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One promising weapon in the battle against cancer is fast-neutron therapy in which a beam of high-energy (hence, fast) neutrons is directed into a cancerous region (here at the laser cross hairs). High-energy neutrons break bonds in the DNA of the cancer cells, causing the cells to die and thus eliminating the cancer. You can produce a beam of high-energy particles by using an electric field along a long path if the particles are charged. A long-enough path is not available in a hospital, however, and neutrons are not electrically charged.

How can a beam of high-energy neutrons be produced?

The answer is in this chapter.

28-1 What Is Physics?

As we have discussed, one major goal of physics is the study of how an electric field can produce an electric force on a charged object. A closely related goal is the study of how a magnetic field can produce a magnetic force on a (moving) charged particle or on a magnetic object such as a magnet. You may already have a hint of what a magnetic field is if you have ever attached a note to a refrigerator door with a small magnet or accidentally erased a credit card by moving it near a magnet. The magnet acts on the door or credit card via its magnetic field.

The applications of magnetic fields and magnetic forces are countless and changing rapidly ...

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