G3 SEC Physics Filament Lamp I–V Graph

Fast answer: a filament lamp does not usually give a straight current–potential-difference graph over a wide range. As its metallic filament becomes hotter, its resistance rises. Calculate resistance at each non-zero operating point using R = V/I, compare several readings, and avoid treating a single quotient as a constant for every condition.

This G3 Physics learner guide follows the wire-resistance investigation in Volumes 0693–0696. It takes the next conceptual step: from an approximately constant-temperature wire to a lamp whose temperature changes during operation. All figures below are invented for teaching, not official examination data. This article is for school-approved learning and contains no instructions for working on household wiring.

2027 G3 Physics: SEC syllabus K323, previously referenced as O-Level 6091.
Paper 1: 40 multiple-choice marks in one hour; Paper 2: 80 marks in 1 hour 45 minutes; Paper 3: practical, 40 marks in 1 hour 50 minutes.
Keep observed readings separate from calculated resistance and inferred temperature.
Study what the curvature shows; do not confuse V/I with a curved graph’s changing steepness.
Use official SEAB syllabuses for exam rules and practice material for practice only.

The next question after wire resistance
What the 2027 examination actually tests
Current means charge passing per second
Potential difference is an energy transfer per charge
Place the ammeter and voltmeter correctly
A working model must match the material
The lamp’s filament becomes hot
Resistance at a point is a quotient
Do not mistake a quotient for a gradient
A graph is an argument, not a decoration
Record data before calculating
The first data pair is not a universal law
Zero voltage needs careful handling
A non-zero intercept requires investigation
Temperature as a confounding variable
Never equate brightness with a temperature reading
Know the roles of voltage, current and power
Different graphs answer different questions
The diode is not the same curve
Check the direction of the applied p.d.
The controlled laboratory setting matters
Repeats help, but do not guarantee validity
Measurements have sensible precision
Explain anomalies without hiding them
What structured questions reward
What practical evaluation rewards
Transfer from PSLE to G3 reasoning
An efficient way to revise this topic
Worked example dataset
Twenty worked transfer checks
Four-week practice plan
Frequently asked questions
Linked learning routes and sources

1. The next question after wire resistance

In Volumes 0693–0696 we learned to change wire length fairly and to distinguish a predicted relationship from measured evidence. A filament lamp moves the question forward. Rather than keeping a metallic conductor at approximately constant temperature, we study a component whose temperature changes markedly as current passes.

A learner who memorised R = V/I may assume the number R is fixed. Ask what conditions held when the formula was used and whether those conditions still hold in a glowing filament.

A formula can remain valid at each operating point even when the resistance itself varies between operating points.

A useful examination habit at this stage is to write three short statements before answering: what the question gives, what relationship or observation is relevant, and what the evidence permits you to conclude. Test that habit on the next question after wire resistance. If you cannot identify the given data, return to the diagram or source. If you cannot state the principle, revise the small concept rather than guessing from familiar words. If the conclusion contains more certainty than the readings allow, narrow it.

For independent practice, change one feature of the example: a unit, an axis label, one reading, or the condition being controlled. Decide which parts of the explanation survive and which must change. This is harder than copying a worked answer, but it prepares you for unfamiliar contexts in a structured paper and for honest evaluation in a practical paper.

2. What the 2027 examination actually tests

The official 2027 SEAB G3 Physics syllabus K323 includes current, potential difference, resistance, metallic conductors, filament-lamp and diode I–V characteristics, circuits and practical evaluation. The earlier 6091 code is the reference code for 2026 and earlier.

K323 Physics has three assessed papers: multiple choice, structured/free response, and practical. This original learner case is practice, not an examination specimen or a forecast of a specific question.

Know the distinction between published syllabus outcomes and a teacher-created investigation used to practise those outcomes.

A useful examination habit at this stage is to write three short statements before answering: what the question gives, what relationship or observation is relevant, and what the evidence permits you to conclude. Test that habit on what the 2027 examination actually tests. If you cannot identify the given data, return to the diagram or source. If you cannot state the principle, revise the small concept rather than guessing from familiar words. If the conclusion contains more certainty than the readings allow, narrow it.

