Lab 12
E x B

A Deflection Tube is positioned at the center of a pair of Helmholtz Coils with the aid of a Tube Support. When current is passed through the Helmholtz Coils a magnetic field is generated perpendicular to an electric field (ExB). The electric field is created between two parallel metal plates in the Deflection Tube.

The goal, today, is to investigate mutually orthogonal electric and magnetic fields and to use ExB to measure e/m (the charge-to-mass ratio of the electron). See Figure 1.

Figure 1. E x B

Review Theory

Suppose a particle of charge, q, enters a region of space containing a uniform electric field of magnitude, E. The particle will experience a force due to the field, where:

If a magnetic field is present in the same region of space, the particle will experience a force due to the magnetic field given by:

where, B, is the magnetic field strength and, v, is the particle velocity. The particle will experience no net force if the electric field, the magnetic field, and the particle’s velocity are mutually perpendicular, provided:

If the electric field is established between parallel plates of length, L, the deflection of an electron, y, at the edge of the plates is:

Eliminating the velocity from the last two equations gives an expression for e/m (the charge-to-mass ratio of the electron):

Joseph John Thomson used this approach in 1897 to measure e/m for cathode rays. He concluded that cathode rays were composed of charged particles and were not a wave property of the aether as most physicists believed. We now know that the aether does not exist and cathode rays are beams of electrons. The word electron, was coined by
G. Johnstone Stoney in 1891, to denote the unit of charge found in electrochemical experiments.

Learn more about J. J. Thomson and the discovery of the electron at the website:
http://www.aip.org/history/electron/