A change matters more than the presence of a magnet
A magnet rests beside a coil connected to a sensitive meter. The needle stays at zero. Someone then pushes the magnet towards the coil, and the needle moves. When the magnet stops, the needle returns to zero. Pull the magnet away and the needle deflects in the opposite direction. Four moments, one essential idea: a changing magnetic field through a circuit can induce an electromotive force, or e.m.f. A strong magnet that is not changing the magnetic flux through a stationary coil will not produce a sustained induced e.m.f. simply because it is nearby.
Many examination mistakes start with the sentence “a magnet creates electricity.” That sentence is too loose to explain the observation. The candidate should say what changed, what responded and what the direction of the response means. In this guide you will use those three questions to interpret induction experiments, understand a simple alternating-current generator, sketch the output against time, calculate ideal transformer ratios and make sensible claims about transmitting electrical energy.
The purpose is not to memorise five separate tricks. It is to connect the changing field, the electrical response and the energy transfer in a chain that remains valid when the diagram or numbers change.
Where this belongs in the official 2027 SEC
The Singapore–Cambridge Secondary Education Certificate (SEC) begins in 2027. The official school-candidate list identifies G3 Physics as syllabus K323, with 6091 supplied as the reference code for 2026 and earlier. In the official 2027 K323 Physics syllabus, Topic 19 is Electromagnetic Induction. Its learning outcomes cover induction from a changing field, the opposition of the induced e.m.f. to the change producing it, factors that affect its magnitude, simple a.c. generators, an output-voltage graph, simple iron-cored transformers and their ideal relationships, and the advantages of high-voltage transmission.
The SEAB G3 school-candidate directory, updated on 26 October 2026, gives the current subject code. The K323 assessment consists of Paper 1 multiple choice, one hour, 40 marks and 30%; Paper 2 structured/free response, one hour 45 minutes, 80 marks and 50%; and Paper 3 practical, one hour 50 minutes, 40 marks and 20%. These official details matter because a candidate must be able both to explain physics and to analyse evidence or practical observations. The worked questions below are original teaching examples, not SEAB examination questions.
Start with the circuit, not the story
Picture a circular coil with its wire ends connected to a sensitive galvanometer. The coil offers a conducting route for charge. A moving magnet changes the magnetic flux through the region enclosed by that coil, so an induced e.m.f. can drive a current while the circuit is closed. The meter reveals the transient current by a deflection. If the circuit is open, an induced e.m.f. may still exist across its ends, but a complete current path is absent. Saying “the circuit produces no induction because it is open” is therefore misleading.
The examiner may change the apparatus. Instead of moving a magnet into the coil, a student moves the coil towards a stationary magnet. The important feature is still change in magnetic flux through the coil. It is relative motion, orientation and field change that matter for the observed effect, not which object has the label “moving.”
Begin every diagram question by identifying the coil, magnet, field direction, relative movement, electrical connections and measured quantity. Then write one short statement: “The magnetic flux linking the coil is increasing,” “decreasing,” or “remaining constant.” This sentence is more useful than reaching immediately for a directional rule without checking what actually changes.
Four stages of a magnet–coil demonstration
At stage one the magnet and coil remain still in the same arrangement. There is no change of flux and therefore no induced e.m.f. from that stationary situation. At stage two the north pole approaches the coil; the flux linked with the coil changes and the galvanometer deflects. At stage three the magnet is momentarily stationary inside the coil; the induced signal returns to zero if the linked flux is not changing. At stage four the magnet withdraws, so the change is reversed and the induced current reverses in the same connected circuit.
The strength of this explanation lies in its time sequence. Do not describe only the final position. A magnet can be farther away but moving rapidly, or nearby but stationary, and these can produce different meter observations. Equally, a magnet can cross the centre of a coil yet fail to give the simple “more distance means more signal” pattern if the geometry and motion are not specified. A timed observation tests a change, not a photograph.
When writing your answer, separate observed facts from explanations. “Needle deflects left” is an observation. “The changing magnetic flux induces a current in that direction” is an explanation. An excellent response does not substitute one for the other.