3. Current means charge passing per second

An ammeter measures electric current in amperes. Current tells us how much charge flows past a point each second, not how quickly an individual electron travels. The relationship Q = It can help check what a steady measured current represents over a specified interval.

If 0.20 A is maintained for 15 s, 3.0 C passes a chosen cross-section. That calculation says nothing by itself about the lamp temperature or how much energy was delivered.

Avoid calling current ‘the electricity used up’. In a simple series circuit, the current through components is the same.

A useful examination habit at this stage is to write three short statements before answering: what the question gives, what relationship or observation is relevant, and what the evidence permits you to conclude. Test that habit on current means charge passing per second. If you cannot identify the given data, return to the diagram or source. If you cannot state the principle, revise the small concept rather than guessing from familiar words. If the conclusion contains more certainty than the readings allow, narrow it.

4. Potential difference is an energy transfer per charge

Potential difference across a lamp describes the work done per unit charge passing through it. A value written on a power supply does not automatically equal the measured p.d. across the lamp if other components or source effects matter.

When a voltmeter reads 2.0 V across the lamp, use that measured value with the current through the lamp when calculating its operating resistance.

A reading is only meaningful when the meter connection and the measured component are correctly identified.

A useful examination habit at this stage is to write three short statements before answering: what the question gives, what relationship or observation is relevant, and what the evidence permits you to conclude. Test that habit on potential difference is an energy transfer per charge. If you cannot identify the given data, return to the diagram or source. If you cannot state the principle, revise the small concept rather than guessing from familiar words. If the conclusion contains more certainty than the readings allow, narrow it.

5. Place the ammeter and voltmeter correctly

An ammeter is connected in series with the path whose current is required. A voltmeter is connected across the component whose potential difference is required. This measurement logic explains the circuit diagram rather than merely decorating it.

In a school-approved low-voltage practical, trace the path through the lamp and ammeter before interpreting a reading. Then identify the two points connected to the voltmeter.

Do not infer valid resistance from two numbers taken at unrelated locations. This is a reasoning guide, not an instruction to modify electrical equipment.

A useful examination habit at this stage is to write three short statements before answering: what the question gives, what relationship or observation is relevant, and what the evidence permits you to conclude. Test that habit on place the ammeter and voltmeter correctly. If you cannot identify the given data, return to the diagram or source. If you cannot state the principle, revise the small concept rather than guessing from familiar words. If the conclusion contains more certainty than the readings allow, narrow it.

6. A working model must match the material

At approximately constant temperature, many metallic conductors behave ohmically and the current is proportional to potential difference. The filament lamp is also metallic, but a change in temperature causes its resistance to change.

A straight I–V line through the origin can describe an ohmic resistor under suitable conditions. A curved lamp graph suggests that a fixed-resistance description is insufficient across the measured range.

The useful question is what variable changed during the investigation, not whether the formula stopped working.

A useful examination habit at this stage is to write three short statements before answering: what the question gives, what relationship or observation is relevant, and what the evidence permits you to conclude. Test that habit on a working model must match the material. If you cannot identify the given data, return to the diagram or source. If you cannot state the principle, revise the small concept rather than guessing from familiar words. If the conclusion contains more certainty than the readings allow, narrow it.

7. The lamp’s filament becomes hot

Electrical energy is transferred within the filament, raising its temperature until losses balance input at an operating condition. For typical metallic filaments, higher temperature increases resistance. A lamp therefore draws less additional current for each added volt at higher operating values.

Look at a progression such as 1.0 V and 0.18 A, then 2.0 V and 0.29 A: the voltage doubled but the current did not.

Do not claim that temperature was measured directly unless a temperature instrument or an independently justified method was used.

A useful examination habit at this stage is to write three short statements before answering: what the question gives, what relationship or observation is relevant, and what the evidence permits you to conclude. Test that habit on the lamp’s filament becomes hot. If you cannot identify the given data, return to the diagram or source. If you cannot state the principle, revise the small concept rather than guessing from familiar words. If the conclusion contains more certainty than the readings allow, narrow it.

