Question:

What are quarks and leptons and how does each group interact?

Answer:

All matter is composed of quarks and leptons. They are subatomic particles that are dictated by four forces. The forces are the strong force, the weak force, the electrometric force, and the gravitational force. There are three small particles characterized as quarks that make up the following: neutrons, protons, mesons, and many other particles. Quarks are bound together by the strong force, which explains why the protons and neutrons can stay tightly packed in a nucleus without the nucleus blowing apart due to the electromagnetic force. The electromagnetic force also dictates the interactions between quarks and the weak force in quarks has the ability to change one type of quark to another. Leptons make up the remaining types of particles which include electrons, muons, and neutrinos. Leptons however are affected by all except for the strong force.

Question:

Explain how a bubble chamber works:

Answer:

The bubble chamber was created with several goals in mind. The detection high energy particles every couple of seconds was a primary goal. In order to increase the chance of seeing meaningful collisions of very small particles, a high density liquid must be used. The liquid is stored in a large container and is heated past its boiling point. Extremely pure liquids will not form bubbles unless they are introduced to contaminants. In the case of the bubble chamber, these contaminants are useful and take the form of ionization. As ionization occurs along the path of a particle, bubbles form. From photographs of the paths, much information can be learned. The charge of the particle is determined by the orientation of the curvature in the path. The momentum is determined by the amount of curvature. The bubble chamber is photographed with high-resolution cameras and analyzed by a computer. Advances in holographic photography have lead to sharper pictures and therefore more spatially-rich information about the path of the bubbles.

Question:

Explain how a cyclotron works.

Answer:

The cyclotron works on the principle that a charged particle will more in a circular orbit with its plane of motion perpendicular to the magnetic field. The particle moves in a larger orbit when it has a higher momentum, but the orbit size is also related to the size of the magnet. A stronger magnet will make the orbit smaller. The cyclotron consists of an evacuated circular container with a tube to inject protons. The tube leads to the center where electrical sparks remove the one electron from the hydrogen atoms turning them into protons. The protons gradually drift outwards and are started accelerating in a circle. The particles start moving faster and faster as they complete more than a thousand loops. They are kept in the circular loop by a magnetic field that is weaker around the edges of the container. Lawrence’s first model accelerated electrons to 80 keV and his second accelerated them to 1.2 MeV. As a side note, on a visit to Argonne National Labs, I saw the Advanced Photon Source. It is the second most powerful source of x-rays in the world. It works by accelerating electrons to huge energies of 7 GeV. It is second to an 8 GeV accelerator in Japan.

Question:

Describe how electricity, magnetism, and light were discovered to be related, and how electromagnetism and the weak force were unified.

Answer:

Maxwell used differential equations to create equations for electricity and magnetism. The equations were based on experiments, but seemed to produce inconsistencies. He knew that changing magnetic fields could produce currents. His question was: Could changing electric fields also produce currents? After Maxwell accounted for this symmetry in his mathematical equations, he was able to get rid of the contradictions. Now, according to his equations, electricity and magnetism are known to be related because they cause each other in the way described above. He also wrote a mathematical equation for the propagation of electromagnetic waves. When he used the constants µ and e, he found that his velocity equation gave the speed of light. From this result, he concluded that “…light consists in the transverse undulations of the same medium which is the cause of electrical and magnetic phenomena (Lederman, 25).” This means that he found that all electromagnetic waves (which include light) are fundamentally the same because they are caused by oscillating electric charges that only vary in frequency.

The weak force was unified with electromagnetism through the boson which is responsible for both forces. It was discovered that they only vary by mass. The nuclear weak force boson is not effective outside of the nucleus because of the massive boson that is about one hundred times the mass of the proton. The massive boson will decay before it escapes far enough to have any kind of electromagnetic force effect. On the other hand, there is a mass-less boson that causes electromagnetism. It does not decay.

Question:

Describe the 1920s to 1930s problem of “lost energy” and its resolution.

Answer:

There was an observation of the violation of the conservation of energy. These problems eventually lead to the observation obey detectors of the neutrino. According to conservation of energy, the sum of an objects kinetic and potential energy should be constant at all times if no energy is lost due to outside forces. There also exist other forms of stored energy. As that energy is released, it can also go into kinetic or potential energy. The main point though, is that the total amount of energy available before and after activity is the same. Chadwick, a student of Rutherford, did experiments to measure the energy of beta particles. He found that each particle had energy somewhere along a continuous scale. The equation A ? B + electron shows us how the energy is seemingly not conserved. A is the radioactive atom before its decay. B is the atom after decay. The energy on both sides of the equation should be the same. It was found that B was independent of the electron energy causing the equation to be unbalanced. An unseen particle was hypothesized to explain the problem. Pauli predicted the neutrino to explain it. Fermi wrote in 1933 the new equation: A ? B + electron + neutrino. The neutrino wasn’t detected previously because it didn’t deposit heat in the measuring device. Twenty-five years later, the neutrino was detected directly.

Question:

Describe the theoretical prediction and experimental discovery of antimatter (i.e. the positron).

Answer:

Paul Dirac was a theorist working out of Cambridge in 1927. He wanted to prove a connection between special relativity and quantum mechanics. No one had been successful in doing this before, and finding a connection between the two theories would allow him to describe the electron consistent with relativity. Previously, people had only talked about particles moving extremely fast, or particles that are extremely small, but never about extremely small particles that move extremely fast. He studied the behavior of the electron in the behavior of electric fields. This study lead to the prediction of the spin of the electron that told him that an electron must have about half a unit of angular momentum. The equation also predicted that a particle with the same spin and mass of an electron must exist with a positive charge. Thus, he predicted the positron, also known as antimatter.

Using the cloud chamber, and winning a Nobel Prize for his discovery, Anderson found evidence for the existence of the positron. The book shows a picture of the positron path through the cloud chamber on page 67. The curvature of the path of the particle indicates that it is a positive particle, and the thinness of the ionization along the path indicated that it had a very small mass like an electron.

Question:

How is the strong force different from the electromagnetic and weak forces?

Answer:

The strong nuclear force is what holds protons inside the nucleus and keeps them from flying away from each other. . The repulsive force is caused by the electromagnetic force, which holds electrons and the proton-containing nucleus together. The strong force seems to result from quarks interacting. Particles in the nucleus are made of quarks. In the 1970s, experiments revealed that these particles called quarks could be combined to make neutrons and protons. The weak force changes one type of quark into another. It has the ability to change to neutrons to protons and protons to neutrons.

Another difference is that the strong force is caused by quarks whereas the weak and electromagnetic forces are caused by bosons. From what I have learned in class, quarks are held together by a sea of gluons. This was not specifically stated in the reading. In the case of the electromagnetic force, the boson is mass-less and has an infinite range. On the other hand, the weak force is carried by a massive boson, and therefore does not travel as far before it decays. The strong nuclear force manifests itself in the binding energy of the nucleus. Every mass in the nucleus added up will be greater than the combined mass because when the masses are combined, some of their mass energy goes into the binding process.