State the meaning of a magnetic field.
Explain why the path taken by the particle in the field is the arc of a circle.
Show that the radius $r$ is given by the expression $r = \frac{mv}{Bq}$.
A particle of mass $m$ and charge $+q$ moves with velocity $v$ in a vacuum. It enters a region of uniform magnetic field with flux density $B$, as shown in Fig. 5.1. The magnetic field is normal to the particle's direction of motion. The particle's path in the field is the arc of a circle of radius $r$.
A thin metal foil is positioned in the magnetic field in (b). A second charged particle enters the magnetic-field region. As it passes through the foil, it loses kinetic energy. The particle follows the path shown in Fig. 5.2.
On Fig. 5.2, draw an arrow to show the particle's direction of travel.
The particle's path has different radii on opposite sides of the foil. The radii are $7.4\,\text{cm}$ and $5.7\,\text{cm}$. Determine the ratio $\frac{\text{final momentum of particle}}{\text{initial momentum of particle}}$ for the particle as it goes through the foil.