Simple Nature - Light and Matter

the object with a net positive charge, which attracts the lost sheephome to the fold. (For objects immersed in air rather than vacuum,there will also be a balanced exchange of electrons between theair and the object.)This interpretation explains the warm and friendly yellow glow ofthe vacuum tubes in an antique radio. To encourage the emissionof electrons from the vacuum tubes’ cathodes, the cathodes areintentionally warmed up with little heater coils.Discussion QuestionsA Today many people would define an ion as an atom (or molecule)with missing electrons or extra electrons added on. How would peoplehave defined the word “ion” before the discovery of the electron?B Since electrically neutral atoms were known to exist, there had to bepositively charged subatomic stuff to cancel out the negatively chargedelectrons in an atom. Based on the state of knowledge immediately afterthe Millikan and Thomson experiments, was it possible that the positivelycharged stuff had an unquantized amount of charge? Could it be quantizedin units of +e? In units of +2e? In units of +5/7e?This chapter is summarized on page 955. Notation and terminologyare tabulated on pages 941-942.8.2 The Nucleus8.2.1 RadioactivityBecquerel’s discovery of radioactivityHow did physicists figure out that the raisin cookie model wasincorrect, and that the atom’s positive charge was concentrated ina tiny, central nucleus? The story begins with the discovery of radioactivityby the French chemist Becquerel. Up until radioactivitywas discovered, all the processes of nature were thought to be basedon chemical reactions, which were rearrangements of combinationsof atoms. Atoms exert forces on each other when they are close together,so sticking or unsticking them would either release or storeelectrical energy. That energy could be converted to and from otherforms, as when a plant uses the energy in sunlight to make sugarsand carbohydrates, or when a child eats sugar, releasing the energyin the form of kinetic energy.Becquerel discovered a process that seemed to release energyfrom an unknown new source that was not chemical. Becquerel,whose father and grandfather had also been physicists, spent thefirst twenty years of his professional life as a successful civil engineer,teaching physics on a part-time basis. He was awarded thechair of physics at the Musée d’Histoire Naturelle in Paris after thedeath of his father, who had previously occupied it. Having now asignificant amount of time to devote to physics, he began studyingthe interaction of light and matter. He became interested in the phe-474 Chapter 8 Atoms and Electromagnetism