§-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: 60
Time: 120 min
Authority: QCAA
Updated: 2026-05-29

Open the official PDF ->Official marking notes

What this paper assessed

The QCAA Physics external assessment (Paper, 60 marks in 120 minutes) 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

60 marks in 120 minutes is a comfortable 2 minutes per mark.

Multiple choice + short response. Aim for ~2 minutes per mark; even calculation MCQs need scratch working.
Graph and data-handling items. Mark gradient/intercept directly on the figure before interpreting.
Extended/complex response. A 6-mark item deserves ~12 minutes, including a short plan and a comment on assumptions or limitations.

A workable plan: work steadily at 2 min/mark and keep a 10-minute buffer 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 $7.0 \times 10^6\ \text{m}$ . Calculate its orbital speed. ( $G = 6.67 \times 10^{-11}\ \text{N m}^2\,\text{kg}^{-2}$ , $M_\text{Earth} = 5.97 \times 10^{24}\ \text{kg}$ .)

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

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

Substitute:

v = \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 = \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 $1.2 \times 10^{15}\ \text{Hz}$ strikes a metal of work function $3.0\ \text{eV}$ . (a) Calculate the maximum kinetic energy of the emitted photoelectrons in joules. (b) Explain why no electrons are emitted below a threshold frequency. ( $h = 6.63 \times 10^{-34}\ \text{J s}$ , $1\ \text{eV} = 1.6 \times 10^{-19}\ \text{J}$ .)

(a) Photon energy:

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

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

By the photoelectric equation $E_{k,\text{max}} = hf - W$ :

E_{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 $hf$ . An electron is emitted only if a single photon supplies at least the work function $W$ . Below the threshold frequency ( $f_0 = W/h$ ), 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 $0.98c$ and has a proper lifetime of $2.2\ \mu\text{s}$ . (a) Calculate its lifetime as measured from Earth. (b) Explain how this supports special relativity.

(a) Use time dilation, with proper time $t_0 = 2.2\ \mu\text{s}$ (measured in the muon's rest frame):

t = \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 = \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 $2.2\ \mu\text{s}$ 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 $t_0$ , 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 in a single timed 120-minute block, keeping a 10-minute end buffer to check units, vector directions and significant figures. Mark each calculation against the official QCAA marking guide at the authority page linked in the frontmatter, 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.

Use this paper well

Sit the paper under exam conditions (120 minutes, 60 marks).
Mark yourself against the official QCAA marking notes.
Compare against the Physics hub to find the syllabus dot points this paper tested.

What this paper assessed

Structure and timing

Worked practice questions (exam-style)

Common errors students made

How to use this paper

Use this paper well

Keep going