Edexcel A-Level Physics Particle Energy and Mass Units

Practise Edexcel A-Level Physics particle energy and mass units by converting between SI, MeV, GeV and mass-energy notation.

Exam points

convert particle masses between kilograms and MeV or GeV over c squared using E equals mc squared
convert particle kinetic or rest energies between electronvolts, joules, MeV and GeV
judge relativistic claims by calculating classical speed from converted energy and mass data

[Maximum number: 3]

In 1932, Carl Anderson published this photograph of a track in a cloud chamber. The cloud chamber contained a lead plate. There was a magnetic field perpendicular to the plane of the track.

Figure for Question 4.5.119 - Particle energy and mass units question 1 — Edexcel A-Level Physics A2

The photograph shows the track of a positron from cosmic rays and is the first photographic record of the existence of an antiparticle.

In the upper part of the photograph the positron had an energy of 23 MeV .

Show that the positron must have been travelling at a relativistic speed. Assume that all of its energy is kinetic energy.

Use of conversion factor $1.6 \times 10^{-19} \mathrm{C}$

- Use of $E_{\mathrm{k}}=1 / 2 m v^{2}$

- Calculated speed $=2.8 \times 10^{9}\left(\mathrm{~m} \mathrm{~s}^{-1}\right)$ , which is greater than the speed of light (so it must be relativistic)

Example of calculation
$\begin{aligned} & E_{\mathrm{k}}=23 \times 10^{6} \mathrm{eV} \times 1.6 \times 10^{-19} \mathrm{C}=3.7 \times 10^{-12} \mathrm{~J} & 3.7 \times 10^{-12} \mathrm{~J}=0.5 \times 9.11 \times 10^{-31} \mathrm{~kg} \times v^{2} & v=2.8 \times 10^{9} \mathrm{~m} \mathrm{~s}^{-1} \end{aligned}$

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4.5.119 - Particle energy and mass units question 1