What the direction of induced e.m.f. opposes
The induced effect opposes the change producing it. That is not the same as saying that induced current is always opposite the magnet’s motion, or that a coil always repels any magnet brought near it. If a north pole approaches the coil, the induced magnetic effect resists that increasing flux; a coil face may behave like a north pole to oppose the approach. If that north pole withdraws, the coil’s induced field tends to oppose the reduction in flux, and the near face may behave like a south pole.
This distinction is subtle but central. The same magnetic pole can be involved in both cases; the direction of motion changed and so the direction of the induced current changed. Always state the change being resisted before drawing the inferred magnetic pole or current direction. A question may also reverse the magnet, reverse the motion or reverse the observation direction. Memorising a clockwise arrow from one diagram is not enough.
The reason the effect opposes the change is consistent with conservation of energy. If an induced response reinforced an approaching magnet without any extra work, the arrangement could apparently gain electrical and mechanical energy for free. In practice, work must be done against the opposing effect to sustain energy generation.
Strength of the induced signal is a rate question
Moving the same magnet faster through the same coil usually produces a larger momentary induced e.m.f. because the linked magnetic flux changes more rapidly. Increasing the number of turns can increase the induced e.m.f., because more turns are linked by the changing flux. A stronger magnetic field can also produce a larger change under otherwise comparable conditions. These are comparisons, not promises that every possible modification will increase every meter reading.
A student often writes “more turns make more current.” That can be wrong without further details: changing the turns may also change the coil’s resistance, and the circuit current depends on the total resistance as well as the e.m.f. The safer syllabus-level statement concerns the magnitude of the induced e.m.f. If the circuit resistance is known or controlled, current comparisons can then be made separately.
A practical answer should identify the independent variable being varied, keep other meaningful variables constant and say how the meter reading is compared. More words are not needed if they do not specify a fair comparison. “Move the magnet through the same distance at controlled speeds and record the peak deflection” is more informative than “change speed and see what happens.”
Do not confuse e.m.f., current and energy
Electromotive force is measured in volts; electric current is measured in amperes. The e.m.f. describes the energy supplied per unit charge around a complete circuit by a source. A galvanometer reading concerns current. Electrical power is a rate of energy transfer. These terms are connected, but they are not interchangeable.
Suppose an experiment produces an induced e.m.f. of 2 V across a complete circuit with total resistance 4 Ω. Under the simple ohmic model, I = V/R = 0.5 A. If the resistance were doubled under the same e.m.f., the current would be 0.25 A. The field change produced the e.m.f.; circuit properties determined the resulting current. That distinction prevents many careless claims about “more voltage equals more electricity.”
A generator does not create energy from nothing. Mechanical work drives the movement of the coil or magnet, and electrical energy becomes available at the terminals. There are losses in actual devices, including resistive heating and friction. When the examiner asks for an energy conversion, be precise: mechanical energy is converted into electrical energy, with unavoidable losses. The physics of the change explains why continuous mechanical input is needed.
From a single pulse to alternating output
In a basic demonstration, moving a magnet into and out of a coil creates brief deflections. A simple a.c. generator arranges continuous rotation so that the magnetic flux through the rotating coil changes repeatedly. The induced e.m.f. changes magnitude and reverses direction over each cycle. That repeated reversal is what makes the output alternating rather than a constant direct-current supply.
An examiner may show a rotating coil between magnetic poles. First identify the axis of rotation, the magnetic field direction and whether the coil’s orientation is changing relative to the field. As it turns, the flux through the coil varies. The voltage against time can be represented by a repeating waveform that alternates above and below zero. In an idealised simple generator, a smooth sinusoidal wave is a useful model.
Do not assume that a bigger wave always means faster rotation. A larger peak may also reflect more turns or a stronger magnetic field. Faster rotation typically changes both the frequency and, with other factors fixed, the induced voltage amplitude. When comparing two waveforms, treat peak value, period and zero crossings as different pieces of evidence.
How to read the voltage–time graph
On a voltage–time graph, the horizontal axis is time and the vertical axis is induced voltage. The height of a peak shows maximum positive voltage, while the depth of a trough shows maximum negative voltage under the indicated sign convention. One full cycle is the interval between equivalent repeating positions, such as consecutive positive peaks. The frequency is the reciprocal of the period: f = 1/T.
For an idealised example, suppose positive peaks occur at t = 0.01 s and t = 0.05 s. The period is 0.04 s, and the frequency is 25 Hz. If the next graph has peaks only 0.02 s apart, its frequency is 50 Hz. The student must check the axis scale before claiming that the second generator spins faster. A compressed-looking horizontal scale can create a false visual impression.
