Beyond the God Particle by Lederman Leon M. & Hill Christopher T

Author:Lederman, Leon M. & Hill, Christopher T. [Lederman, Leon M.]
Language: eng
Format: mobi, epub
Publisher: Prometheus Books
Published: 2013-10-08T00:00:00+00:00

FIGURE 8.31. Bar Magnet. The magnetic field of a bar magnet.

The poles of a magnet are called north (N) and south (S). If a small iron bar magnet is hung from its middle by a string, it becomes a compass needle, and its N end will point northward, thus its S end points southward. The N end of a magnet will repel the N end of another magnet, S will repel S, but N and S attract each other. Hence, N and S are like positive and negative charges (we call N and S magnetic monopoles). However, we can never have an isolated N without, somewhere, having a compensating S; all magnets are therefore “dipoles,” i.e., having two opposite poles, with equal but opposite N and S. This is a consequence of the magnetic field being set up by electrical currents, rather than having magnetic charges, or “magnetic monopoles,” as their sources.15 Either pole of a magnet will induce magnetization in a nearby magnetic material. Therefore, either pole can attract iron-containing objects, such as paper clips, because the magnet will induce magnetization in the paper clip. The paper clip becomes itself a temporary magnet, with its N pole facing an S pole, or vice versa.

If we arrange a flat white sheet of paper over a bar and sprinkle over the paper little iron filings, the filings will align with the magnetic field and allow us to visualize the magnetic field itself!16

CYCLOTRONS

We'll only mention cyclotrons in passing, since they are rather passé in modern particle physics, and will instead refer the interested reader to the extant literature, e.g., search online for “cyclotrons” or see the Wikipedia entry.17 The idea of a cyclotron is to accelerate charged particles but to hold them in circular spiral motion with a constant magnetic field. For example, we can inject particles into the center of a circular machine with a perpendicular magnetic field. We give the particles a little kick in energy, and they will move in a circle. Each time the particles complete one full turn, they are given another “kick” of energy from the same electric field, and then the cycle repeats. As the particle receives each kick in energy, it will tend to spiral outward into a circular orbit with a larger radius.

The cyclotron was invented in 1932 by Ernest Lawrence of the University of California, Berkeley, with much of the development in collaboration with his student, M. Stanley Livingston. The cyclotron was an improvement over the linac of the 1920s, when it was invented, being more compact and cost-effective due to the circular repetitive acceleration process.

For several decades, cyclotrons were the best source of high-energy beams for nuclear physics experiments; several cyclotrons are still in use for this type of research. Cyclotrons are still actively used in medical applications to treat cancer and to produce radioactive isotopes for medical imaging. Ion beams from cyclotrons can be used, as in proton therapy, to penetrate the body and kill tumors by radiation damage, while minimizing damage to healthy tissue along their path.

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