Which relationship governs the motion of a charged particle crossing the magnetic field of a detector?

The force on the particle is F = qvB when it moves at right angles to the field, and this acts as the centripetal force on a circular path.

What does the direction of curvature of a track tell you about the particle?

It gives the sign of the charge, because positive and negative particles moving the same way through the same field are pushed in opposite directions.

Why does a neutral particle leave no track in a detector of this kind?

Detection relies on the particle ionising the material it passes through, and an uncharged particle exerts no electric force to strip electrons from atoms.

OCR A-Level Physics B H557

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Probing Deep Into Matter

Charged particles in detectors and magnetic fields

3 things to know
Components 01, 02 and 03

Overview

Recall questions on why tracks curve and what the curvature direction reveals, momentum from a track radius, tracks that spiral or appear from nowhere, invisible neutral particles, and charge-to-mass ratios.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

Why do the tracks of charged particles in a detector curve?

Key term

What quantity is obtained from the radius of a curved track in a known magnetic field?

Numbers

A singly charged particle leaves a track of radius 0.25 m in a field of flux density 1.5 T. Calculate its momentum.

Questions from this subtopic

Each one with the model answer underneath.

1
Which relationship governs the motion of a charged particle crossing the magnetic field of a detector?
Model answer
The force on the particle is F = qvB when it moves at right angles to the field, and this acts as the centripetal force on a circular path.
2
Why do the tracks of charged particles in a detector curve?
Model answer
A magnetic field is applied across the detector, and the force qvB on each moving charge is always perpendicular to its velocity, so it is deflected onto a curved path.
3
What does the direction of curvature of a track tell you about the particle?
Model answer
It gives the sign of the charge, because positive and negative particles moving the same way through the same field are pushed in opposite directions.
4
What quantity is obtained from the radius of a curved track in a known magnetic field?
Model answer
The momentum of the particle is obtained, since r = mv/(qB) rearranges to p = qBr.
5
A singly charged particle leaves a track of radius 0.25 m in a field of flux density 1.5 T. Calculate its momentum.
Model answer
p = qBr = 1.60 × 10⁻¹⁹ × 1.5 × 0.25 = 6.0 × 10⁻²⁰ N s.
6
Why does a neutral particle leave no track in a detector of this kind?
Model answer
Detection relies on the particle ionising the material it passes through, and an uncharged particle exerts no electric force to strip electrons from atoms.

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