A line crossing zero does not mean the generator has stopped. It means the instantaneous e.m.f. is zero at that moment within a changing cycle. Positive and negative values express direction or polarity, not “good and bad” electricity. A graph must be read as a measurement of a named quantity.
What slip rings do, and what they do not do
A simple a.c. generator may use slip rings and brushes to connect a rotating coil to an external circuit while the coil continues to turn. The rings provide ongoing electrical contact; they do not themselves create the alternating e.m.f. The changing flux through the rotating coil is responsible for induction. If the diagram uses a rotating magnet with a stationary coil, the way electrical connections are arranged can differ.
This is a helpful place to compare earlier electrical learning. In a d.c. motor, a split-ring commutator changes connections to maintain a turning effect in a consistent rotational direction. In the simple a.c. generator, slip rings allow the alternating output to be passed to the external circuit. These components serve different purposes even though both diagrams contain circles, brushes and wires.
For a concise explanation, identify the component and its function, then describe the physical process that produces the output. A candidate who merely labels “slip rings” may earn fewer explanation marks than one who states that they maintain contact during rotation while the changing magnetic flux induces a varying e.m.f.
Transformers need a changing magnetic field
A simple transformer has a primary coil, a secondary coil and a common soft-iron core. Alternating current in the primary coil produces a changing magnetic field in the core. The changing flux linking the secondary coil induces an alternating e.m.f. across it. This is why an ordinary transformer does not provide a sustained secondary output from a steady direct current in the primary under normal operation.
The core guides the changing magnetic flux between the coils. The two coils are electrically separate in an ideal simple transformer, yet energy is transferred magnetically. Be careful with the everyday phrase “the electricity travels through the iron.” That is not the operating explanation. The changing field links the circuits and transfers energy.
A diagram question may label primary and secondary wrongly if you rely only on which side is drawn left. The primary is connected to the input supply. The secondary delivers the output. Their positions on the page do not define their functions. Identify the power path first, then attach the labels.
The turns ratio is a voltage relationship
For an ideal transformer, Vp/Vs = Np/Ns, where V represents voltage and N represents number of turns, with p and s denoting primary and secondary. A secondary coil with more turns than the primary delivers a higher voltage than the input under the ideal model; fewer secondary turns gives a lower voltage. The ratio matters, not the absolute counts alone.
Suppose Np = 200 and Ns = 600, with a primary voltage of 12 V. Then Vs = 12 × 600/200 = 36 V. This is a step-up transformer. If instead Ns = 50, then Vs = 3 V, a step-down transformer. You can check the direction before calculating: more secondary turns should correspond to more secondary voltage. An answer of 4 V in the first example would fail that immediate reasonableness test.
Do not manipulate symbols before labelling them. Write a short key for primary and secondary values and state what is being asked. In multi-stage problems, the output of one stage may become the input of another. A clean labelled equation prevents swapped ratios.
Ideal power gives a current relationship
In an ideal transformer, input power equals output power, so VpIp = VsIs. If voltage rises, current falls in the corresponding ideal comparison. A tenfold step-up in voltage accompanies a tenfold reduction in current for equal transmitted power. It is not correct to conclude that both voltage and current rise simply because the secondary has more turns.
For example, an ideal transformer takes 24 V at 3 A. Its input power is 72 W. If the secondary voltage is 144 V, the secondary current must be 72/144 = 0.5 A. Check by multiplying again: 144 × 0.5 = 72 W. The word ideal matters. Actual transformers lose energy, so real output power is smaller than input power; do not substitute perfect equality into a question that explicitly gives efficiency data unless the model permits it.
Treat the two equations as different tools. The turns ratio determines the voltage ratio. Conservation of power in the ideal model gives the current relation. Choose the equation whose quantities are provided and check whether the model is ideal before proceeding.
High-voltage transmission is not high-energy magic
Electrical power P = VI. If the same power can be transmitted at a higher voltage, the current can be lower. The heating loss in a transmission cable with resistance R is P_loss = I²R. Lower current therefore reduces that loss strongly: reducing current to one-tenth makes resistive loss one-hundredth, provided the cable resistance is unchanged.
