About electromagnetic radiation, http://www.phy6.org/stargaze/Sun5wave.htm.
Quantum phenomena, http://www.phy6.org/stargaze/Q1.htm and the 7 sections, Q2…Q7, that follow it.
"Spectral lines" of various elements, emitted when they descend from a high-energy level to a lower one, http://www.phy6.org/stargaze/Sun4spec.htm.
Why planets have negative energy, http://www.phy6.org/stargaze/Skepl2nd.htm.
Supernovas, http://www.phy6.org/stargaze/Sun7enrg.htm - (near the end).
Units of particle energies, http://www.phy6.org/Education/wenpart1.html.
Section on nuclear fission in "Hyperphysics" by Rod Nava, http://hyperphysics.phy-astr.gsu.edu/hbase/nucene/u235chn.html.
Also, on the curve of binding energy, http://hyperphysics.phy-astr.gsu.edu/hbase/nucene/nucbin.html.
The photon, http://www.phy6.org/stargaze/Sun5wave.htm (at the end)
"Nuclear Power," http://www.phy6.org/stargaze/Snuclear.htm. Related site on nuclear weapons, http://www.phy6.org/stargaze/Snucweap.htm. Also on the Sun's energy, http://www.phy6.org/stargaze/Sun7enrg.htm.
Nuclear power in space, http://www.eoearth.org/article/Nuclear_reactors_for_space.
The natural reactor at Oklo, http://en.wikipedia.org/wiki/Oklo_phenomenon.
Rhodes, Richard. The Making of the Atomic Bomb. Simon and Schuster: 1988. 886 pp. Nuclear Renewal, is a short book about nuclear energy by the same author, reviewed at http://www.phy6.org/outreach/books/NuclEnrg.htm.
Allen, Leslie. If Nuclear Power has a More Promising Future "Washington Post Magazine" Sunday supplement, 2 August 2009 http://www.phy6.org/stargaze/Sthorium.htm.
Virginia Physics Standards of Learning
This chapter fulfills sections PH.4 and PH.8 of the Virginia Physics Curriculum.
Chapter 4: The Standard Model of Particle Physics
Michael Fetsko. "The Standard Model", 21st Century Physics FlexBook.
Visual Overview for The Standard Model
Figure 4.1
The Standard Model Cribsheet #1
Figure 4.2
The Standard Model Cribsheet #2
In the Beginning
“I can see no escape from the conclusion that [cathode rays] are charges of electricity carried by particles of matter. What are these particles? Are they atoms, or molecules, or matter in a still finer state of subdivision?'" 1897 Experiments, J. J. Thomson
And so it begins, the modern search for the building blocks of matter. What are we made of? What are the smallest constituents of all matter? What do they all have in common? What is different? What holds all the matter together? Where did we come from and where are we going? The search for the building blocks goes back to the days of Aristotle and has always had one goal: to simplify our understanding of nature.
Aristotle believed that there were four elements that comprised nature: earth, water, air, and fire. Democritus, a contemporary of Aristotle, stated that matter could be cut into smaller and smaller halves until you could cut the piece no smaller and it became indivisible. Our present word atom comes from Democritus’ use of the Greek word for indivisible, atomos. Aristotle’s theory of the four elements survived until the and century when these four elements were replaced by our modern chemical elements.
In the beginning there were a couple dozen elements, but this number soon grew to nearly . It appears that science went from a simple model (four building blocks) to a much more complex model (nearly building blocks). Would building blocks all be fundamental? Another change was about to occur with the discovery of the atom and the idea of the indivisible nature of matter returned. The atom was made up of three building blocks and it appeared that a simpler model was restored. This is where our chapter truly begins…with the discovery of these three “fundamental” particles.
