AQA A Level Physics
AQA A Level Physics 3.12 Turning Points in Physics (Option D) Exam Questions
Free AQA A Level Physics exam questions for 3.12 Turning Points in Physics (Option D), with a mark scheme for every pack. Covers cathode rays, specific charge, Millikan's oil drop experiment, theories of light, de Broglie waves and special relativity.
3.12 Turning Points in Physics (Option D) exam questions
3 question packs of real past exam questions, each with its mark scheme. Do not stop after a few: answer as many as you can without notes, then mark every answer with the mark scheme. The more questions you do, the more quickly you will spot how examiners ask about each idea.
StemUp mark scheme flashcards
A-Level Physics Flashcards for 2027 Onwards
Every answer is written in mark scheme wording, so you learn exactly what examiners give marks for. Then test yourself with the questions on this page.
From £10.99View flashcards
About AQA A Level Physics 3.12 Turning Points in Physics (Option D)
Turning Points in Physics is optional topic 3.12 in AQA A Level Physics and is assessed in Section B of Paper 3 if your school chooses it. It focuses on experiments that changed physics. Section 3.12.1 covers the discovery of the electron: cathode rays, thermionic emission, Thomson's specific charge measurement and Millikan's oil drop experiment. Section 3.12.2 covers wave-particle duality: Newton's and Huygens' theories of light, Maxwell and Hertz, the photoelectric effect, de Broglie's hypothesis and electron microscopes. Section 3.12.3 covers special relativity: the Michelson-Morley experiment, Einstein's postulates, time dilation, length contraction and mass-energy. Examiners favour extended answers about how evidence changed scientific models.
What you need to know
- 3.12.1 Cathode rays Cathode rays are beams of electrons produced in a discharge tube or by thermionic emission from a heated filament. The speed of electrons accelerated through pd V is given by 1/2 mv^2 = eV.
- 3.12.1 Specific charge Thomson measured e/m using crossed electric and magnetic fields (eE = Bev so v = E/B) and the radius of the path in a magnetic field. The value was about 1800 times that of the hydrogen ion, showing the electron is a much smaller particle.
- 3.12.1 Millikan's oil drop For a stationary drop QV/d = mg; terminal speed with no field gives the mass using Stokes' law F = 6 pi eta r v. The charges were always whole number multiples of 1.6 x 10^-19 C, showing charge is quantised.
- 3.12.2 Theories of light Newton's corpuscular theory predicted light travels faster in glass; Huygens' wave theory explained refraction and diffraction. Young's double slit supported waves. Maxwell predicted EM waves travelling at c = 1/square root of (mu0 epsilon0), and Hertz detected radio waves.
- 3.12.2 Photoelectric effect and de Broglie The wave theory could not explain the threshold frequency; Einstein's photon model could. De Broglie proposed lambda = h/mv, confirmed by electron diffraction. TEMs and STMs use electron wave properties to see fine detail.
- 3.12.3 Special relativity Michelson-Morley found no shift in fringes, so no evidence for the ether. Einstein's postulates: physical laws are the same in all inertial frames, and the speed of light in free space is invariant. Time dilation t = t0 / square root of (1 - v^2/c^2); length contraction l = l0 square root of (1 - v^2/c^2); muon decay is evidence. E = mc^2 with relativistic mass.
Common mistakes in 3.12 Turning Points in Physics (Option D)
- Saying Michelson-Morley proved relativity. It gave a null result showing no detectable ether, which Einstein's theory later explained.
- Mixing up proper time and dilated time. The proper time t0 is measured in the frame where the events happen at the same place, and it is always the shorter time.
- Stating that Millikan measured the mass of the electron. He measured charge and showed it was quantised; combined with e/m this gave the electron mass.
- Claiming Newton's theory was rejected because of diffraction alone. The key test was the speed of light in water, which was found to be slower, supporting the wave theory.
- Using the non-relativistic kinetic energy at speeds close to c. Above about 0.1c use relativistic energy E = mc^2 with the gamma factor.
Try three 3.12 Turning Points in Physics (Option D) exam-style questions
1. An electron is accelerated from rest through a pd of 1500 V. Calculate its final speed, ignoring relativistic effects. (3 marks)
Show the mark scheme answer
1/2 mv^2 = eV; v = square root of (2 x 1.60 x 10^-19 x 1500 / 9.11 x 10^-31); v = 2.3 x 10^7 m s-1.
2. An unstable particle has a half-life of 2.0 microseconds in its rest frame and travels at 0.98c. Calculate its half-life measured in the laboratory. (2 marks)
Show the mark scheme answer
gamma = 1 / square root of (1 - 0.98^2) = 1 / 0.199 = 5.0; t = 5.0 x 2.0 = 10 microseconds.
3. Explain how Millikan's oil drop experiment showed that charge is quantised. (3 marks)
Show the mark scheme answer
The pd was adjusted until each drop was held stationary so QV/d = mg; the mass was found from the terminal speed with the field off using Stokes' law; the charges found were always whole number multiples of about 1.6 x 10^-19 C.
3.12 Turning Points in Physics (Option D) FAQs
What does AQA A Level Physics 3.12 Turning Points in Physics cover?
It covers the discovery of the electron and its specific charge, Millikan's experiment, the history of wave and particle theories of light, de Broglie waves, electron microscopes and Einstein's special relativity.
Which paper is the Turning Points option examined in?
If your school takes this option, it is assessed in Section B of Paper 3, after the practical skills section.
How should I prepare for long answers in Turning Points?
Learn each key experiment as a story: what was predicted, what was done, what was observed and what it showed. Examiners reward clear links between evidence and the model it supported or overturned.
