Edexcel A-Level Physics Charged-Particle Radius

Practise Edexcel A-Level Physics charged-particle radius by connecting magnetic force, momentum, curvature and charge-to-mass ratio.

Exam points

derive or use r equals p over BQ and r equals mv over BQ for circular particle tracks
calculate magnetic flux density from particle energy, momentum and curvature radius
use track radius to compare isotopes or judge an experimental charge-to-mass ratio

[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 4.5.115 - Charged-particle radius in a magnetic field 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 .

For relativistic particles such as this positron, momentum obeys the relationship

E=pc

where E = particle energy, p = particle momentum and c = speed of light.

Determine the magnetic flux density of the magnetic field.

Radius of curvature of path = $3.7\,\mathrm{cm}$

Magnetic flux density =

Use of E=p c (ecf for E from (c)(i))

- Use of r=p / B q

- $B=2.1 \mathrm{~T}$

Do not award MP1 if p=m v calculated using v from part (i)

Example of calculation
$\begin{aligned} & 3.7 \times 10^{-12} \mathrm{~J}=p \times 3.00 \times 10^{8} \mathrm{~m} \mathrm{~s}^{-1} & p=1.2 \times 10^{-20} \mathrm{~N} \mathrm{~s} & 0.037 \mathrm{~m}=1.2 \times 10^{-20} \mathrm{~N} \mathrm{~s} / B \times 1.6 \times 10^{-19} \mathrm{C} & B=2.1 \mathrm{~T} \end{aligned}$

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4.5.115 - Charged-particle radius in a magnetic field question 1