Discovery of the Electron
In the mid– century, many scientists traveled the country presenting lectures on various scientific ideas. One of the topics that most delighted the audiences at that time involved a glass tube and high voltage. By pumping out most of the air from the glass tube and connecting wires on either side of an evacuated tube, a high voltage would be applied across the tube and to the amazement of the audience the interior of the tube would glow! This device was called a Crooke’s tube or, a cathode ray tube. Now, to the audience all that mattered was the incredible mysterious glow that appeared within the tube, but to the scientists the main question was “What caused the glow?” To most, the notion was that there was some kind of ray being emitted from the cathode. But, what was this ray made up of…was it a wave or a particle? The dominant theory of the time was that light was a wave, but there was also the idea that maybe the ray was some type of unknown particle. What was this mysterious ray? Was this some type of wave traveling through the invisible fluid known as ether or a particle that developed out of the ether? The search for an answer was the mission of the British physicist J. J. Thomson.
Figure 4.3
Cathode Ray Tube
As a result of Maxwell’s work in the 1860s it was known that all electromagnetic waves, including visible light, travel at a speed of in a vacuum. Experimentation with cathode rays showed that their direction of travel could be altered by placing the tube in a magnetic field. With these two ideas in mind, J. J. Thomson began his experimentation on the mystery of the cathode rays. In 1894, he decided to experimentally determine the velocity of the cathode rays. The measured velocity could then be compared to the speed of an electromagnetic wave, which could help possibly determine something about its structure. Through the use of mirrors and the cathode rays, Thomson was able to determine the velocity of the rays to be approximately meters per second, which is significantly less than the speed of light. So, it appeared that cathode rays were not electromagnetic waves, but actually small particles. This result was not widely celebrated by the scientific community, but it did lead to further experimentation by other scientists.
The rays are influenced by a magnetic field and they travel much more slowly than an electromagnetic wave. From this experimental evidence, one might conclude that the rays are particles. Thomson did not stop at this point. He continued to use electric fields and magnetic fields to determine how much they influenced the motion of the rays. The first conclusion that he reached through this line of inquiry was that the rays must be particles or, as he called them, “corpuscles.” Thomson found that the mysterious stream would bend toward a positively charged electric plate. Thomson theorized, and was later proven correct, that the stream was in fact made up of small particles, pieces of atoms that carried a negative charge. These particles later became known as electrons. Thomson was unable to determine the mass of the electron, but he was able to determine the charge-to-mass ratio, or . He knew the for the hydrogen ion and it was much smaller than the for the cathode rays. He assumed that the mass of the particle was much smaller than the mass of the charged hydrogen atom. Thomson went on and “… made a bold speculative leap. Cathode rays are not only material particles, he suggested, but in fact the building blocks of the atom: they are the long-sought basic unit of all matter in the universe.” 1897 Experiments, J. J. Thomson.
Based on Thomson’s belief that the atom is divisible and consists of smaller blocks, namely the electron, he then developed a model for the atom. His finding has been called the “plum pudding model” in which the atom is represented as a positively charged ball with negatively charged particles inside. This model was the accepted explanation for the structure of the atom until Ernest Rutherford and his gold foil experiment in 1911.
Figure 4.4
Thomsons Plum Pudding Model
Discovery of the Proton
In 1909, an experiment intended to verify Thomson’s plum pudding model was conducted under the guidance of Ernest Rutherford. Hans Geiger and Ernest Marsden, Rutherford’s students, directed alpha particles (the nuclei of helium atoms) at a very thin sheet of gold foil. Based on the plum pudding model the alpha particles should have
barely been deflected, if at all. The reason for this is that the momentum of the alpha particles was so large that the particles should not be influenced by the relatively small mass of the electrons and the positive charge spread throughout the atom. However, they observed that a small number of the particles were deflected through large angles, including some reflecting back to the source.
Figure 4.5
Images showing the expected and the actual results from Rutherfords gold foil experiment.