8. Resistance at a point is a quotient

For each measured pair, R = V/I gives the resistance under that measured operating condition. Calculate the quotient using matching values for the same lamp at the same time, with volts divided by amperes yielding ohms.

At 2.0 V and 0.29 A, resistance is approximately 6.90 ohms. At 3.0 V and 0.36 A, the quotient is 8.33 ohms.

These are not contradictory measurements. They show that the simple constant-resistance assumption does not hold for all points.

A useful examination habit at this stage is to write three short statements before answering: what the question gives, what relationship or observation is relevant, and what the evidence permits you to conclude. Test that habit on resistance at a point is a quotient. If you cannot identify the given data, return to the diagram or source. If you cannot state the principle, revise the small concept rather than guessing from familiar words. If the conclusion contains more certainty than the readings allow, narrow it.

9. Do not mistake a quotient for a gradient

When current is on the vertical axis and voltage on the horizontal, the quantity I/V is a ratio between coordinates and the origin. On a curved graph it must not casually be identified with the gradient of the curve everywhere.

At 2.0 V and 0.29 A the coordinate ratio is 0.145 A/V. Its reciprocal is about 6.90 ohms. The changing steepness of the curve tells a related, but different, story.

G3 learners need clear axes and proportionality reasoning; more advanced instantaneous slope language is unnecessary here.

A useful examination habit at this stage is to write three short statements before answering: what the question gives, what relationship or observation is relevant, and what the evidence permits you to conclude. Test that habit on do not mistake a quotient for a gradient. If you cannot identify the given data, return to the diagram or source. If you cannot state the principle, revise the small concept rather than guessing from familiar words. If the conclusion contains more certainty than the readings allow, narrow it.

10. A graph is an argument, not a decoration

Label axes with both quantities and units, choose a sensible scale and plot measured points before describing any trend. A smooth curve can help readers notice a gradual change, but it cannot certify a particular physical cause alone.

A plot of the invented lamp measurements should rise as p.d. increases, becoming less steep in the first quadrant. Describe what is directly seen before explaining it.

Do not draw a perfect straight line merely because you have practised straight-line gradients in Mathematics.

A useful examination habit at this stage is to write three short statements before answering: what the question gives, what relationship or observation is relevant, and what the evidence permits you to conclude. Test that habit on a graph is an argument, not a decoration. If you cannot identify the given data, return to the diagram or source. If you cannot state the principle, revise the small concept rather than guessing from familiar words. If the conclusion contains more certainty than the readings allow, narrow it.

11. Record data before calculating

A good practical record distinguishes the measured p.d., measured current and resistance calculated afterwards. It also records repeat observations when appropriate. Transparency lets another student detect a copied entry or an impossible unit.

Use separate columns headed p.d. / V, current / A and R / ohm. Write a note for any flicker, unstable reading or change to the selected range.

Do not silently replace an unusual measurement with one that makes the curve look more beautiful.

A useful examination habit at this stage is to write three short statements before answering: what the question gives, what relationship or observation is relevant, and what the evidence permits you to conclude. Test that habit on record data before calculating. If you cannot identify the given data, return to the diagram or source. If you cannot state the principle, revise the small concept rather than guessing from familiar words. If the conclusion contains more certainty than the readings allow, narrow it.

12. The first data pair is not a universal law

A single voltage/current pair is enough to calculate one operating resistance, but insufficient to decide whether R stays fixed over a range. A pattern requires several distinct, reliable operating readings.

A class may report 1.0 V and 0.18 A accurately. It still cannot justify ‘this lamp always has resistance 5.56 ohms’ from that result.

Use words such as ‘at this operating point’ when the evidence covers only one condition.

A useful examination habit at this stage is to write three short statements before answering: what the question gives, what relationship or observation is relevant, and what the evidence permits you to conclude. Test that habit on the first data pair is not a universal law. If you cannot identify the given data, return to the diagram or source. If you cannot state the principle, revise the small concept rather than guessing from familiar words. If the conclusion contains more certainty than the readings allow, narrow it.