Consider a simplified transmission example. A power station transfers 100 kW. At 1 kV the current would be 100 A; at 10 kV it would be 10 A in the idealised comparison. For a cable with 2 Ω resistance, I²R losses would be 20,000 W in the first case and 200 W in the second. The figures illustrate the reason for using high voltages. They are not a recommendation to handle high-voltage equipment, which is dangerous and requires professional systems.
Do not claim the transformer eliminates all loss. There are real transformer losses and other network constraints. The valid examination conclusion is narrower: for the same transmitted power and cable resistance, increasing the voltage reduces current, which reduces resistive heating loss.
The hidden assumption in a transformer calculation
A calculation can be numerically correct while its explanation is incomplete. Suppose the question says an ideal transformer. Then it is appropriate to set input and output power equal. If it says a transformer operates at 80% efficiency, then output power equals 0.80 times input power. For input power 150 W, output is 120 W, not 150 W. If the output voltage is 30 V, the output current is 4 A. You should not quietly change models midway through the question.
Another assumption concerns the source. The simple transformer explanation relies on a changing magnetic field generated by alternating current. A direct-current input can create transients while it switches, but a steady d.c. input does not maintain the changing flux needed for ordinary continuous transformer action. That does not contradict the existence of electronic power converters; it simply defines the mechanism of the simple iron-cored transformer in this syllabus.
Write down what the question gives as fact, what its model assumes and what you infer. This habit is worth more than memorising a particular diagram.
A practical investigation must control the comparison
A student claims that faster movement of a magnet through a coil produces a greater induced e.m.f. A fair test needs the same coil and magnet, the same path and orientation, and a defined way to compare speed. One approach is to move the magnet through a specified distance at controlled speeds while recording the peak meter response. Repeats help identify random variation, but they do not fix a poorly controlled route.
The dependent variable must be named. If the meter reads current, do not write that it directly measures e.m.f. without explaining the link and assumptions. A voltmeter across a suitable open circuit may measure induced potential difference, subject to instrument response. The measurement instrument must be sensitive and fast enough to capture transient changes. If the change happens faster than a slow instrument can respond, the observed peak may not represent the real peak.
For a practical answer, state the question, variables, apparatus, method, results table headings with units, repeats, likely limitation and improvement. Keep safety proportionate: use low-voltage educational apparatus under appropriate supervision, not mains electricity or improvised transformer builds.
How a careful examiner can change the question
An examination might replace the familiar magnet-and-coil setup with a stationary coil beside a time-varying electromagnet. The induction principle survives: the field through the coil is changing. Another question might move both magnet and coil at the same speed so their relative arrangement stays constant; if the linked flux stays unchanged, the simple demonstration does not produce the expected signal merely because both objects have moved in the room.
Questions can also present two graphs with different vertical scales. A waveform drawn taller on paper may actually show a lower peak voltage if the axis markings differ. A transformer problem can reverse which coil is primary without changing the ratio formula. A high-voltage transmission question may change the cable resistance, requiring you to evaluate both current and R rather than quoting I²R by memory.
The best defence is to name the invariant concept for each family: changing flux for induction; measured amplitude and period for generator graphs; labelled ratios for transformers; and current-dependent resistive loss for transmission. The details may change, but the reasoning remains traceable.
A worked generator comparison
Generator A produces a peak voltage of 6 V with a period of 0.02 s. Generator B produces a peak voltage of 9 V with a period of 0.04 s. First compare amplitude: B has the larger peak magnitude. Then compare frequency: A is 1/0.02 = 50 Hz, whereas B is 1/0.04 = 25 Hz. A has the higher frequency, even though B has the taller peak.
Now consider what cannot be inferred. Without the generator designs, you cannot prove that B rotates faster or uses the stronger magnet. Different numbers of turns or field strengths could affect output magnitude. If the problem states that both generators are identical except for rotation rate, then a speed inference becomes defensible. Notice how the conclusion changes when the controlled variables are supplied.
For a graph explanation, give a calculated number before a comparative adjective. “A has half the period and twice the frequency” is stronger than “A has more waves.” A good answer also identifies the units and does not confuse peak voltage with electrical energy output.
