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Every year, Edexcel examiners' reports highlight the same recurring mistakes. These are not gaps in knowledge — most students know the physics — but errors in execution that cost marks. By learning to recognise these common traps, you can avoid them in the exam.
This is the single most common source of lost marks in physics calculations. Students use values straight from the question without converting to SI base units.
The critical conversions:
| Common mistake | Correct conversion | Typical context |
|---|---|---|
| Using cm instead of m | 1 cm = 0.01 m | Plate separation |
| Using mm instead of m | 1 mm = 0.001 m | Wire extension, slit width |
| Using g instead of kg | 1 g = 0.001 kg | Small masses |
| Using °C instead of K | K = °C + 273 | Gas law equations |
| Using kJ instead of J | 1 kJ = 1000 J | Energy calculations |
| Using mA instead of A | 1 mA = 0.001 A | Small currents |
| Using μF instead of F | 1 μF = 10⁻⁶ F | Capacitance |
| Using nm instead of m | 1 nm = 10⁻⁹ m | Light wavelengths |
| Using hours instead of s | 1 hour = 3600 s | Orbital periods |
| Using eV instead of J | 1 eV = 1.60 × 10⁻¹⁹ J | Particle energies |
| Using MeV instead of J | 1 MeV = 1.60 × 10⁻¹³ J | Nuclear energies |
Prevention strategy: Before substituting any value, write it down with its unit and convert it to SI. Make this a habit — even when it seems unnecessary. Write the conversion explicitly: "d = 5.0 cm = 5.0 × 10⁻² m."
Vectors have direction, and direction matters. Common sign errors include:
Prevention strategy: At the start of every vector problem, explicitly state your positive direction. For example: "Taking upward as positive" or "Taking rightward as positive." Then apply this consistently.
Speed is a scalar quantity (magnitude only); velocity is a vector (magnitude and direction).
Common mistakes:
Students sometimes pick a SUVAT equation that involves the unknown they need AND another unknown. This creates two unknowns and an unsolvable equation.
Correct approach:
graph TD
A["SUVAT Equation Selection"] --> B["List knowns: u, v, a, s, t"]
B --> C["Identify the unknown you WANT"]
C --> D["Identify the variable NOT involved"]
D --> E{"Which variable is\nnot involved?"}
E -->|"s not needed"| F["v = u + at"]
E -->|"v not needed"| G["s = ut + ½at²"]
E -->|"u not needed"| H["s = vt − ½at²"]
E -->|"a not needed"| I["s = ½(u+v)t"]
E -->|"t not needed"| J["v² = u² + 2as"]
When an object is on a slope at angle θ to the horizontal, the weight must be resolved into two components:
The most common mistake is using mg sin θ when you should use mg cos θ, or vice versa. Remember: the component of weight acting down the slope uses sin θ, and the component into the slope uses cos θ.
A useful check: when θ = 0° (flat surface), the component down the slope should be zero (mg sin 0° = 0) and the normal reaction should equal mg (mg cos 0° = mg). This confirms the correct assignment.
In both gravitational and electric fields, students confuse field strength and potential:
| Quantity | Gravitational | Electric | Key difference |
|---|---|---|---|
| Field strength | g = GM/r² (vector, N kg⁻¹) | E = kQ/r² (vector, N C⁻¹) | Force per unit mass/charge |
| Potential | V = −GM/r (scalar, J kg⁻¹) | V = kQ/r (scalar, J C⁻¹) | Energy per unit mass/charge |
| Depends on | 1/r² | 1/r² | Field strength: 1/r², Potential: 1/r |
| Nature | Always attractive | Attractive or repulsive | Gravitational potential always negative |
Key differences:
A satellite moves from orbit radius r to orbit radius 2r. A student claims that both the gravitational field strength and the gravitational potential halve.
The field strength at distance r is g = GM/r². At distance 2r: g = GM/(2r)² = GM/4r². The field strength drops to one quarter, not one half.
The potential at distance r is V = −GM/r. At distance 2r: V = −GM/2r. The potential halves (becomes less negative — closer to zero).
The student confused 1/r² with 1/r. This is one of the most common field calculation errors.
Common mistake: calculating energy when the question asks for power, or vice versa. Always check the units of your answer — if you are asked for power and get an answer in joules, you have calculated energy and need to divide by time.
Students often assume what a graph shows rather than reading the axis labels. Common errors:
Prevention strategy: Before analysing any graph, read both axis labels, note the scale and units, and check whether the axes start from zero.