13. Zero voltage needs careful handling

At the origin, I and V may both be zero. The expression V/I then becomes 0/0, which is undefined. A graph may pass through the origin without giving a numerical resistance from that single point.

The starting point on a lamp graph is useful for recognising the absence of applied p.d. and current, but it is not a licence to divide zero by zero.

Students often treat the origin as another resistance row. Instead, use a non-zero measured pair to calculate R.

A useful examination habit at this stage is to write three short statements before answering: what the question gives, what relationship or observation is relevant, and what the evidence permits you to conclude. Test that habit on zero voltage needs careful handling. If you cannot identify the given data, return to the diagram or source. If you cannot state the principle, revise the small concept rather than guessing from familiar words. If the conclusion contains more certainty than the readings allow, narrow it.

14. A non-zero intercept requires investigation

Measurement errors, meter resolution, contact effects or misreading can create a small apparent intercept. Do not force the data through a preferred mathematical shape without explaining why. Review the experimental arrangement and uncertainties.

If an instrument indicates a small current with no recorded applied p.d., the first task is to check what the instruments actually measured, not to invent an exotic material property.

Distinguish a data pattern from an artefact of method, transcription or instrument zero.

A useful examination habit at this stage is to write three short statements before answering: what the question gives, what relationship or observation is relevant, and what the evidence permits you to conclude. Test that habit on a non-zero intercept requires investigation. If you cannot identify the given data, return to the diagram or source. If you cannot state the principle, revise the small concept rather than guessing from familiar words. If the conclusion contains more certainty than the readings allow, narrow it.

15. Temperature as a confounding variable

A student may propose changing voltage to study how current varies while claiming the temperature is controlled. For a filament lamp that assumption is usually unrealistic. Instead describe heating as part of the phenomenon being investigated.

A design can investigate the observed I–V relationship of a lamp while acknowledging that temperature is not fixed. The question should match what can honestly be varied and observed.

A fair test does not always mean that every variable stays constant; it means relevant conditions and mechanisms are handled thoughtfully.

A useful examination habit at this stage is to write three short statements before answering: what the question gives, what relationship or observation is relevant, and what the evidence permits you to conclude. Test that habit on temperature as a confounding variable. If you cannot identify the given data, return to the diagram or source. If you cannot state the principle, revise the small concept rather than guessing from familiar words. If the conclusion contains more certainty than the readings allow, narrow it.

16. Never equate brightness with a temperature reading

A glowing lamp may appear brighter when more power is delivered, but human vision and surroundings affect apparent brightness. Brightness can support a qualitative account; it is not a calibrated numerical thermometer.

If the lamp appears brighter at 3.0 V than at 1.0 V, report the observation separately from the inferred temperature rise and calculated resistance.

This distinction matters in science explanations: observation, calculation and inference are not interchangeable.

A useful examination habit at this stage is to write three short statements before answering: what the question gives, what relationship or observation is relevant, and what the evidence permits you to conclude. Test that habit on never equate brightness with a temperature reading. If you cannot identify the given data, return to the diagram or source. If you cannot state the principle, revise the small concept rather than guessing from familiar words. If the conclusion contains more certainty than the readings allow, narrow it.

17. Know the roles of voltage, current and power

Power is the rate of energy transfer and can be calculated from P = VI. Higher voltage does not mean precisely doubled power if the current also changes non-proportionally. Work with the measured pair.

At 1.0 V and 0.18 A, P = 0.18 W. At 3.0 V and 0.36 A, P = 1.08 W, which is six times as large in this illustrative dataset.

A lamp’s power at an operating point does not by itself establish efficiency, lifetime or suitability for a different circuit.

A useful examination habit at this stage is to write three short statements before answering: what the question gives, what relationship or observation is relevant, and what the evidence permits you to conclude. Test that habit on know the roles of voltage, current and power. If you cannot identify the given data, return to the diagram or source. If you cannot state the principle, revise the small concept rather than guessing from familiar words. If the conclusion contains more certainty than the readings allow, narrow it.

