Turning Points in Physics — A-Level Physics Revision

Topic has curated content entry with explanation, mistakes, and worked example. [auto-gate:promote; score=75.25]

What is Turning Points in Physics?

This optional topic explores the key experiments and conceptual shifts that revolutionised physics. It often starts with J.J. Thomson's determination of the specific charge of the electron, which established the existence of subatomic particles. It then moves to Einstein's theory of special relativity, exploring its two fundamental postulates and their consequences, such as time dilation and length contraction, and the equivalence of mass and energy (E=mc²).

Board notes: 'Turning Points in Physics' is a historical and conceptual optional topic in the AQA specification. It provides a deeper dive into the experimental and theoretical foundations of modern physics, linking many different areas of the course.

Worked example

Common mistakes

• 1Confusing the setup for determining the electron's velocity with that for its specific charge. A velocity selector uses crossed electric and magnetic fields to allow particles of a specific speed to pass through undeflected, whereas measuring the deflection in just one of the fields allows the specific charge (e/m) to be determined.
• 2Misunderstanding the postulates of special relativity. The first postulate (laws of physics are the same in all inertial frames) and the second (the speed of light in a vacuum is constant for all inertial observers) are the foundation for all the counter-intuitive consequences.
• 3Thinking that relativistic effects like time dilation are just an illusion. Time dilation is a real, measurable effect; time genuinely passes at a slower rate for a moving clock compared to a stationary one.

Turning Points in Physics exam questions

Exam-style questions for Turning Points in Physics with mark-scheme style solutions and timing practice. Aligned to AQA, Edexcel and OCR specifications.

Frequently asked questions

• What was the significance of Thomson's experiment?

  Thomson's experiment to measure the specific charge (e/m) of cathode rays was significant because it showed that these 'rays' were in fact particles with a consistent and very large charge-to-mass ratio, much larger than any known ion. This was the first discovery of a subatomic particle, the electron.

• What is time dilation?

  Time dilation is a consequence of special relativity which states that a clock that is moving relative to an observer will be measured to tick slower than a clock that is at rest. The effect only becomes significant at speeds approaching the speed of light.