Skip to main content
ExamExplained
Past papers
§-Past paper
QLDPhysics2021

QCE Physics 2021

Walkthrough of the 2021 QCE physics exam: what it assessed, strategy tips, and the common errors flagged in the official marker report.

Marks
97
Time
180 min
Authority
QCAA
Updated
Open the official PDF Official marking notes
Key fact

This paper was set on the 2019 QCAA syllabus. From the 2026 external assessments the 2025 syllabus applies, so some content and the exam format have changed. It is still useful practice, but check the current QCAA Physics syllabus and QCAA's 2026 key changes before relying on it. The mark and timing figures at the top of this page are a guide based on the 2024 papers (two papers, each with 90 minutes of working time), not QCAA's official structure for every year. Marks and question types vary, so use QCAA's own paper.

What this paper assessed

The QCAA Physics external assessment examines Unit 4: Revolutions in modern physics, drawing on Unit 3 (gravity and electromagnetism). The 2021 paper covered:

  • Gravity and motion (Unit 3): projectile motion, gravitational fields and orbital motion.
  • Electromagnetism (Unit 3): electric and magnetic fields, the motor effect, and electromagnetic induction (Faraday's and Lenz's laws).
  • Special relativity: the postulates, time dilation, length contraction and mass-energy equivalence.
  • Quantum theory and the standard model: the photoelectric effect, atomic spectra and the structure of matter.

Many items required extracting data from a graph, applying a relationship, then commenting on the limitations of the model. Extended responses rewarded structured reasoning over volume of writing, with the formula, substitution and unit-bearing answer shown clearly.

Structure and timing

QCAA sets the external assessment as more than one paper, and the number of marks, the sections and the question types differ from year to year. Open the official paper for the exact structure of this year; the marks and time shown at the top of this page are a guide only.

  • Pace by marks. Share your working time in proportion to the marks on offer, so a 6-mark item gets about three times the time of a 2-mark item. Flag any multiple choice item that runs long and return to it.
  • Use perusal time. Scan for the data-heavy items and the longest calculations or extended responses so you can plan them early.
  • Subject habits. Set out every calculation on three lines: the relationship, the substituted values with units, and the final answer with units and appropriate significant figures. For graph-based items, mark the gradient and intercept on the page before stating their physical meaning, and keep a few minutes at the end to check units, vector directions and significant figures.

Worked practice questions (exam-style)

Worked example

Question 1 (4 marks): A satellite orbits Earth in a circular orbit of radius 7.0×106 m7.0 \times 10^6\ \text{m}. Calculate its orbital speed. (G=6.67×10−11 N m2 kg−2G = 6.67 \times 10^{-11}\ \text{N m}^2\,\text{kg}^{-2}, MEarth=5.97×1024 kgM_\text{Earth} = 5.97 \times 10^{24}\ \text{kg}.)

For a circular orbit the gravitational force provides the centripetal force, giving the orbital speed:

v=GMrv = \sqrt{\frac{GM}{r}}

Substitute:

v=(6.67×10−11)(5.97×1024)7.0×106=3.982×10147.0×106v = \sqrt{\frac{(6.67 \times 10^{-11})(5.97 \times 10^{24})}{7.0 \times 10^6}} = \sqrt{\frac{3.982 \times 10^{14}}{7.0 \times 10^6}}

v=5.69×107=7.5×103 m s−1v = \sqrt{5.69 \times 10^{7}} = 7.5 \times 10^{3}\ \text{m s}^{-1}

The orbital speed is about 7.5 km s⁻¹.

Worked example

Question 2 (5 marks): Light of frequency 1.2×1015 Hz1.2 \times 10^{15}\ \text{Hz} strikes a metal of work function 3.0 eV3.0\ \text{eV}. (a) Calculate the maximum kinetic energy of the emitted photoelectrons in joules. (b) Explain why no electrons are emitted below a threshold frequency. (h=6.63×10−34 J sh = 6.63 \times 10^{-34}\ \text{J s}, 1 eV=1.6×10−19 J1\ \text{eV} = 1.6 \times 10^{-19}\ \text{J}.)

(a) Photon energy:

E=hf=(6.63×10−34)(1.2×1015)=7.96×10−19 JE = hf = (6.63 \times 10^{-34})(1.2 \times 10^{15}) = 7.96 \times 10^{-19}\ \text{J}

Work function in joules: W=3.0×1.6×10−19=4.8×10−19 JW = 3.0 \times 1.6 \times 10^{-19} = 4.8 \times 10^{-19}\ \text{J}.

By the photoelectric equation Ek,max=hf−WE_{k,\text{max}} = hf - W:

Ek,max=7.96×10−19−4.8×10−19=3.16×10−19 JE_{k,\text{max}} = 7.96 \times 10^{-19} - 4.8 \times 10^{-19} = 3.16 \times 10^{-19}\ \text{J}

(b) Light delivers energy in discrete photons of energy hfhf. An electron is emitted only if a single photon supplies at least the work function WW. Below the threshold frequency (f0=W/hf_0 = W/h), each photon has insufficient energy, so no electron escapes regardless of the light's intensity - confirming the particle model of light over the wave model.

Worked example

Question 3 (6 marks): A muon created in the upper atmosphere travels at 0.98c0.98c and has a proper lifetime of 2.2 μs2.2\ \mu\text{s}. (a) Calculate its lifetime as measured from Earth. (b) Explain how this supports special relativity.

(a) Use time dilation, with proper time t0=2.2 μst_0 = 2.2\ \mu\text{s} (measured in the muon's rest frame):

t=t01−v2c2=2.21−0.982=2.21−0.9604=2.20.0396t = \frac{t_0}{\sqrt{1 - \dfrac{v^2}{c^2}}} = \frac{2.2}{\sqrt{1 - 0.98^2}} = \frac{2.2}{\sqrt{1 - 0.9604}} = \frac{2.2}{\sqrt{0.0396}}

t=2.20.199=11.1 μst = \frac{2.2}{0.199} = 11.1\ \mu\text{s}

The Earth-frame lifetime is about 11 μs, roughly five times longer.

(b) Classically, a muon living only 2.2 μs2.2\ \mu\text{s} could not travel far enough to reach the ground before decaying. The observed flux of muons at sea level is explained by time dilation: from Earth's frame the fast-moving muon's "clock" runs slow, extending its lifetime so it survives the journey. (Equivalently, from the muon's frame the atmosphere is length-contracted.) This experimental observation is direct evidence for special relativity.

Common errors students made

The 2021 QCAA marker report noted missing units, mixing scalar and vector quantities, treating projectile motion as one-dimensional, and confusing induced EMF with flux rather than rate of change of flux. Adding to those:

  • Wrong frame in relativity - applying time dilation with the dilated time as t0t_0, or mixing up which observer measures the proper quantity.
  • Photoelectric slips - forgetting to convert eV to joules, or treating intensity as if it raised electron energy rather than electron count.
  • Energy-unit confusion - mixing eV and J within one calculation.
  • No comment on limitations/assumptions in extended items, where the marking scheme expected it.

How to use this paper

Work the full paper under timed exam conditions, keeping a short buffer at the end to check units, vector directions and significant figures. Mark each calculation against the official QCAA marking guide at the authority page linked at the top of this page, noting where method marks sit - stating the correct relationship earns marks even when arithmetic slips. Re-attempt any item that lost more than half its marks on a fresh page, writing the formula and substitution on separate lines before solving. Keep an error log of recurring traps (frame choice in relativity, eV↔J conversion, flux vs rate of change) and re-test those item types after a week.

Keep going

ExamExplained