Geiger and Marsden spent many hours in a darkened room using a low–powered microscope to “see” tiny flashes of light on a scintillator screen. A variety of different foils were used as well as different thicknesses. Given the relatively high momentum of the alpha particles they expected that the particles would pass through without any, or minimal, deflection. For the majority of events, this held to be true. Amazingly, they found that approximately in every particles were reflected through angles greater than degrees. Rutherford later remarked, "It was almost as incredible as if you fired a fifteen-inch shell at a piece of tissue paper and it came back and hit you." This observation was completely unexpected and appeared to contradict Thomson’s plum pudding model.
Figure 4.6
Rutherfords Gold Foil Scattering Experiment
In 1911, Rutherford published a new atomic model that stated that the atom contained a very small positive charge that could repel the alpha particles if they came close enough. He also went on to state that the atom is mostly empty space, with most of the atom’s mass concentrated in the center, and that the electrons were held in orbit around it by electrostatic attraction. The center of the atom is called the nucleus. The idea of a massive, positively charged nucleus supported the observations of Geiger and Marsden. The alpha particles that came close to the nucleus had been deflected through varying angles, but the majority of alpha particles passed relatively far away and therefore experienced no deflection at all.
Over the next 10 years, Rutherford and many other physicists continued to explore the components of the atom. It was widely accepted that positively charged particles were contained within the nucleus. It was believed that the positive charge of any nucleus could be accounted for by an integer number of hydrogen nuclei. Rutherford was the first to refer to these hydrogen nuclei as protons in 1920.
Discovery of the Neutron
Ernest Rutherford continued to play a significant role in the discovery of the building blocks of matter. As physicists continued to study atomic events, they noticed that the atomic number of the atoms and the atomic mass did not match up. They were finding that the atomic number (number of protons) was typically less than the atomic mass (mass of atom). Due to the electron’s small mass, the prevailing thought was that there must be something besides the proton adding to the overall mass of the atom. The main theory put forward by Rutherford stated that additional electrons and protons, coupled together inside the nucleus, formed a neutral particle. This new particle, called the neutron, would not influence the overall charge of the atom, but would account for the missing mass.
At this point, Rutherford appointed a former student, James Chadwick, to the post of Assistant Director of his lab at Cambridge University. Chadwick spent the next ten years tracking down this elusive particle. It was not until some experiments carried out in Europe came to his attention that Chadwick achieved some success with his endeavor. Chadwick repeated their experiments with the goal of looking for a neutral particle—one with the same mass as a proton, but with zero charge. His experiments were successful. He was able to determine that the neutron did exist and that its mass was slightly greater than the proton’s. The third component of the atom had been discovered. The model of the atom now consisted of the positively charged proton and the neutral neutron that made up the nucleus and the negatively charged electron that moved around the empty space surrounding the nucleus.
Figure 4.7
Rutherfords Planetary Model of the Atom
One More Particle—the Photon
Long before the proton and neutron were discovered another fundamental particle was found—the photon. In 1900, Max Planck presented the revolutionary theory that energy was not actually continuous, but existed in tiny, discrete chunks. Each tiny chunk, or quantum, has a magnitude equal to . The energy of the quanta, , is determined by multiplying Planck’s constant, , by the frequency of oscillation, , of the electromagnetic wave. The value of is . These energy packets are so small that we don’t notice their size in our everyday experiences. On our normal scale of events energy seems continuous. In other words, the motion of a ball down an inclined plane looks continuous, but according to quantum theory it is actually rolling down a set of extremely tiny stairs jumping from one level to the next.
At approximately the same time another phenomenon was discovered that connected electricity, light, and atomic theory. It was found that when light is shone on certain metallic surfaces, electrons are ejected from the surfaces. This is known as the photoelectric effect. In some way the light is giving up its energy to the electrons in the metal and causing them to be released and produce a current. However, not all colors of light will cause a current to flow. Two aspects of this experiment cannot be explained with the classical theory of light [i.e., electricity and magnetism]. (1) No matter how bright a red light one used, a current was never produced. But, a very dim blue light would allow for a current to be produced. (2) The current is observed immediately, and not several minutes as predicted by classical theory.
