UNIT FIVE: Electricity and Magnetism  Chapter 16 Electricity  Chapter 17 Magnetism

Chapter Seventeen: Magnetism  17.1 Properties of Magnets  17.2 Electromagnets  17.3 Electric Motors and Generators  17.4 Generating Electricity

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.

Investigation 17A  Key Question: How do magnets and compasses work? Magnetism

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  Whether the two magnets attract or repel depends on which poles face each other.

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

17.1 Properties of Magnets  Magnetic forces are used in many applications because they are relatively easy to create and can be very strong.  Large magnets create forces strong enough to lift a car or a moving train.

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 Geographic and magnetic poles  The names of Earth’s poles were decided long before people understood how a compass needle worked. The compass needle’s “north” end is actually attracted to Earth’s “south” magnetic pole!

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 Declination and “true north”  Magnetic declination is measured in degrees and is indicated on topographical maps.

17.1 Declination and “true north”  Magnetic declination is measured in degrees and is indicated on topographical maps.  Most good compasses contain an adjustable ring with a degree scale used compensate for 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.