Chapter 14: Magnetism

Section 1: Magnetism

Magnets * Magnetism: refers to the properties and interactions of magnets. * Magnets can be found in many devices you use every day, such as TVs, video games, and telephones. Headphones and CD players also contain magnets. * Depending on which ends of the magnets are close together, magnets either repel or attract each other. * The strength of the force between two magnets increases as magnets move closer together and decreases as the the magnets move farther apart. * Magnetic Field: exerts a force on other magnets and objects made of magnetic materials. * The magnetic field is strongest close to the magnet and weaker farther away. * A magnet is surrounded by a magnetic field. * A magnet’s magnetic field is represented by magnetic field lines. * Magnetic Poles: where the magnetic force exerted by the magnet is strongest. * All magnets have a north pole and a south pole. * The magnetic field lines around horseshoe and disk magnets begin at each magnet’s north pole and end at the south pole. * Two magnets can attract or repel each other, depending on which poles are closest together. * When a compass is brought near a bar magnet, the compass needle rotates. The compass needle is a small bar magnet with a north pole and a south pole * A compass can help determine direction because the north pole of the compass needle points north. * The north pole of a magnet is defined as the end of the magnet that points toward the geographic north. * A compass needle aligns with the magnetic field lines of Earth’s magnetic field.
Magnetic Materials * Only a few metals, such as iron, cobalt, and nickel, are attracted to magnets or can be made into permanent magnets. * Electrons have magnetic properties. * In iron, cobalt, nickel, and some other magnetic materials, the magnetic field created by each atom exerts a force on the other nearby atoms. * Magnetic Domains: The groups of atoms with aligned magnetic poles * Magnetic materials contain magnetic domains. * A permanent magnet can be made by placing a magnetic material, such as iron, in a strong magnetic field. * The strong magnetic field causes the magnetic domains in the material to line up. * If a permanent magnet is heated enough, its atoms may be moving fast enough to jostle the domains out of alignment. * Each piece of a broken magnet still has a north and a south pole.

Section 2: Electricity and Magnetism

Electricity Current and Magnetism * In 1820, Hans Christian Oersted, a Danish physics teacher, found that electricity and magnetism are related. * Moving charges, such as those in an electric current, produce magnetic fields. * When an electric current flows through a wire, a magnetic field forms around the wire. The direction of the magnetic field depends on the direction of the current in the wire.
Electromagnets: a temporary magnet made by wrapping a wire coil carrying a current around an iron core. * An electromagnet is made from a current-carrying wire. * Solenoid: A single wire wrapped into a cylindrical wire coil * A solenoid wrapped around an iron core forms an electromagnet. * The sound is produced by a loudspeaker that contains an electromagnet connected to a flexible speaker cone that is usually made from paper, plastic, or metal. * The electromagnet in a speaker converts electrical energy into mechanical energy to produce sound. * An electromagnet can be made to rotate in a magnetic field. * One way to change the forces that make the electromagnet rotate is to change the current in the electromagnet. * Galvanometers: devices that use an electromagnet to measure electric current. * The rotation of the needle in a galvanometer depends on the amount of current flowing in the electromagnet. The current flowing into the galvanometer in a car’s fuel gauge changes as the amount of fuel changes.
Electric Motor: a device that changes electrical energy into mechanical energy. * Electric motors are used in all types of industry, agriculture, and transportation, including airplanes and automobiles. * The main parts of a simple electric motor include a wire coil, a permanent magnet, and a source of electric current, such as a battery. * The battery produces the current that makes the coil an electromagnet. * In a simple electric motor, a coil rotates between the poles of a permanent magnet. To keep the coil rotating, the current must change direction twice during each rotation.

Section 3: Producing Electric Current

From Mechanical to Electrical Energy * Electromagnetic Induction: The generation of a current by a changing magnetic field * Generator: uses electromagnetic induction to transform mechanical energy into electrical energy. * The coil in a generator is rotated by an outside source of mechanical energy. * The current in the coil changes direction each time the ends of the coil move past the poles of the permanent magnet. * As the generator’s wire coil rotates through the magnetic field of the permanent magnet, current flows through the coil. * There is a relationship between electricity and magnetism. * This connection occurs because the electric force and the magnetic force are two different aspects of the same force. * An electromagnetic force exists between all objects that have electric charge. Just like the magnetic and the electric force, the electromagnetic force can be attractive or repulsive. * Turbine: a large wheel that rotates when pushed by water, wind, or steam. * Some power plants first produce thermal energy by burning fossil fuels or using the heat produced by nuclear reactions. * Other power plants use the mechanical energy in falling water to rotate the turbines.
Direct and Alternating Currents * Modern society relies heavily on electricity. * Some devices, such as this radio, can use either direct or alternating current. Electronic components in these devices change alternating current from an electric outlet to direct current. * Direct Current: flows only in one direction through a wire. * Alternating Current: reverses the direction of the current in a regular pattern * Electronic devices that use batteries as a backup energy source usually require direct current to operate.
Transmitting Electrical Energy * The alternating current produced by an electric power plant carries electrical energy that is transmitted along electric transmission lines * One way to reduce the heat produced in a power line is to transmit the electrical energy at high voltages, typically around 150,000 V.
Transformer: a device that increases or decreases the voltage of an alternating current. * Transformers can increase or decrease voltage. * The voltage in the primary coil is the input voltage, and the voltage in the secondary coil is the output voltage. * A transformer that increases the voltage so that the output voltage is greater than the input voltage is a step-up transformer. * A step-up transformer increases voltage. The secondary coil has more turns than the primary coil does. * A transformer that decreases the voltage so that the output voltage is less than the input voltage is a step-down transformer. * A step-down transformer decreases voltage. The secondary coil has fewer turns than the primary coil does. * Power plants commonly produce alternating current because the voltage can be increased or decreased with transformers.