Magnetism

Chapter 17.1 Learning Goals Identify properties of magnetic materials and use interactions between magnets to explain attraction and repulsion. Describe the source of Earth’s magnetism. Explain how a compass works.

17.1 Properties of Magnets If a material is magnetic, it has the ability to exert forces on magnets or other magnetic materials nearby. A permanent magnet is a material that keeps its magnetic properties.

17.1 Properties of Magnets All magnets have two opposite magnetic poles, called the north pole and south pole. If a magnet is cut in half, each half will have its own north and south poles.

17.1 Properties of Magnets Magnetic forces can pass through many materials with no apparent decrease in strength.

17.1 Magnetic fields The force from a magnet gets weaker as it gets farther away. Separating a pair of magnets by twice the distance reduces the force by 8 times or more.

17.1 Magnetic fields A special kind of diagram is used to map the magnetic field. The force points away from the north pole and towards the south pole.

17.1 Magnetic fields You can actually see the pattern of the magnetic field lines by sprinkling magnetic iron filings on cardboard with a magnet underneath.

17.1 Magnetic field lines A compass needle is a magnet that is free to spin. Because the needle aligns with the local magnetic field, a compass is a great way to “see” magnetic field lines.

17.1 Geographic and magnetic poles The planet Earth has a magnetic field that comes from the core of the planet itself.

17.1 Declination and “true north” Because Earth’s geographic north pole (true north) and magnetic south pole are not located at the exact same place, a compass will not point directly to the geographic north pole. The difference between the direction a compass points and the direction of true north is called magnetic declination.

17.1 Earth’s magnetism Studies of earthquake waves reveal that the Earth’s core is made of hot, dense molten metals. Huge electric currents flowing in the molten iron produce the Earth’s magnetic field.

17.1 Earth’s magnetism The gauss is a unit used to measure the strength of a magnetic field. The magnetic field of Earth (.5 G) is weak compared to the field near the ceramic magnets you have in your classroom. (300- 1,000 G). For this reason you cannot trust a compass to point north if any other magnets are close by.

17.1 Earth’s magnetism Today, Earth’s magnetic field is losing approximately 7 percent of its strength every 100 years. If this trend continues, the magnetic poles will reverse sometime in the next 2,000 years.