Question

A mass spectrometer is designed to calculate masses of atoms. A source produces positive ions of
mass m and charge q. The ions move with a velocity v to the right, pass through a narrow slit S1 and
enter a region between two charged plates. In this region they experience both an electric and a
magnetic field. The electric field E is caused by the 2 charged plates; the upper plate is (-) charged
and the lower plate is (+) charged. The magnetic field is B1 and points out of the page. The ions
exit this region through slit S2 . When they exit, they experience a new magnetic field B2, that is also
directed out of the page. The ions are then collected inside a collector.
a) Draw the electric field between the charged plates.
b) With what velocity should ions enter in order to follow a straight trajectory from S1 to S2?
c) Draw the trajectory followed by the ions in the region of magnetic field B2? Why does the
trajectory look like this?
d) Where should the collector be placed in the plane containing S2 to collect the ions?
e) What is the mass of the ion in terms of field strengths, charge, and S2-collector distance?

Answer 1