6. Explain how atoms can be arranged in such a way to create a permanent magnet.
Atoms have magnetic moments. Conceptually, this can be thought of as the result of electrons orbitin the nuclei, creating current loops. Current loops of course have magnetic dipole moments, meaning they tend to align with an external magnetic field. As a result, an atom will experience a magnetic torque if it is not aligned with the external magnetic field, and this will tend to rotate the atom to line up with the field.
In magnets, the atoms’ magnetic moments are aligned parallel to each other, and their small individual fields combine to form a sensible macroscopic magnetic field. In many materials, the atoms will align parallel to an applied external magnetic field, as discussed above. But in some materials (ferromagnetic materials, for example) the atoms are naturally aligned, even without any external field. In these materials the aligned configuration is the lowest energy state. In other materials, the atoms will align when an external field is applied. Some such materials will remain magnetized, with the atoms in an aligned configuration, for some period after the field is removed. But the atoms eventually decay back into a random, unaligned configuration and the material loses its magnetization.
7. Identify 10 objects in your home that use semiconductors. What other kinds of materials with special electrical properties are found in all of these 10 objects?
Computer, refrigerator, microwave, TV, phone, clock, coffeemaker, camera, dvd player, cable modem.
All of these share digital electronics, so all make use of electrical conductors (copper and aluminum, for example) and insulating materials (rubber and plastic for example).
8. Research the status of magnetically levitated trains like the one now operating in China. How does it operate? How fast might it go?
The Shanghai Maglev Train (Shanghai Transrapid) was the first commercially operated high-speed magnetic levitation train line. It connects the region with the Shanghai International Airport and the Shanghai Metro. The line opened for business in 2004 after three years of construction, at a cost $1.2 billion. At 268 mph it is the world's fastest commercial passenger train.
The two major types of maglev technology are EMS (electromagnetic suspension)and EDS (electrodynamic suspension). In EMS, electromagnets are mounted in the train and attract it to a magnetically conductive track. EDS uses superconducting electromagnets or strong permanent magnets which create a magnetic field that induces currents in nearby metallic conductors when there is relative movement which pushes and pulls the train towards the designed levitation position on the guide way. Most, but not all, maglev designs are based on the monorail configuration.
Maglev is highly efficient, more so than conventional wheeled trains. Much of the energy expended in accelerating the train can be recaptured from the braking process. The main source of energy loss is in air drag. For this reason, conceptual designs have been proposed for vacuum train lines, where a maglev train runs through an evacuated tube or tunnel. In such a train, enormous speeds are possible. One study indicated a transcontinental US train could make the trip in 21 minutes!
9. How does a nuclear reactor work?
A nuclear reactor uses controlled nuclear fission to generate heat, which is used to create steam, which in turn drives turbine engines. The fission process is, initiated by bombarding uranium-235 or plutonium-239 fuel rods with neutrons. Some of the atoms in the fuel rods will absorb these neutrons, become unstable, and split into two lighter elements, emitting more neutrons in the process. The emitted neutrons then collide with yet other fuel atoms, and cause a cascading chain reaction. Each split atom releases heat by virtue of a slight loss of mass. The chain reaction is kept in check by introducing a material like water that absorbs the neutrons, slowing down the reactions.
10. What types of researchers and scientists use carbon-14 radiometric dating? What type of researcher would use the other isotopes such as uranium-238?
Archeologists use carbon-14 dating to determine the age of organic materials up to about 60 thousand years old. Uranium-238 dating can be used as far back as 500 thousand years, so is more useful for researchers studying pre-historic eras. Uranium, however, offers less precision than carbon-14 dating for the range covered by that method.
L. Potassium-40 decays into argon-40 with a half-life of 1.2 billion years. A particular mineral grain is known to have had a million potassium atoms in its crystal structure when it formed. When the grain was ground up, scientists discovered 750,000 atoms of argon-40. When did the grain form? (Hint: Use Figure 12-10 on page 272 to obtain a qualitative estimate).
M. What are the half-lives of the following common isotopes:
a. Carbon-14: 5730 years
b. Uranium-238: 4.468 billion years
c. Uranium-235: 704 million years
12. Name the three kinds of strength used to characterize materials AND give an example of a material that are strong in each of these modes.
13. What kinds of bonding occurs in each of the three materials you listed in question #12?