The Figure Shows a Velocity Selector That Can Be Used $+ q$

Multiple Choice

The figure shows a velocity selector that can be used to measure the speed of a charged particle. A beam of particles of charge $+ q$ is directed along the axis of the instrument. A parallel plate capacitor sets up an electric field $E$ which is oriented perpendicular to a uniform magnetic field $B$ . If the plates are separated by $3.0 \mathrm {~mm}$ and the value of the magnetic field is $0.30 \mathrm {~T}$ , what potential difference between the plates will allow particles of speed $v = 5.0 \times 10 ^ { 5 } \mathrm {~m} / \mathrm { s }$ to pass straight through without deflection?

A) $2800 \mathrm {~V}$
B) $70 \mathrm {~V}$
C) $450 \mathrm {~V}$
D) $1400 \mathrm {~V}$
E) $140 \mathrm {~V}$