18. Different graphs answer different questions

Plotting I against V makes current the response variable. Plotting V against I changes the axes and the interpretation of a slope. Plotting R against V highlights the changing operating resistance but requires a calculated series.

Before answering a gradient question, say out loud which quantity is on each axis. At a curved plot, use precise wording about changing steepness instead of treating one ratio as a universal constant.

The graph should be chosen for the question, not for the method the student happened to memorise.

A useful examination habit at this stage is to write three short statements before answering: what the question gives, what relationship or observation is relevant, and what the evidence permits you to conclude. Test that habit on different graphs answer different questions. If you cannot identify the given data, return to the diagram or source. If you cannot state the principle, revise the small concept rather than guessing from familiar words. If the conclusion contains more certainty than the readings allow, narrow it.

19. The diode is not the same curve

Both a filament lamp and a semiconductor diode may have non-linear I–V relationships, but their explanations differ. A filament-lamp curve is linked to heating; diode behaviour includes strong direction-dependent conduction.

A question might present a diode curve and a lamp curve without labels. First compare the shapes and axes, then use the syllabus models to identify a plausible component.

Do not use the phrase ‘non-ohmic’ as though it names a single physical mechanism.

A useful examination habit at this stage is to write three short statements before answering: what the question gives, what relationship or observation is relevant, and what the evidence permits you to conclude. Test that habit on the diode is not the same curve. If you cannot identify the given data, return to the diagram or source. If you cannot state the principle, revise the small concept rather than guessing from familiar words. If the conclusion contains more certainty than the readings allow, narrow it.

20. Check the direction of the applied p.d.

An I–V characteristic may include positive and negative values. For a symmetrical filament lamp, reversing the applied voltage reverses conventional current; the magnitude relationship can be approximately mirrored under comparable thermal conditions.

A diode, by contrast, generally shows very different current in forward and reverse directions within its operating limits.

Avoid sketching the diode as though it behaves like a lamp simply because both are electronic components.

A useful examination habit at this stage is to write three short statements before answering: what the question gives, what relationship or observation is relevant, and what the evidence permits you to conclude. Test that habit on check the direction of the applied p.d.. If you cannot identify the given data, return to the diagram or source. If you cannot state the principle, revise the small concept rather than guessing from familiar words. If the conclusion contains more certainty than the readings allow, narrow it.

21. The controlled laboratory setting matters

A teaching investigation should use appropriate school equipment, permitted ratings and direct teacher supervision. Do not give learners mains-voltage wiring instructions. Real installation, appliance repair and protective-device selection belong to qualified adults.

The published case uses invented low-voltage observations to teach examination reasoning. It does not imply that a particular live circuit arrangement is safe to build at home.

Scientific explanation and practical authorisation are two separate responsibilities.

A useful examination habit at this stage is to write three short statements before answering: what the question gives, what relationship or observation is relevant, and what the evidence permits you to conclude. Test that habit on the controlled laboratory setting matters. If you cannot identify the given data, return to the diagram or source. If you cannot state the principle, revise the small concept rather than guessing from familiar words. If the conclusion contains more certainty than the readings allow, narrow it.

22. Repeats help, but do not guarantee validity

Repeating readings may reveal random variation. It cannot remove a systematic problem such as a mispositioned meter, an instrument offset or a lamp with changing conditions between readings.

If repeated current readings at 2.0 V are 0.28, 0.29 and 0.30 A, their mean is 0.29 A. That looks stable but does not establish that the meter was correctly connected.

A strong evaluation names what repeated observations can and cannot improve.

A useful examination habit at this stage is to write three short statements before answering: what the question gives, what relationship or observation is relevant, and what the evidence permits you to conclude. Test that habit on repeats help, but do not guarantee validity. If you cannot identify the given data, return to the diagram or source. If you cannot state the principle, revise the small concept rather than guessing from familiar words. If the conclusion contains more certainty than the readings allow, narrow it.