A worked transformer decision
A school demonstration uses a transformer whose primary has 300 turns and secondary has 75 turns. The input is an a.c. supply of 48 V. The turns ratio gives Vs = 48 × 75/300 = 12 V. Therefore it is a step-down transformer. If the device is ideal and the primary current is 0.25 A, input power is 12 W, so secondary current is 12/12 = 1 A. The higher output current does not violate conservation of energy because the output voltage is one quarter of the input.
A common wrong route is to copy the turns ratio without labels and calculate 48 × 300/75 = 192 V. The sign of the error is visible before final calculation: the secondary has fewer turns, so its ideal voltage should not be higher. Another mistake is to say that the secondary current must be 0.0625 A because all quantities are reduced by four. Ideal power balance shows why that is incorrect.
Use a two-line check after the algebra: compare the number of turns and voltages, then compare input and output powers. Both checks must agree.
Electricity and magnetism: two directions of explanation
The magnetic effect of a current and electromagnetic induction are related but distinct. Current through a wire produces a magnetic field. In induction, a change of magnetic flux through a circuit can produce an e.m.f. A motor makes useful mechanical motion from electrical input through magnetic forces, while a generator converts mechanical input into electrical output through induction. These are not perfect mirror images in every component, but the contrast helps orient unfamiliar diagrams.
Ask first what drives the apparatus. If a power source sends current through a coil to make it turn, you are probably dealing with motor action. If a moving coil or magnet generates a voltage at terminals, induction is central. If alternating input to a primary coil creates a changing magnetic field that induces a voltage in a secondary, you are looking at transformer action.
A student who identifies the energy input, change and useful output can often recover the physics even after forgetting a component name. This is a practical method for a demanding question when the drawing looks new.
Explain without pretending the evidence is stronger than it is
Imagine an experiment showing that the galvanometer has a larger peak deflection when the magnet moves faster. It supports an association between movement speed and the measured response in that controlled arrangement. It does not independently tell you the exact shape of the magnetic flux–time curve or guarantee the same numerical relationship under all geometries. Strong scientific writing leaves room for what was not measured.
Similarly, a generator graph can prove that the voltage reverses sign and reveal the period. It cannot, by itself, prove which mechanical component was rotating or what material made the core. A transformer calculation using ideal equations answers an ideal-model question; it is not a direct measurement of efficiency. Avoid inferring hidden equipment details merely because the story sounds familiar.
The habit is simple: distinguish the observed quantity, the supported explanation and the untested possibility. In Physics, modest, accurate statements earn more trust than confident sentences that exceed the data.
A six-step solution routine for structured questions
Read the command word first. “Describe” requires the relevant observable or structural detail. “Explain” needs a mechanism or linked reason. “Deduce” must follow from provided evidence. Next, identify which electrical or mechanical quantity is changing. Third, label the apparatus, axes, voltages, currents and units. Fourth, select the appropriate principle: changing flux, turns ratio, ideal power balance or I²R loss. Fifth, work one step at a time and write the reason beside the calculation when it matters. Sixth, test the result against the physical situation.
This method is especially helpful in Paper 2, where a long stimulus can tempt candidates to start writing before deciding what is asked. A question with three marks may require three different ideas, not three repetitions of “the magnetic field changes.” The mark value is not a licence to invent extra claims; it is a reminder to check that the response covers change, mechanism and observable consequence where relevant.
When you finish, reread the response as if the examiner did not know your intention. Every symbol should be defined, and every conclusion should be defensible from the question.
Common mistakes worth catching before the exam
First, “a stationary magnet beside a coil produces current” omits the needed change in flux. Second, “induced current always repels a magnet” confuses opposition to change with a permanent direction of force. Third, “more turns always produce more current” ignores the resistance and measured quantity. Fourth, “zero crossing means no generation” confuses an instant of zero voltage with a whole rotating process. Fifth, “slip rings generate a.c.” gives the connector credit for the induction mechanism.
Sixth, mixing up primary and secondary reverses transformer calculations. Seventh, claiming voltage and current both increase in an ideal step-up transformer violates power equality. Eighth, treating all transformers as lossless ignores a stated efficiency. Ninth, saying high voltage “makes the cable have no resistance” misses the I²R mechanism. Tenth, using a graph’s appearance without axis scales can reverse a comparison.
Write these ten mistakes on one side of a revision card. On the other, write a one-sentence correction for each. The aim is not to learn a list of prohibitions. It is to notice the earliest point where a plausible story stopped matching the physics.
