State what the term field of force means.
Explain how a uniform magnetic field and a uniform electric field can be used to select the velocity of charged particles. A diagram may be included if you wish.
A beam of charged particles enters a region containing uniform magnetic and electric fields, as shown in Fig. 9.1. The magnetic field points into the plane of the paper. As the particles enter the field, their velocity is perpendicular to the magnetic field. A particle in the beam has mass $m$, charge $+q$ and speed $v$. It passes undeviated through the region where the two fields overlap. On Fig. 9.1, sketch the path of a particle with (i) mass $m$, charge $+2q$ and speed $v$ (label this path Q), (ii) mass $m$, charge $+q$ and speed slightly greater than $v$ (label this path V).