23. Measurements have sensible precision

Report a calculated result to a precision justified by the original readings and the examination instruction. Displaying 6.896551724 ohms from 2.0 V and 0.29 A suggests accuracy not supported by the measurements.

Write approximately 6.9 ohms in an explanatory paragraph, or 6.90 ohms where three significant figures are requested and justified by the exam convention.

Mathematical arithmetic can be exact while the physical measurement remains uncertain.

A useful examination habit at this stage is to write three short statements before answering: what the question gives, what relationship or observation is relevant, and what the evidence permits you to conclude. Test that habit on measurements have sensible precision. If you cannot identify the given data, return to the diagram or source. If you cannot state the principle, revise the small concept rather than guessing from familiar words. If the conclusion contains more certainty than the readings allow, narrow it.

24. Explain anomalies without hiding them

An unexpected point can arise from transcription, temporary temperature change, unsuitable range or other experimental limitations. It may also reflect a real effect. The evidence should guide the explanation.

If the 2.5 V reading is unexpectedly below the 2.0 V current, first check whether the values were recorded at the correct time and whether the circuit was stable.

The examiner rewards a defensible inference from evidence, not a polished story invented after seeing a graph.

A useful examination habit at this stage is to write three short statements before answering: what the question gives, what relationship or observation is relevant, and what the evidence permits you to conclude. Test that habit on explain anomalies without hiding them. If you cannot identify the given data, return to the diagram or source. If you cannot state the principle, revise the small concept rather than guessing from familiar words. If the conclusion contains more certainty than the readings allow, narrow it.

25. What structured questions reward

In K323 Paper 2, students may need to interpret a data-based stem, combine quantities and give reasons. The source supplies the facts; the answer should use the relevant physics rather than assume facts not stated.

A two-mark explanation can be clear: as the applied p.d. increases, the filament becomes hotter and its resistance increases, so current does not rise in direct proportion to p.d.

Use the wording of the specific prompt. Do not write a long essay where a calculation plus one reason is enough.

A useful examination habit at this stage is to write three short statements before answering: what the question gives, what relationship or observation is relevant, and what the evidence permits you to conclude. Test that habit on what structured questions reward. If you cannot identify the given data, return to the diagram or source. If you cannot state the principle, revise the small concept rather than guessing from familiar words. If the conclusion contains more certainty than the readings allow, narrow it.

26. What practical evaluation rewards

K323 Paper 3 assesses planning, manipulation and observation, data presentation, analysis and evaluation. Learning to distinguish apparatus errors from changing filament properties supports a good experimental discussion.

When asked for an improvement, identify a plausible weakness and state how the change addresses it. ‘Be more careful’ does not specify a method.

Never claim an experimental procedure is mandatory merely because it appears in a practice exercise.

A useful examination habit at this stage is to write three short statements before answering: what the question gives, what relationship or observation is relevant, and what the evidence permits you to conclude. Test that habit on what practical evaluation rewards. If you cannot identify the given data, return to the diagram or source. If you cannot state the principle, revise the small concept rather than guessing from familiar words. If the conclusion contains more certainty than the readings allow, narrow it.

27. Transfer from PSLE to G3 reasoning

At PSLE, pupils learn to observe changes and compare variables. G3 work requires tighter measurement language, symbolic relationships and examination-specific explanations. The underlying discipline is continuous even when the complexity grows.

A simple PSLE observation such as ‘the lamp gets brighter’ becomes a G3 account distinguishing observed light output, current, p.d., power and resistance under changing conditions.

A student who can say what was measured, calculated and inferred has a reliable bridge across subjects.

A useful examination habit at this stage is to write three short statements before answering: what the question gives, what relationship or observation is relevant, and what the evidence permits you to conclude. Test that habit on transfer from psle to g3 reasoning. If you cannot identify the given data, return to the diagram or source. If you cannot state the principle, revise the small concept rather than guessing from familiar words. If the conclusion contains more certainty than the readings allow, narrow it.

