chapter_19 - Aurora borealis the Northern Lights Displays...

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624 19 CHAPTER Magnetism O U T L I N E 19.1 Magnets 19.2 Earth’s Magnetic Field 19.3 Magnetic Fields 19.4 Magnetic Force on a Current-Carrying Conductor 19.5 Torque on a Current Loop and Electric Motors 19.6 Motion of a Charged Particle in a Magnetic Field 19.7 Magnetic Field of a Long, Straight Wire and Ampère’s Law 19.8 Magnetic Force between Two Parallel Conductors 19.9 Magnetic Fields of Current Loops and Solenoids 19.10 Magnetic Domains Johnny Johnson/Stone/Getty In terms of applications, magnetism is one of the most important fields in physics. Large elec- tromagnets are used to pick up heavy loads. Magnets are used in such devices as meters, motors, and loudspeakers. Magnetic tapes and disks are used routinely in sound- and video- recording equipment and to store computer data. Intense magnetic fields are used in mag- netic resonance imaging (MRI) devices to explore the human body with better resolution and greater safety than x-rays can provide. Giant superconducting magnets are used in the cyclotrons that guide particles into targets at nearly the speed of light, and magnetic bottles hold antimatter, possibly the key to future space propulsion systems. Magnetism is closely linked with electricity. Magnetic fields affect moving charges, and moving charges produce magnetic fields. Changing magnetic fields can even create electric fields. These phenomena signify an underlying unity of electricity and magnetism, which James Clerk Maxwell first described in the 19th century. The ultimate source of any magnetic field is electric current. 19.1 MAGNETS Most people have had experience with some form of magnet. You are most likely familiar with the common iron horseshoe magnet that can pick up iron-containing objects such as paper clips and nails. Several commercially available magnets are shown in Figure 19.1. In the discussion that follows, we assume the magnet has the shape of a bar. Iron objects are most strongly attracted to either end of such a bar magnet, called its poles . One end is called the north pole and the other the south pole . The names come from the behavior of a magnet in the presence of Earth’s magnetic field. If a bar magnet is suspended from its midpoint by a piece of string so that it can swing freely in a horizontal plane, it will rotate until its north pole points to the north and its south pole points to the south. The same idea is used to construct a simple compass. Magnetic poles also exert attractive or repulsive forces on each other similar to the electrical forces between charged objects. In fact, simple Aurora borealis, the Northern Lights. Displays such as this one are caused by cosmic ray particles trapped in the magnetic field of Earth. When the particles collide with atoms in the atmosphere, they cause the atoms to emit visible light.
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19.1 Magnets 625 experiments with two bar magnets show that like poles repel each other and unlike poles attract each other .
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