relationship the question tests and make the conclusion no broader than the conditions permit. In a timed answer, show the relevant arrow or label as well as the short explanation.
Common error and transfer: Guessing from the visible pole positions without reading labels is unreliable. State the field as a direction, not merely ‘towards the magnet’. Return to the original statement and ask whether the same reasoning would still hold after just one variable was changed. This extra check is especially useful when a diagram looks familiar but is rotated or labelled differently.
23. The stationary coil
Question: The circuit supplies current but the demonstration motor does not start.
Answer and reasoning: A stationary observation can have several explanations; a simple diagram alone is insufficient to diagnose a specific fault. The secure approach is to name what is fixed, identify the one relationship the question tests and make the conclusion no broader than the conditions permit. In a timed answer, show the relevant arrow or label as well as the short explanation.
Common error and transfer: Do not blame the commutator without evidence of its contacts or supply. Offer a testable hypothesis and say what observation would distinguish it. Return to the original statement and ask whether the same reasoning would still hold after just one variable was changed. This extra check is especially useful when a diagram looks familiar but is rotated or labelled differently.
24. A final two-mark explanation
Question: The question asks why a rectangular current-carrying coil turns in a magnetic field.
Answer and reasoning: Current in the coil sides interacts with the field to produce oppositely directed forces, giving a turning effect. The secure approach is to name what is fixed, identify the one relationship the question tests and make the conclusion no broader than the conditions permit. In a timed answer, show the relevant arrow or label as well as the short explanation.
Common error and transfer: ‘Magnets make it spin’ omits the causal interaction and separation of forces. Answer with current, field, opposing forces and turning effect, then stop. Return to the original statement and ask whether the same reasoning would still hold after just one variable was changed. This extra check is especially useful when a diagram looks familiar but is rotated or labelled differently.
Four-week practice plan
Week 1 — directions and reversals
Build nine small diagrams on paper: baseline current and field, each single reversal, both reversed, a parallel case and several different wire orientations. For each diagram, draw the field arrow before the current arrow, then deduce the force. Explain aloud why reversing just one of the input directions changes the outcome. Mark each error as a field-reading error, a current-reading error or a hand-rule error. Repeat two days later without looking at the previous answers.
Week 2 — the coil and components
Draw an elementary rectangular coil and trace the current around it. Determine forces on the active sides one at a time, then say how they create a turning effect. Add labels for brushes, the split-ring commutator and soft-iron core. Write one function sentence for each. Test yourself by covering the labels and replacing them from memory; then explain why a core cannot replace a commutator. This is the week’s central distinction.
Week 3 — unfamiliar questions
Ask someone to rotate diagrams, replace a bar magnet with a uniform field arrow, omit one direction or write the question as an experiment instead of a labelled picture. Answer without the chapter title. For each wrong answer, write the first incorrect assumption rather than copying the correct final arrow. Revisit three questions after forty-eight hours. Keep a small record of which errors reappear and which have disappeared.
Week 4 — exam-style transfer
Complete a mixed set involving poles, magnetic field, force on a wire, a d.c. motor diagram and a practical fair-test description. Include a short written question on component functions. Time the set only once the explanations are stable. End with a five-minute check for correct conventional-current direction, named components, unambiguous force arrows and unsupported claims. If an area is weak, return to the earliest relevant practice step rather than increasing the quantity of papers.
Frequently asked questions
Do I need to memorise every internal component?
Learn the syllabus components that explain the single-coil d.c. motor: field magnets, coil, brushes, split-ring commutator and soft-iron core. Focus on function, not a catalogue of industrial parts. A labelled sketch with one accurate sentence per part is more useful than a long list of names.
Is this about the same motor as the earlier motor-ramp articles?
No. Earlier volumes addressed work, power, efficiency and energy costs in a motorised ramp. This volume addresses the electromagnetic origin of the turning effect in a simple d.c. motor. Keep the links between topics, but do not confuse energy accounting with the force-direction mechanism.
What changes when current is reversed?
With the external field direction unchanged, the force on a transverse current-carrying conductor reverses. In a simple rotating coil, the commutator times current reversal to sustain rotation. The comparison depends on the stated geometry; do not assume every quantity in the apparatus reverses.
Does increasing current guarantee a particular speed?
No. The syllabus supports an increase in turning effect under suitable conditions. Rotational speed depends on other factors, including load and resistance. Describe any measured speed relationship only within the observations that support it.
Must I build a working motor to revise?
No. Diagram interpretation, safe classroom demonstrations and teacher-approved experiments can develop the assessed reasoning. Do not use household mains wiring or dismantle powered equipment for home practice.
How should I prepare for Paper 3?
Practise following instructions, recording observations consistently, evaluating control of variables and explaining a sensible improvement. Physics Paper 3 assesses practical skills; it is not merely a written test of component vocabulary. Your school will determine appropriate safe apparatus.
How is SEC different from the old O-Level Physics?
From 2027, SEC combines the former N- and O-Level examinations under G1, G2 and G3 subject levels. G3 Physics uses the 2027 code K323, with the previous 6091 code shown by SEAB for reference. The overall standard and mode of assessment remain aligned with the earlier O-Level route, subject to the official syllabus.
What should I learn next?
After force on a conductor and the d.c. motor, the Physics syllabus moves to electromagnetic induction, the a.c. generator and transformers. Master the contrast between motor effect and induction before extending the numerical models.
Where to go next
The next guide develops a controlled mathematical reading of motor tests: turns, current settings, repeated observations and the limits of a comparison. In Physics, the next major mechanism is induction. Build that distinction carefully: a motor uses forces on a current-carrying coil in a magnetic field; an induction problem asks how a changing magnetic field produces an e.m.f. The common words do not mean the mechanisms are interchangeable.