28. An efficient way to revise this topic

Read the diagram first, name the measured quantities, calculate one resistance correctly, then compare at least two operating points. Explain the curve only after checking the numerical pattern. Finish by naming the limits of the conclusion.

Give a partner a small table, hide the component label and ask them to explain which physical model could produce the curve. Change the dataset afterwards to test whether they truly understood.

Build accuracy before speed. A short correct explanation can beat several unsupported paragraphs.

A useful examination habit at this stage is to write three short statements before answering: what the question gives, what relationship or observation is relevant, and what the evidence permits you to conclude. Test that habit on an efficient way to revise this topic. If you cannot identify the given data, return to the diagram or source. If you cannot state the principle, revise the small concept rather than guessing from familiar words. If the conclusion contains more certainty than the readings allow, narrow it.

A complete fictional filament-lamp dataset

The following six current–p.d. pairs are made for practice. They are not published experimental results. The operating conditions, component ratings and apparatus are not specified. Their purpose is to let a learner calculate ratios and evaluate a trend without guessing a cause from a lone number.

p.d. / V	Current / A	R = V/I / ohm (approx.)	Electrical power / W
0.5	0.11	4.55	0.055
1.0	0.18	5.56	0.180
1.5	0.24	6.25	0.360
2.0	0.29	6.90	0.580
2.5	0.33	7.58	0.825
3.0	0.36	8.33	1.080

Read the table from left to right: the first two columns are assumed observations, while the last two are calculated. Resistance rises from about 4.55 ohms to about 8.33 ohms. The physical model of a warming metallic filament offers an explanation, but temperature was not independently measured in this constructed exercise. That distinction is what makes the argument honest.

Twenty worked transfer checks

Practice 1: Resistance at 0.5 V

Try this without looking at the explanation: take the relevant figures from the fictional data or the wording of this mini-question. Name the measured quantities, select the appropriate relation, show any necessary numerical working and write a conclusion whose scope matches the evidence.

Compute 0.5 divided by 0.11, giving about 4.55 ohms. This is an operating resistance from one non-zero pair, not a permanent value for the component.

To check your answer, look for a complete chain rather than a remembered phrase. Does the calculation use consistent units? Does your conclusion describe a single operating point or the whole measured range accurately? Has a physical explanation been labelled as an inference where the measurements do not directly observe it? Make the smallest repair that addresses the first error you find.

Practice 2: Resistance at 1.0 V

Divide 1.0 by 0.18 to obtain about 5.56 ohms. Compare with 4.55 ohms, and describe the increase in the calculated quotient without claiming a directly measured temperature.

Practice 3: Resistance at 1.5 V

Divide 1.5 by 0.24 to obtain 6.25 ohms. If a classmate says the lamp must obey a constant 5.56-ohm rule, this additional reading provides counterevidence.

Practice 4: Resistance at 2.0 V

Use 2.0 V and 0.29 A to calculate about 6.90 ohms. The voltage and current are measured quantities; the resistance is a derived quantity under the stated condition.

Practice 5: Resistance at 2.5 V

Use 2.5 V and 0.33 A to calculate about 7.58 ohms. Ask whether a direct proportion describes every pair. The changing ratios show it does not.

Practice 6: Resistance at 3.0 V

Use 3.0 V and 0.36 A to calculate about 8.33 ohms. A justified conclusion is that calculated operating resistance rises over the displayed range.

Practice 7: The doubling test

If current were directly proportional to applied p.d., doubling 1.0 V would double 0.18 A to 0.36 A. The table instead records 0.29 A at 2.0 V, so direct proportionality fails in this range.

Practice 8: Power at the first point

Multiply 0.5 V by 0.11 A to obtain 0.055 W. Resist the temptation to infer the lamp’s efficiency; light output and the energy distribution have not been measured.

Practice 9: Power at the final point

Calculate 3.0 × 0.36 = 1.08 W. This is the electrical input at the recorded operating point, assuming the measurements correspond to the same condition.

Practice 10: A changing increment

From 1.0 to 2.0 V, current rises 0.11 A. From 2.0 to 3.0 V, it rises only 0.07 A. Compare equal voltage changes before discussing the curve’s flattening.

