Physics 9702 · AS & A Level · Force on a moving charge
Force on a moving charge — practice question
An electron moves in a vacuum in a straight line at speed $v$. It then enters a uniform magnetic field with flux density $8.0 \times 10^{-4}\,\text{T}$. At the instant it enters, the electron travels at right angles to the magnetic field, as shown in Fig. 9.1. Inside the magnetic field, the electron follows a circular arc of radius $6.4\,\text{cm}$.
(a(i))[2]
State the direction of the magnetic field, and explain your answer.
(a(ii))[3]
Show that the electron speed $v$ is $9.0 \times 10^{6}\,\text{m s}^{-1}$.
(b(i))[1]
On Fig. 9.2, indicate the direction of the uniform electric field with an arrow.
(b(ii))[2]
Use the data in (a) to calculate the magnitude of the electric field strength.
(c)[2]
In part (b), the electron is replaced by an $\alpha$-particle moving at the same speed $v$ along the same initial path as the electron. Describe the path it takes in the region of the magnetic and electric fields, and explain your answer.
Worked solution & mark scheme
This 10-mark question has a full step-by-step worked solution and mark scheme. One marking point: “The force acts downwards” …