Question

A magnetic field set up using Helmholtz coils (described in Exercise 4.16) is uniform in a small region and has a magnitude of 0.75 T. In the same region, a uniform electrostatic field is maintained in a direction normal to the common axis of the coils. A narrow beam of (single species) charged particles all accelerated through 15 kV enters this region in a direction perpendicular to both the axis of the coils and the electrostatic field. If the beam remains undeflected when the electrostatic field is 9.0 × 10⁻⁵ V m⁻¹, make a simple guess as to what the beam contains. Why is the answer not unique?

Answer 1

Magnetic field, B = 0.75 T
Accelerating voltage, V = 15 kV = 15 × 10³ V
Electrostatic field, E = 9 × 10⁵ V m⁻¹
Mass of the electron = m
Charge of the electron = e
Velocity of the electron = v
Kinetic energy of the electron = eV

This value of specific charge e/m is equal to the value of deuteron or deuterium ions. 

This is not a unique answer. Other possible answers are He⁺⁺, Li⁺⁺, etc.