Practice 11: Changing graph axes

When the vertical axis is current and horizontal is p.d., a straight ohmic line has gradient I/V. If the axes are reversed, that interpretation changes; always read labels first.

Practice 12: A missing reading

A class loses the current entry for 2.5 V. It should mark that measurement missing, not interpolate a number and label it observed. An estimate must remain visibly identified as an estimate.

Practice 13: An untrustworthy precision

A calculator gives 6.896551724 for 2.0/0.29. Report a justified rounded value, not every digit, and carry enough digits through intermediate work where necessary.

Practice 14: A mistaken cause

A student writes: ‘The resistance rose because the voltage entered the lamp.’ Repair it by explaining that increased electrical input heats a metallic filament, changing its resistance.

Practice 15: A contradictory headline

An exhibition headline says ‘Double the voltage, double the current.’ The displayed 1.0 V and 2.0 V pairs contradict it. A better caption says that current increased by less than a factor of two.

Practice 16: A practical limitation

Three repeated readings near 0.29 A do not certify correct meter placement. State one independent way the experimental arrangement could still be checked by the supervising teacher.

Practice 17: Short-answer explanation

Question: why is a filament-lamp graph curved? Answer: as the filament gets hotter, its resistance rises; consequently current is not proportional to p.d. across the measured range.

Practice 18: Unjustified comparison

A claim that ‘Lamp A is better than Lamp B’ cannot follow from one resistance comparison. Better at what—brightness, energy use, longevity, or a specified function? Define the comparison criterion.

Practice 19: Zero-over-zero

At V = 0 and I = 0, do not calculate R = 0/0. That expression is undefined. Use the non-zero pairs to compare resistance values.

Practice 20: Examination transfer

A new dataset may have different values or missing units. Start again from its graph and stated measurements; do not substitute the numbers from this invented practice set.

Four-week practice plan

Week 1 — foundation: practise identifying current, potential difference, resistance and the role of meters on annotated diagrams. Make ten paired V/I calculations and explain each unit.
Week 2 — graphs: plot the supplied dataset on sensible axes. Compare equal changes in p.d. and write short descriptions that do not claim direct proportionality.
Week 3 — scientific reasoning: compare ohmic conductors, heated filament lamps and diodes using syllabus-level qualitative models; then critique unsupported statements and sources of experimental uncertainty.
Week 4 — exam transfer: complete short timed structured explanations and practical evaluations using fresh values. Rework errors, then explain one new graph aloud without notes.

Frequently asked questions

Is a filament lamp an ohmic conductor?

A typical filament lamp does not have constant resistance across a wide applied-voltage range because its temperature changes. This is different from an ohmic metallic conductor tested at approximately constant temperature.

Does R = V/I stop being correct?

No. It gives the operating resistance for a valid measured pair. The error is assuming that the value must remain unchanged under every condition.

Why is the current–voltage graph curved?

As current and input power increase, the filament becomes hotter, raising resistance. The current therefore does not increase in direct proportion to voltage.

Can brightness prove temperature numerically?

No. It may support a qualitative account, but a numerical temperature requires an appropriate method and assumptions.

Does a diode have the same explanation?

No. Although both can have non-linear I–V characteristics, a diode’s direction-dependent conduction differs from the heating-dominated behaviour of a filament lamp.

Are these measurements official exam data?

No. They are deliberately invented examples. Refer to the published 2027 G3 syllabus and official specimen materials for assessment rules.

What is the best first move in a data question?

Check the axes, quantities, units and experimental condition before selecting a formula or describing a trend.

How should a student prepare from Primary 6?

Keep the PSLE habits of precise observations and fair comparisons, then extend them with algebraic ratios, accurate measurement language and evidence-based explanations.

Small Group Tutorials

How to Perform in the new G3 SEC Examinations | Learner’s Guide Vol 0697 | Physics Filament-Lamp Evidence — Explain Heating, Resistance and Curved I–V Data

Contents