Physics 9702 · AS & A Level · Force on a moving charge
Force on a moving charge — practice question
Electrons travel in a vacuum as a narrow beam. They have speed $v$ and then enter a region where the magnetic field is uniform, with flux density $B$. At first, the electrons move at right angles to the magnetic field. The trajectory of one electron is shown in Fig. 7.1. In the magnetic field, the electrons take a curved route. A uniform electric field of field strength $E$ is then applied in the same region as the magnetic field. The electrons pass through the region without any deflection. Gravitational effects can be ignored.
(a)[3]
Derive a relationship between $v$, $E$ and $B$ so that the electrons are not deflected. Show your working.
(b)[2]
An $\alpha$-particle has speed $v$ and enters the region containing the two fields along the same path as the electron. Describe and explain the route taken by the $\alpha$-particle as it moves through the region of the two fields.
Worked solution & mark scheme
This 5-mark question has a full step-by-step worked solution and mark scheme. One marking point: “the forces caused by the $E$ and $B$ fields are equal in size and opposite in direction” …