James N. Lloyd

What’s Going On Here?

From earliest times humans have speculated about the nature of matter. The Greeks with their characteristic genius developed a highly systematic set of ideas about matter. They called these ideas “physics,” but physics in the modern sense of the word comes into being only in the seventeenth century.

The Heisenberg Uncertainty Principle

The photoelectric effect showed that waves behave like particles. A wave with a frequency f has a minimum packet, or quantum, of energy E = hf, where h is Planck’s constant. Compton showed that when hf is comparable to the rest mass energy mc2 of an electron, the scattering of electromagnetic radiation from electrons behaves like the scattering of one compact object from another. The particle-like behavior of light seems so prominent in these cases that the quantum of light has been given the particle-like name of “photon.” Individual photons can be detected with a photomultiplier tube; such detection also suggests a degree of localization in space that is characteristic of particles rather than waves.

Radioactivity and the Atomic Nucleus

In 1896 Henri Becquerel discovered that compounds containing uranium emit radiations that can penetrate opaque paper and even thin sheets of metal and cause photographic plates to darken. Like x-rays, these emissions ionized air and caused electroscopes to discharge, but unlike x-rays, they occurred without any external source of excitation. Becquerel’s student, Marie Curie, named this spontaneous emission of ionizing radiation “radioactivity.”

Spectra and the Bohr Atom

We come now to a new aspect of atoms: the existence of discrete energy states. Niels Bohr’s idea that atoms can possess only certain well–defined amounts of energy was a major development in our understanding of atoms. In 1911 Bohr, a young Dane who had just received his Ph.D. in physics from the University in Copenhagen, came to England to visit for a year. He worked for a while in J.J. Thomson’s laboratory in Cambridge, and then in early 1912 Bohr transferred to Manchester to work with Rutherford. Inspired by Rutherford’s concept of the atomic nucleus, Bohr subsequently developed a nuclear model of the hydrogen atom that predicted the wavelengths emitted in the spectrum of atomic hydrogen. The agreement of his predictions with observations was startlingly good.

Electric Fields and Electric Forces

This chapter introduces you to the electric field — an important and useful way to describe electric forces.

Atoms, Photons, and Quantum Mechanics

Quantum mechanics was the outcome of physicists’ twenty-five year struggle to understand the behavior of matter and light at the atomic level. This struggle began in 1900 when Max Planck explained the spectrum of light from a hot body by an ad hoc assumption that atoms absorb and emit light in bundles of energy. In 1905 Einstein argued convincingly that light is itself quantized in bundles of energy and used the idea to explain the photoelectric effect (Chap. 13). Rutherford and Moseley showed (Chaps. 16 and 17) that the atom is made of discrete elements, and Bohr showed (Chap. 17) that atoms take on definite, or as we say today, quantized states of energy.

Magnetic Field and Magnetic Force

In this chapter you meet another field of force, the magnetic field. It is quite different from the electric field. Electric fields produce forces on electrical charges whether they are moving or sitting still. The magnetic field exerts a force on an electric charge only if the charge is moving. Equally strange, the strength of the exerted force depends upon the direction of the charge’s motion. Whenever you see such peculiar behavior, you know there is a magnetic field present.

Energy and Momentum at High Speeds

Einstein’s special theory of relativity modifies the Newtonian concepts of energy and momentum so that they correctly describe bodies moving at high speeds. The modifications lead to the best-known prediction of the theory of relativity: Energy has mass and vice versa, E = mc2, and they also show that the relationship between kinetic energy and momentum that you have frequently used \(K\, = \,\frac{{{p^2}}}{{2m}}\, = \,\frac{1}{2}m{v^2}\), is only an approximation of the equations that are exact at all speeds. You now need to become familiar with the relativistically correct relationships and how they are used to extract information about atoms and the particles they are made of.

Electric Charges and Electric Forces

By imagining a gas to be a collection of tiny spheres, physicists and chemists were able to explain many features of the behavior of gases and estimate the number and size of molecules. Their results gave further credibility to the idea that atoms exist, but their numbers were imprecise, yielding estimates of Avogadro’s number and atomic sizes accurate only to within an order of magnitude, i.e., only to within a factor of ten.

Entanglement and Non-Locality

Before you finish this book you need to learn about experiments that support the belief that quantum indeterminacy is a fundamental feature of atoms, indeed, of the entire physical world. To understand these experiments you need to know about a remarkable and important feature of quantum superposition called “entanglement.” As you will see, entanglement not only helps to establish that indeterminacy is a basic feature of reality, it also reveals surprising, non-local connections between quantum systems far apart from each other. It shows that quantum mechanics is a non-local theory. Non-locality is as strange as fundamental indeterminacy. This chapter discusses both.