Many physics definitions require "per unit" phrasing:
| Term | Correct definition | Common incorrect version |
|---|---|---|
| Electric field strength | Force per unit positive charge | "The electric force" |
| Gravitational field strength | Force per unit mass | "The gravitational force" |
| Potential difference | Energy transferred per unit charge | "The energy transferred" |
| Specific heat capacity | Energy per unit mass per unit temperature rise | "Energy to heat something up" |
| Specific latent heat | Energy per unit mass to change state | "Energy to melt/boil" |
Omitting "per unit" changes the meaning entirely and costs marks in definition questions. The word "specific" in physics nearly always means "per unit mass."
Before submitting any calculation, run through these checks:
graph TD
A["Finished calculation?\nRun the checklist:"] --> B["✓ All units converted to SI?"]
B --> C["✓ Signs correct for vectors?"]
C --> D["✓ Positive direction stated?"]
D --> E["✓ Correct equation chosen?"]
E --> F["✓ Units included in final answer?"]
F --> G["✓ Sig figs appropriate?"]
G --> H["✓ Answer seems reasonable?"]
H --> I{"All checks pass?"}
I -->|Yes| J["Move to next question"]
I -->|No| K["Re-check the flagged step"]
The errors that drag a working A-grade candidate down to a B, or a B-grade candidate down to a C, are not usually gaps in physics knowledge. They are execution failures — small, consistent leaks that bleed two or three marks per page across a 2-hour paper. The arithmetic of this is brutal: a candidate losing one easily-avoidable mark per page across the three Edexcel 9PH0 papers gives away enough to drop a full grade. The sections below treat error-elimination as a separately learnable skill and set out the structural habits that close the leaks.
Mistakes in physics exams fall into four broad categories, and the distribution across the three Edexcel 9PH0 papers is broadly stable. Recognising which category a mistake belongs to is the first step to eliminating it — different categories respond to different remedies, and a candidate trying to fix everything with "more practice" wastes time on errors that need a different intervention.
| Error category | Typical share of lost marks | What it looks like |
|---|---|---|
| Arithmetic | 25–35% | Slips in calculator entry, lost factors of 2, sign flips during rearrangement |
| Conceptual | 20–30% | Confusing field with potential, treating speed as velocity, mis-applying conservation laws |
| Presentational | 20–25% | Missing units, wrong sig figs, no positive direction stated, ambiguous final line |
| Exam-technique | 15–25% | Misreading a demand verb, ignoring a stated form, blank space on a "show that" |
The mix shifts across papers. Paper 1 leans more heavily on conceptual mistakes because its content (mechanics, materials, waves, electricity, particle physics) requires explicit definitions and careful unit work. Paper 2 amplifies presentational mistakes — its longer questions on fields, capacitance, and oscillations punish candidates who carry sloppy notation through several lines of working. Paper 3 magnifies arithmetic and exam-technique mistakes because the synoptic mix forces candidates to switch topic strands inside a single question, and the practical-skills content rewards clear stating of variables, uncertainties and sources of error.
A useful diagnostic is to mark three of your own past papers and tag every lost mark by category. After three papers a personal pattern emerges, and one or two categories typically dominate. The remediation differs sharply: arithmetic leaks respond to slow-motion calculator drills, conceptual leaks respond to definition flashcards and worked-example replay, presentational leaks respond to a one-page checklist run on every answer, and exam-technique leaks respond to demand-verb practice and stem-underlining drills.
Mistakes cost time as well as marks. A sign error in the second line of an 8-mark question that propagates to the wrong final answer is not a 1-mark loss — it is closer to 4 marks lost (the A-marks on the answer line) plus the 60 seconds spent re-checking the working before deciding to move on. The candidate who catches the sign error in the first 30 seconds of a self-check loses zero marks; the candidate who never re-reads loses the four marks and may also lose the next question's pacing.
This is why the time spent on careful checking is one of the highest-value uses of paper time. A 90-second margin on a 100-mark paper used purely for checking — running a quick units-and-signs sweep on every multi-line calculation — typically recovers 4 to 8 marks. The marks gained per minute of checking are higher than the marks gained per minute of attempting the next question, because the next question carries the risk of compounding new errors. The strategic implication: do not surrender the final 8 to 12 minutes to "trying one more part". Use them to sweep arithmetic, units, signs, and final-answer form on the questions you have already answered.
The check itself should be patterned, not improvised. A four-pass sweep — pass 1 for units, pass 2 for signs, pass 3 for sig figs and final-answer form, pass 4 for demand-verb compliance — catches more leaks than a single open-ended re-read because each pass focuses the eye on one error category at a time. Candidates who try to "check everything at once" reliably miss the very leaks they were looking for.
The error-elimination habits below close the largest leaks identified across the four error categories.
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