Physics 9702 · AS & A Level · Force on a moving charge
Force on a moving charge — practice question
A particle carrying charge $+q$ and having mass $m$ moves through a vacuum at a steady speed of $1.6 \times 10^5\,\text{m s}^{-1}$. It enters a uniform magnetic field of flux density $9.7 \times 10^{-2}\,\text{T}$, as shown in Fig. 9.1. The magnetic field is perpendicular to the particle’s initial velocity and is also perpendicular to, and out of, the plane of the paper. A uniform electric field is applied in the same region as the magnetic field so that the particle travels through the fields without deviation.
(a)[2]
State and explain the direction of the electric field.
(b)[3]
Calculate the magnitude of the electric field strength. Explain your working.
(c)[3]
The electric field is now removed so that the positively-charged particle follows a curved path in the magnetic field. This path is an arc of a circle of radius $4.0\,\text{cm}$. Calculate, for the particle, the ratio $\frac{q}{m}$.
(d(i))[2]
The particle has a charge of $3e$ where $e$ is the elementary charge. Use your answer in (c) to determine the mass, in $u$, of the particle.
(d(ii))[1]
The particle is the nucleus of an atom. State how many protons and how many neutrons this nucleus contains.
Worked solution & mark scheme
This 11-mark question has a full step-by-step worked solution and mark scheme. One marking point: “Electric force acts opposite to magnetic force” …