Topic 8: Nuclear and Particle Physics — Edexcel A-Level Physics
Test yourself on Topic 8: Nuclear and Particle Physics with PEARSON EDEXCEL A-Level practice questions.
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Topic 8: Nuclear and Particle Physics explained
Fast-moving unstable particles live longer than identical particles at rest, because time dilation stretches the interval measured in the laboratory frame.
Read the full explanation
A muon created high in the atmosphere has a proper lifetime of about 2.2 μs; at rest it would travel only a few hundred metres, yet many reach detectors at ground level because their laboratory lifetime is extended and their decay is delayed. Recognise the signature: a particle speed close to c, a measured lifetime or decay distance far longer than the rest-frame value predicts, or a beam of short-lived particles surviving over a long flight path. The effect is negligible when speed is small compared with c. You are not required to use relativistic equations, so reason qualitatively: greater speed means greater time dilation and a longer observed lifetime.
Your focus
- Identify situations where relativistic lifetime increase is significant.
- Explain qualitatively why a fast particle's laboratory lifetime exceeds its rest-frame lifetime.
- Apply the idea to examples such as atmospheric muons reaching the ground.
Topic 8: Nuclear and Particle Physics exam tips
Marking Points
- Identifies that an unstable particle's lifetime measured in the laboratory is longer than its proper (rest-frame) lifetime when it moves at speeds close to c.
- Recognises time dilation as the cause: the moving particle's clock runs slow relative to the laboratory frame.
- Applies the idea to a concrete case, such as atmospheric muons reaching the ground despite a short rest-frame lifetime.
- States that the effect becomes significant only when the speed is a substantial fraction of the speed of light, and negligible at low speeds.
- Distinguishes proper lifetime (measured in the particle's rest frame) from the dilated lifetime measured in the laboratory frame.
Examiner Tips
- 💡Look for the cue of a speed close to c combined with a survival distance or lifetime far exceeding the rest-frame prediction.
- 💡Reason qualitatively about direction: faster motion gives a longer observed lifetime, never a shorter one.
- 💡Link the concept to a named example such as atmospheric muons to justify your choice.
- 💡Reject options that claim relativistic equations are needed or that the lifetime is unchanged.
Common Mistakes
- Thinking the particle's lifetime physically shortens at high speed: correct this by stating the laboratory-measured lifetime is longer, not shorter.
- Confusing time dilation with length contraction as the explanation for survival: correct by attributing the extended lifetime to time dilation of the moving clock.
- Assuming the effect matters at everyday speeds: correct by noting it is significant only when speed approaches c.
- Treating the rest-frame lifetime as the one observed in the lab: correct by identifying the proper lifetime as that in the particle's own frame.