Build a study route from observation to transfer
For the first study session, draw a magnet and coil in four motion states and explain each meter response. For the second, interpret three voltage–time graphs, calculating periods and frequencies while keeping peak voltage separate. For the third, work on step-up and step-down transformer calculations with independent checks. For the fourth, take a transmission problem and explain why lowering current reduces cable heating. For the fifth, mix all four families without topic labels so that choosing the principle becomes part of the challenge.
Do not move straight from watching a worked solution to an identical question. Change one feature each time: reverse the pole, reverse the movement, change the graph’s time scale, swap the primary and secondary labels, or give an efficiency below 100%. If you can still explain what is happening, you are transferring understanding rather than copying a route.
A student coming from Primary Science may remember magnets and electrical circuits; secondary Physics asks for a more exact explanation of change, measurement and conservation. The PSLE Learning Guide offers the earlier bridge; the Science learning hub retains the broader subject route.
Further links and the next step in the series
For circuit-reading foundations, revisit Volume 0171 on ammeters and voltmeters and Volume 0167 on electrical power and energy; their exact links may be confirmed by the related-post route if the site’s permalink structure has moved. For graph-reading discipline, Volume 0270 on power-function graphs reinforces the principle that axes, values and model assumptions matter.
Use the Secondary English Learning Hub to make explanations precise and the Primary 6 Mathematics Learning Hub if ratio calculations still need repair. The next guide will ask you to audit public claims about a school energy demonstration rather than simply solve a familiar textbook diagram. That is a different level of performance: knowing when a scientific claim is justified, not merely knowing a formula.
Original mini-exam clinic: fifteen changes of condition
Clinic 1: Magnet stops mid-coil
Question. A student moves a bar magnet into a coil, holds it still for three seconds, and then pulls it out.
Reasoned answer. During the stationary interval, linked flux is no longer changing, so the induced e.m.f. from that motion returns to zero. On withdrawal the change reverses, so the meter deflection reverses under the same connection convention.
Common trap. A weak response says the current continues because the magnet is inside. That describes position, not change.
Transfer task. What if the coil is moved while the magnet remains fixed? Explain in terms of changing flux.
Clinic 2: A larger deflection
Question. Two trials use the same coil and magnet. Trial B moves the magnet through the same route faster and shows a larger peak deflection.
Reasoned answer. The faster trial changes the linked flux more rapidly and can produce a larger induced e.m.f. The observed meter response must still be interpreted according to the circuit and instrument.
Common trap. Do not claim this proves that magnetic field strength increased. The field source may be unchanged.
Transfer task. Name a method to keep the path comparable and a reason to repeat the trial.
Clinic 3: An open circuit
Question. A coil has its terminals separated. A magnet moves rapidly through it.
Reasoned answer. A changing field can induce a potential difference across the ends, but a sustained loop current cannot flow because the conducting circuit is incomplete.
Common trap. The mistaken statement that an open coil has no induced e.m.f. confuses source effect with current flow.
Transfer task. What equipment would you use to detect potential difference without completing a large load circuit?
Clinic 4: Opposing an approach
Question. The north pole of a magnet approaches one face of the coil.
Reasoned answer. The changing linked flux induces an effect that resists the increase; with a suitable closed circuit, the near face acts as a north pole during the approach.
Common trap. Merely saying clockwise current is risky because clockwise depends on viewing direction.
Transfer task. State which face and viewing direction the question specifies before assigning a current direction.
Clinic 5: Reverse the movement
Question. After approaching the coil, the same north pole moves away.
Reasoned answer. The induced direction reverses to oppose the reduction of flux, so the near face tends to behave as a south pole during withdrawal.
Common trap. The same magnet pole does not imply the same induced coil polarity for all motion.
Transfer task. Sketch both intervals and mark the change being opposed.
Clinic 6: Period to frequency
Question. Consecutive positive voltage peaks are 0.05 s apart.
Reasoned answer. One complete period is 0.05 s; frequency is 1/0.05 = 20 Hz.
Common trap. Counting the time from peak to next zero crossing would give a quarter-period in a sinusoidal graph, not a full period.
Transfer task. Describe where you would measure the period on a trough-to-trough graph.
