the object with a net positive charge, which attracts the lost sheep
home to the fold. (For objects immersed in air rather than vacuum,
there will also be a balanced exchange of electrons between the
air and the object.)
This interpretation explains the warm and friendly yellow glow of
the vacuum tubes in an antique radio. To encourage the emission
of electrons from the vacuum tubes’ cathodes, the cathodes are
intentionally warmed up with little heater coils.
Discussion Questions
A Today many people would define an ion as an atom (or molecule)
with missing electrons or extra electrons added on. How would people
have defined the word “ion” before the discovery of the electron?
B Since electrically neutral atoms were known to exist, there had to be
positively charged subatomic stuff to cancel out the negatively charged
electrons in an atom. Based on the state of knowledge immediately after
the Millikan and Thomson experiments, was it possible that the positively
charged stuff had an unquantized amount of charge? Could it be quan-
tized in units of +e? In units of +2e? In units of +5/7e?
This chapter is summarized on page 1080. Notation and terminology
are tabulated on pages 1066-1067.8.2 The nucleus
8.2.1 Radioactivity
Becquerel’s discovery of radioactivity
How did physicists figure out that the raisin cookie model was
incorrect, and that the atom’s positive charge was concentrated in
a tiny, central nucleus? The story begins with the discovery of ra-
dioactivity by the French chemist Becquerel. Up until radioactivity
was discovered, all the processes of nature were thought to be based
on chemical reactions, which were rearrangements of combinations
of atoms. Atoms exert forces on each other when they are close to-
gether, so sticking or unsticking them would either release or store
electrical energy. That energy could be converted to and from other
forms, as when a plant uses the energy in sunlight to make sugars
and carbohydrates, or when a child eats sugar, releasing the energy
in the form of kinetic energy.
Becquerel discovered a process that seemed to release energy
from an unknown new source that was not chemical. Becquerel,
whose father and grandfather had also been physicists, spent the
first twenty years of his professional life as a successful civil engi-
neer, teaching physics on a part-time basis. He was awarded the
chair of physics at the Mus ́ee d’Histoire Naturelle in Paris after the
death of his father, who had previously occupied it. Having now a
significant amount of time to devote to physics, he began studying
the interaction of light and matter. He became interested in the phe-494 Chapter 8 Atoms and Electromagnetism