Figure 4.8
The Photoelectric Effect
The problem was that these results could not be explained if light was thought of as a wave. Waves can have any amount of energy you want—big waves have a lot of energy, small waves have very little. And if light is a wave, then the brightness of the light affects the amount of energy—the brighter the light, the bigger the wave, the more energy it has. The different colors of light are defined by the amount of energy they have. If all else is equal, blue light has more energy than red light with yellow light somewhere in between. But this means that if light is a wave, a dim blue light would have the same amount of energy as a very bright red light. And if this is the case, then why won't a bright red light produce a current in a piece of metal as well as a dim blue light? In 1905, Einstein used Planck’s revolutionary idea about the quantization of energy and applied it to the photoelectric effect. Although it was universally agreed that light was a wave phenomenon, he realized that the only way to explain the photoelectric effect was to say light was actually made up of lots of small packets of energy called photons that behaved like particles http://www.lon-capa.org/~mmp/kap28/PhotoEffect/photo.htm Photoelectric [Effect Applet].
Einstein was able to explain all the observations of the photoelectric effect. The ejection of an electron occurs when a photon hits an electron and the photon gives its entire energy to the electron. If the photon has sufficient energy to transfer to the electron, the electron may be ejected from the atom and a current will start. If the photon does not have enough energy, then the electron will not be supplied enough energy and no current will be produced. The amount of energy each photon can transfer is dependent upon the frequency (color) of the light and not on its brightness. The energy of a photon is determined by Planck’s relationship, . So, no matter how bright the red light may be, the frequency of the red light will not provide it with enough energy to ever eject a photon, no matter how bright or how long that red light shines on the metal. Whereas dim blue light will eject electrons, because the frequency of blue light is large enough to provide enough energy to the photon to eject the electron.
With the discovery of the three fundamental particles of the atom and the development of the idea of the photon, it appeared that by 1932 the building blocks of matter had been rediscovered. The hundred different building blocks of matter had been replaced by a much simpler view of the physics world. This elegant picture of the physical world did not last for long, though. As technology im
proved and more questions were posed and eventually answered, many new and rather strange observations were made. The first and perhaps most bizarre discovery happened right after the neutron was discovered in 1932 and it represented an entirely new type of matter.
Not so Fast—Antimatter is Found!
In 1927, Paul Dirac, a British theoretical physicist, was able to formulate a special equation describing the motion of electrons. This equation was applied to Einstein’s theory of relativity to predict that there must be a particle that has the same mass as the electron, but with the opposite charge. This theory led to the conceptualization of antiparticles or broadly speaking, antimatter. Not only does the electron have an antiparticle, but Dirac’s equations predicted that all matter has a corresponding antiparticle.
Figure 4.9
An actual cloud chamber picture from Carl Andersons experiment.
In the early 1930s, Carl Anderson was investigating cosmic rays using a cloud chamber. Charged particles produced by cosmic rays would leave “tracks” in the cloud chamber. These tracks would bend in circles because the chamber was surrounded by a strong magnetic field. As a result, positively charged particles bent one way and negatively charged particles bent in the opposite direction. During his investigation Anderson encountered unexpected particle tracks in his cloud chamber. He found equal numbers of positive and negative particles following very similar, yet oppositely directed paths. He assumed that the negatively charged particles were electrons, but what were the positively charged particles—protons? Anderson correctly interpreted the pictures as having been created by a particle with the same mass as the electron, but with an opposite charge. This discovery, announced in 1932, validated Dirac’s theoretical prediction of the positron, the first observed antimatter particle. Anderson obtained direct proof that positrons exist by shooting gamma rays, high–energy photons, into nuclei. This resulted in the creation of positron-electron pairs. This pair production exemplifies Einstein’s equation ; energy (the massless gamma ray) is converted into mass (the pair of particles). Interestingly enough, the reverse is true as well. If an electron and a positron collide, their mass is converted into energy. This process is true for any matter-antimatter pair and is called pair annihilation.
CK-12 21st Century Physics: A Compilation of Contemporary and Emerging Technologies Page 8