Physics 9702 · AS & A Level · Force on a moving charge
Force on a moving charge — practice question
A beam of positive ions passes through a vacuum in a fine stream. The ions then move into a region where the magnetic field has uniform flux density $B$, and they are bent into a semi-circular path, as shown in Fig. 5.1. These ions, moving at speed $1.40 \times 10^5\,\text{m s}^{-1}$, are recorded by a fixed detector when the arc formed in the magnetic field has a diameter of $12.8\,\text{cm}$.
(a)[1]
Using Fig. 5.1 as a reference, state the direction of the magnetic field.
(b)[3]
The ions have mass $20\,u$ and charge $+1.6 \times 10^{-19}\,\text{C}$. Show that the magnetic flux density is $0.454\,\text{T}$. Explain your working.
(c)
There are also ions of mass $22\,u$ in the beam, and they have the same charge and speed as the ions in (b).
(c(i))[1]
On Fig. 5.1, sketch the route of these ions in the magnetic field of magnetic flux density $0.454\,\text{T}$.
(c(ii))[2]
To make sure these ions are detected at the fixed detector, the magnetic flux density must be altered. Calculate this new magnetic flux density.
Worked solution & mark scheme
This 7-mark question has a full step-by-step worked solution and mark scheme. One marking point: “magnetic field directed into the plane of the paper” …