CHAPTER OUTLINE Section 1 Magnets and Magnetic Fields Key Idea questions > What happens when the poles of two magnets are brought close together? > What causes a magnet to attract or repel another magnet? > How is Earth s magnetic field oriented? Magnets > What happens when the poles of two magnets are brought close together? > Two like poles repel each other. Two unlike poles attract each other. magnetic pole: one of two points, such as the ends of a magnet, that have opposing magnetic qualities All magnets have at least one pair of poles, a north pole and a south pole. It is impossible to isolate a magnet s south pole from the magnet s north pole. Some materials can be made into permanent magnets. A magnetized piece of iron is called a permanent magnet, but its magnetism can be weakened or even removed. Iron is a soft magnetic material. It is easily magnetized. It tends to lose its magnetic properties easily. Cobalt is a hard magnetic material. It more difficult to magnetize. Once magnetized, it doesn t lose its magnetism easily. Super Summary Chapter Outline p. 1
Magnetic Fields > What causes a magnet to attract or repel another magnet? > Magnets repel or attract each other because of the interaction of their magnetic fields. magnetic field: a region where a magnetic force can be detected Magnets are sources of magnetic fields. Moving charges create magnetic fields. magnetic domains: groups of atoms that all line up the same way and form small, magnetized regions within a material Magnetic field lines are used to represent a magnetic field. Field lines always form closed loops. The magnetic field gets weaker with distance from the magnet. Magnetic field lines that are close together indicate a strong magnetic field. Field lines that are farther apart indicate a weaker field. Magnetic field lines begin at the north pole of a magnet and end at the south pole of the magnet. Compasses align with Earth s magnetic field. The compass points in the direction that lies along the magnetic field line at a given point. By convention, the pole of a magnet that points north is painted red. Earth s Magnetic Field > How is Earth s magnetic field oriented? > Earth s magnetic field lines run from geographic south to geographic north. Super Summary Chapter Outline p. 2
The pole in Antarctica is a magnetic north pole. The pole in northern Canada is a magnetic south pole. Earth s magnetic field has both direction and strength. The source of Earth s magnetism is not yet fully understood. The iron in the core is too hot to retain any magnetic properties. The circulation of ions or electrons in the liquid layer of Earth s core may be the source of the magnetism. The sun also has a magnetic field and ejects charged particles into space. Super Summary Chapter Outline p. 3
Section 2 Magnetism from Electric Currents Key Idea questions > What happens to a compass near a wire that is carrying a current? > Why are electric motors useful? Electromagnetism > What happens to a compass near a wire that is carrying a current? > When the wire carries a strong, steady current, all of the compass needles move to align with the magnetic field created by the electric current. Hans Christian Oersted found that magnetism is produced by moving electric charges. Electric currents produce magnetic fields. Use the right-hand rule to find the direction of the magnetic field produced by a current. right-hand rule: If you hold a wire in your right hand and point your thumb in the direction of the positive current, the direction that your fingers curl is the direction of the magnetic field. Solenoids and bar magnets have similar magnetic fields. solenoid: a coil of wire with an electric current in it In a solenoid, the magnetic field of each loop of wire adds to the strength of the magnetic field of any neighboring loops. The strength of a solenoid can be increased. More loops or more current can create a stronger magnetic field. electromagnet: a coil that has a soft iron core and that acts as a magnet when an electric current is in the coil The magnetic field of the rod adds to the coil s field. Moving charges cause magnetism. Super Summary Chapter Outline p. 4
Negatively charged electrons moving around the nuclei of atoms make magnetic fields. Atomic nuclei also have magnetic fields because protons move within the nucleus. Each electron has a property called electron spin, which also produces a tiny magnetic field. The magnetism of the uncanceled fields in certain materials combines to make the materials magnetic overall. Electromagnetic Devices > Why are electric motors useful? > A motor can perform mechanical work when it is attached to an external device. electric motor: a device that converts electrical energy to mechanical energy Galvanometers detect current. galvanometer: an instrument that detects, measures, and determines the direction of a small electric current ammeter: measures current voltmeter: measures voltage Motors use a commutator to spin in one direction. commutator: a device used to make the current change direction every time the flat coil makes a half revolution. brushes: devices that connect the wires to the commutator Super Summary Chapter Outline p. 5
Section 3 Electric Currents from Magnetism Key Idea questions > What happens when a magnet is moved into or out of a coil of wire? > How are electricity and magnetism related? > What are the basic components of a transformer? Electromagnetic Induction > What happens when a magnet is moved into or out of a coil of wire? > Moving a magnet into and out of a coil of wire causes charges in the wire to move. electromagnetic induction: the process of creating a current in a circuit by changing a magnetic field Faraday s law: An electric current can be produced in a circuit by a changing magnetic field crossing the circuit. As the loop moves in and out of the magnetic field of the magnet, a current is induced in the circuit. Rotating the circuit or changing the strength of the magnetic field will also induce a current in the circuit. Electromagnetic induction obeys conservation of energy. Pushing a loop through a magnetic field requires work. The magnetic force acts on moving electric charges. The force is at its maximum value when the charge moves perpendicularly to the field. As the angle between the charge s direction and the direction of the magnetic field decreases, the force on the charge decreases. The magnetic force acts on wires carrying a current. Generators convert mechanical energy into electrical energy. Super Summary Chapter Outline p. 6
generator: a machine that converts mechanical energy to electrical energy alternating current (AC): an electric current that changes direction at regular intervals For each half rotation of the loop, the current produced by the generator reverses direction. AC generators produce the electrical energy you use in your home. The Electromagnetic Force > How are electricity and magnetism related? > Electricity and magnetism are two aspects of a single force, the electromagnetic force. The energy that results from these two forces is called electromagnetic (EM) energy. Light is a form of electromagnetic energy. EM waves are made up of oscillating electric and magnetic fields that are perpendicular to each other. Transformers > What are the basic components of a transformer? > In its simplest form, a transformer consists of two coils of wire wrapped around opposite sides of a closed iron loop. transformer: a device that increases or decreases the voltage of alternating current primary coil: wire attached to a source of alternating current secondary circuit: wire attached to an appliance Current in the primary coil, this current creates a changing magnetic field that magnetizes the iron core. Super Summary Chapter Outline p. 7
The changing magnetic field of the iron core then induces a current in the secondary coil. Transformers can increase or decrease voltage. The voltage induced in the secondary coil of a transformer depends on the number of loops, or turns, in the coil. In a step-up transformer, the primary coil has fewer turns than the secondary coil does. The voltage across the secondary coil is greater than the voltage across the primary coil. In a step-down transformer, the secondary coil has fewer loops than the primary coil does. The voltage across the secondary coil is lower than the voltage across the primary coil. Transformers must obey the law of conservation of energy. The current in the secondary coil of a step-up transformer is always less than the current in the primary coil. Transformers are used in the transfer of electrical energy. Super Summary Chapter Outline p. 8