Edexcel A-Level Physics Mass-Energy Equivalence

Practise Edexcel A-Level Physics mass-energy equivalence by converting mass changes into energy for particles, photons and the Sun.

Exam points

use E equals mc squared to calculate mass loss, particle mass or energy released
analyse pair production and annihilation by converting between rest mass and photon energy
combine mass-energy with hf and wavelength equations for gamma photon calculations

[Maximum number: 4]

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 4.5.118 - Mass-energy equivalence 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.

A positron travelling at a non-relativistic speed of $1.5 \times 10^{7} \mathrm{~m} \mathrm{~s}^{-1}$ collides with an electron travelling at the same speed in the opposite direction. This collision results in the production of gamma radiation.

Calculate the frequency of the gamma radiation produced.

Frequency =

Use $E_k=\frac12mv^2$ , $\Delta E=c^2\Delta m$ , and E=hf.

For the electron and positron, total kinetic energy is $2.0\times10^{-16}\,\mathrm{J}$ , and rest energy is $1.64\times10^{-13}\,\mathrm{J}$ . Each of the two gamma photons has energy $8.2\times10^{-14}\,\mathrm{J}$ .

$8.2\times10^{-14}=6.63\times10^{-34}f$

Therefore $f=1.2\times10^{20}\,\mathrm{Hz}$ .

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4.5.118 - Mass-energy equivalence question 1