Explain what a magnetic field means.
A particle of mass $m$, charge $+q$ and speed $v$ enters a uniform magnetic field of flux density $B$ and, at entry, it is moving normal to the magnetic field, as shown in Fig. 8.1. The magnetic field is directed perpendicular to the plane of the paper and into it. On Fig. 8.1, sketch the particle’s path through the magnetic field region and after it has left the field.
A force acts on the particle, so it accelerates. Explain why the particle’s speed on leaving the magnetic field is $v$.
In (b), the particle loses an electron so that its charge becomes $+2q$. Its change in mass is negligible. Determine, in terms of $v$, the particle’s initial speed so that its path through the magnetic field is unchanged. Explain your working.