Clinic 7: Peak is not frequency
Question. Graph A peaks at 8 V and repeats every 0.02 s. Graph B peaks at 5 V and repeats every 0.01 s.
Reasoned answer. A has greater peak magnitude; B has higher frequency, 100 Hz compared with 50 Hz.
Common trap. The taller wave on a page is not necessarily the faster alternating signal.
Transfer task. List both axis labels and scales before comparing waveforms.
Clinic 8: Step-up ratio
Question. An ideal transformer has 100 primary turns and 500 secondary turns with 6 V input.
Reasoned answer. Secondary voltage is 6 × 500/100 = 30 V, so it is step-up.
Common trap. Multiplying by the inverse ratio gives 1.2 V and contradicts the turns comparison.
Transfer task. If the ideal primary current were 2 A, calculate the output current.
Clinic 9: Current in an ideal transformer
Question. Input power is 120 W, secondary voltage 24 V, ideal conditions.
Reasoned answer. The output current is 120/24 = 5 A. Check that volts multiplied by amperes returns watts.
Common trap. The result cannot be found from voltage ratio alone unless a current or power value is also supplied.
Transfer task. Explain why a question about current needs additional information.
Clinic 10: Efficiency is stated
Question. A real transformer takes 200 W and is 85% efficient.
Reasoned answer. Output power is 0.85 × 200 = 170 W, and 30 W is lost in the device.
Common trap. Do not set input power equal to output power after being told that efficiency is 85%.
Transfer task. If output voltage is 34 V, calculate output current.
Clinic 11: Cable heating
Question. Current falls from 12 A to 3 A in a cable of unchanged resistance.
Reasoned answer. The new current is one quarter; I²R heating becomes one sixteenth of the original.
Common trap. A factor-of-four decrease in current does not produce only a factor-of-four decrease in resistive loss.
Transfer task. Explain which assumption keeps R the same.
Clinic 12: What the waveform cannot prove
Question. A generator voltage plot has two positive peaks separated by 0.02 s.
Reasoned answer. Frequency is 50 Hz. Without further information, the plot cannot prove the number of coil turns, field strength or mechanical rotational arrangement.
Common trap. Do not infer unmeasured design details from the shape alone.
Transfer task. Write a valid conclusion and one question for which the graph provides insufficient evidence.
Clinic 13: Model scope
Question. A question gives a transformer and says the secondary current is twice the primary current.
Reasoned answer. Under ideal conditions the secondary voltage must be half the primary voltage, because input and output power match.
Common trap. Do not assume that an output current doubling means energy is created.
Transfer task. Explain how the conclusion would change if efficiency were 90%.
Clinic 14: Experimental improvement
Question. A magnet is moved by hand at two supposed speeds, with only one trial at each speed.
Reasoned answer. Hand speed and path may vary; repeat a controlled motion along a marked path and record the response with a suitable sensor.
Common trap. Repeating uncontrolled trials without recording their conditions does not necessarily isolate speed.
Transfer task. Identify a specific dependent quantity and one additional controlled variable.
Clinic 15: Energy-flow test
Question. A student argues that a generator creates voltage without anyone doing work because its coil already contains copper.
Reasoned answer. Induction converts mechanical energy into electrical energy. Sustained energy delivery requires a mechanical input, with losses in real equipment.
Common trap. Copper is a conductor, not an unlimited energy store.
Transfer task. Describe what you would notice about the work required when more electrical load is supplied.
Final revision checklist
Before leaving an induction question, ask whether the magnetic flux actually changed; whether the response was an e.m.f. or a measured current; whether the circuit was complete; and whether the direction shown opposes the change. Before leaving a generator graph, identify peak voltage, period, frequency and axis scales separately. Before leaving a transformer calculation, label primary and secondary quantities, identify whether ideal conditions are assumed, test the ratio direction and check input against output power. Before leaving a transmission explanation, state what power is held constant and why reducing current reduces I²R heating. These questions make errors visible while they are still repairable.
Read the official specification rather than relying on inherited notes: the 2027 SEC G3 Physics code is K323. A school may use its own practice question sequence, but the statutory-looking question numbers and diagrams in this article are original examples for learning. A strong learner can give the physical mechanism in ordinary language, demonstrate it with a correctly labelled relation, and refuse to claim more than the evidence establishes.
