Foundations of Physics CPO Science Foundations of Physics Unit 7, Chapter 22

Unit 7: Electricity and Magnetism Chapter 22 Electric Charges and Forces 22.1 Properties of Magnets 22.2 Magnetic Properties of Materials 22.3 The Magnetic Field of the Earth

Chapter 22 Objectives Describe the forces between two permanent magnets. Sketch the magnetic field of a single permanent magnet. Predict the direction of the force on a magnet placed in a given magnetic field. Explain why ferromagnetic materials always attract magnets of either pole. Describe the theory behind why a compass works. Use a compass to find the direction of true north.

Chapter 22 Vocabulary Terms magnet north pole south pole magnetization demagnetization magnetic field compass magnetic field lines diamagnetic paramagnetic ferromagnetic gauss soft magnet magnetic declination magnetic domain hard magnet permanent magnet

22.1 Properties of Magnets Key Question: How do magnets interact with each other? *Students read Section 22.1 AFTER Investigation 22.1

22.1 What is a magnet? If a material is magnetic, it has the ability to exert forces on magnets or other magnetic materials. A permanent magnet is a material that keeps its magnetic properties even when it is NOT close to other magnets.

22.1 Properties of Magnets Magnets have two opposite poles. north south Magnets exert forces on each other. The forces depend on the alignment of the poles.

22.1 Properties of Magnets Plastics, wood, and most insulating materials are virtually transparent to magnetic forces. Conducting metals, like aluminum, also allow magnetic forces to pass through, but may change the forces.

22.1 The force between two magnets The strength of the force between magnets depends on the distance between them. The magnetic force decreases with distance much faster than does either gravity or the electric force.

22.1 The force between two magnets Two magnets near each other often feel a twisting force, or torque. This is a result of having two poles. The combination of attractive and repulsive forces on the same magnet creates a torque.

22.1 The magnetic field All magnets create a magnetic field in the space around them, and the magnetic field creates forces on other magnets. The number of field lines in a certain area indicates the relative strength of the magnetic field in that area. The closer the lines are together, the stronger the field. The arrows on the field lines indicate the direction of the force

22.1 The magnetic field The number of field lines in a certain area indicates the relative strength of the magnetic field in that area. The arrows on the field lines indicate the direction of the force. The closer the lines are together, the stronger the field. Magnetic field lines always point away from a magnet’s north pole and toward its south pole.

22.2 Magnetic Properties of Materials Key Question: How do magnets interact with different materials? *Students read Section 22.2 AFTER Investigation 22.2

22.2 Magnetic Properties of Materials The sources of nearly all magnetic effects in matter are the electrons in atoms. There are two ways in which electrons create magnetism: Electrons around the nucleus and their motion makes the entire atom a small magnet. Electrons themselves act as though they were magnets.

22.2 Magnetic Properties of Materials All atoms have electrons, so you might think that all materials should be magnetic, but there is great variability in the magnetic properties of materials. The electrons in some atoms align to cancel out one another’s magnetic influence. While all materials show some kind of magnetic effect, the magnetism in most materials is too weak to detect without highly sensitive instruments.

22.2 Magnetic Properties of Materials In diamagnetic materials, the electrons are oriented so their individual magnetic fields cancel each other out. Individual atoms in paramagnetic materials are magnetic but the atoms themselves are randomly arranged so the overall magnetism of a sample is zero. When paramagnetic materials are placed in a magnetic field, the atoms align so that the material is weakly magnetic.

22.2 Magnetic Properties of Materials A small group of metals have very strong magnetic properties, including iron, nickel, and cobalt. These metals are the best known examples of ferromagnetic materials. Atoms with similar magnetic orientations line up with neighboring atoms in groups called magnetic domains.

22.2 Magnetic Properties of Materials Magnetic domains in a ferromagnetic material will always orient themselves to attract a permanent magnet. If a north pole approaches, domains grow that have south poles facing out. If a south pole approaches, domains grow that have north poles facing out.

22.2 Properties of magnets Materials that make good permanent magnets are called hard magnets. Steel, which contains iron and carbon, is a common and inexpensive material used to create hard magnets. Materials that lose their magnetism quickly are called soft magnets.

22.3 The Magnetic Field of the Earth Key Question: How do we use Earth’s magnetic field to tell direction? *Students read Section 22.3 AFTER Investigation 22.3

22.3 The Magnetic Field of the Earth As early as 500 B.C. people discovered that some naturally occurring materials— such as lodestone and magnetite—have magnetic properties. By 1200, explorers from Italy were using a compass to guide ocean voyages beyond the sight of land.

22.3 The Magnetic Field of the Earth When you use a compass, the north-pointing end of the needle points toward a spot near (but not exactly at) the Earth’s geographic north pole. The Earth’s magnetic poles are defined by the planet’s magnetic field. That means the south magnetic pole of the planet is near the north geographic pole.

22.3 The Magnetic Field of the Earth The gauss is a unit used to measure the strength of a magnetic field. The magnetic field of the Earth is very weak (0.5 gauss) compared with the strength of the field on the surface of the classroom ceramic magnets (1000 gauss). Historical data shows that both the strength of the Earth’s magnetic field and the location of the north and south magnetic poles can switch places. Today, the Earth’s magnetic field is losing approximately 7 percent of its strength every 100 years.

22.3 The Magnetic Field of the Earth Depending on where you are, a compass will point slightly east or west of true north. The difference between the direction a compass points and the direction of true north is called magnetic declination. After correcting for the declination, you rotate the whole compass until the north-pointing end of the needle lines up with zero degrees on the ring. The large arrow points in the direction you want to go.

Application: Magnetic Resonance Imaging