204 Week 6: Moving Charges and Magnetic Force
charges are themselvesparticles, and if so, what the charges and masses of those particles are. There
are a variety of models for atoms, most of them “static” models that have negative and positive
charge glued together in some way that keeps the negative and positive charge from having toorbit
one another the way the electrostatic force suggests that theyshould, as James Clerk Maxwell has
shown that classical atoms made up of orbiting charged particles would radiate all of their energy
away in a very, very short time and collapse. One of the favorite models is in fact called the “plum
pudding model” portrayed in figure 67 (no kidding!) where negative charge is scattered like raisins
in a gooey pudding of positive charge.
The so-called “cathode ray tube” (or Crooke’s tube) has been invented for twenty or thirty years,
and a mere two years earlier a gentleman named R ̈ontgen discovered that cathode rays hitting the
glass of the screen at high enough energies producex-rays, capable of penetrating the human hand
and forming images of the bones within (see figure 68 above) for which he received thefirstNobel
Prize in physics in 1901.
Figure 68: The first “medical x-ray” ever taken, of the bones in Anna Berthe R ̈ontgen’s hand. She
was the wife of Wilhelm R ̈ontgen, the discoverer of x-rays.
The question is: Just whatarecathode rays? Are they particles? Do they have arbitrary
amounts of charge and mass? Are they a fixed fraction of the massof e.g. a hydrogen atom? Is
the mass of a hydrogen atom split evenly between cathode (negatively charged) material and anode
(positively charged) material? J. J. Thomson set out to try to answer these questions by using
a specially modified Crooke’s tube to deflect cathode raysin flightonce they were produced at a
heated electrical filament and accelerated by an applied potential difference so that they formed a
beam.
Initially the deflection was accomplished only by the application of an electric field in between
special plates built right into the tube (which was sufficient, as we shall see, to measure the ratio of
e/mfor cathode ray particles orelectrons(as they turned out to be) and thereby show that they were
atiny fractionof the total mass of a hydrogen atom, so that nearly all of the mass was associated
with thepositivecharge only. Later Thomson added a uniform magnetic field to his apparatus by
means of a pair of “Helmholtz coils”. As we have seen above, magneticfields canalsodeflect moving
charged particles, and indeed if a region of crossed fields is created, theE~andB~fields together can
be used to measure the actual velocity of the particles, which permits their kinetic energy and/or
mass to be estimated and the consistency of all of the (many, not too accurate yet) measurements
to be checked.