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No Slide TitleChapter 18
Concept Mapping
Bellringer
Given the following objects, a piece of aluminum foil, a nickel, a plastic figure, a piece of wood, a glass vase, and some paper clips, predict which objects will be attracted to a magnet. How do you know?
Record your answers in your science journal.
Chapter 18
Objectives
Explain why some materials are magnetic and some are not.
Describe four kinds of magnets.
Give two examples of the effect of Earth’s magnetic field.
Chapter 18
Properties of Magnets
Magnetic Poles are points on a magnet that have opposite magnetic qualities.
North and South The pole of a magnet that points to the north is called the magnet’s north pole. The opposite end of the magnet, called the south pole, points to the south.
Chapter 18
Properties of Magnets, continued
Magnetic Forces When you bring two magnets close together, the magnets each exert a magnetic force on the other. These magnetic forces result from spinning electric charges in the magnets. The force can either push the magnets apart of pull them together.
Chapter 18
Chapter 18
Properties of Magnets, continued
Magnetic Fields A magnetic field exists in the region around a magnet in which magnetic forces can act.
Chapter 18
The Cause of Magnetism
Atoms and Domains In materials such as iron, nickel, and cobalt, groups of atoms are in tiny areas called domains. The north and south poles of the atoms in a domain line up and make a strong magnetic field.
Chapter 18
Section 1 Magnets and Magnetism
The Cause of Magnetism, continued
Losing Alignment When domains move, the magnet is demagnetized, or loses its magnetic properties.
Making Magnets You can make a magnet from demagnetized material if you line up its domains with another magnet.
Chapter 18
Section 1 Magnets and Magnetism
The Cause of Magnetism, continued
Cutting a Magnet When you cut a magnet in half, you end up with two magnets.
Chapter 18
Kinds of Magnets
Atoms and Domains Some magnets, called ferromagnets, are made of iron, nickel, cobalt, or mixtures of those metals. Another kind of magnet is the electromagnet. This is a magnet made by an electric current.
Temporary and Permanent Magnets Temporary magnets are made from materials that are easy to magnetize. But they tend to lose their magnetization easily. Permanent magnets are difficult to magnetize, but tend to keep their magnetic properties longer.
Chapter 18
Earth as a Magnet
One Giant Magnet Earth behaves as if it has a bar magnet running through its center.
Poles of a Compass Needle The point of a compass needle is attracted to the south pole of a magnet. Opposite poles of magnets attract each other.
South Magnetic Pole near North Geographic Pole A compass needle points north because the magnetic pole of Earth that is closest to the geographic North Pole is a magnetic south pole.
Chapter 18
Chapter 18
Section 1 Magnets and Magnetism
Earth as a Magnet, continued
The Core of the Matter Scientists think that the Earth’s magnetic field is made by the movement of electric charges in the Earth’s core.
A Magnetic Light Show Earth’s magnetic field plays a part in making auroras. An aurora is formed when charged particles from the sun hit oxygen and nitrogen atoms in the air.
Chapter 18
Chapter 18
Visual Concept
Bellringer
Suppose you had a machine that could levitate heavy objects with the flick of a switch. Write a paragraph in your science journal about how this machine would make your life easier.
Chapter 18
Objectives
Identify the relationship between an electric current and a magnetic field.
Compare solenoids and electromagnets.
Describe how electromagnetism is involved in the operation of doorbells, electric motors, and galvanometers.
Chapter 18
The Discovery of Electromagnetism
Hans Christian Oersted Danish physicist Hans Christian Oersted discovered the relationship between electricity and magnetism in 1820.
More Research From his experiments, Oersted concluded that an electric current produces a magnetic field. His research was the first research in electromagnetism—the interaction between electricity and magnetism.
Chapter 18
Chapter 18
Using Electromagnetism
A solenoid is a coil of wire that produces a magnetic field when carrying an electric current.
An electromagnet is made up of a solenoid wrapped around an iron core.
Turning Electromagnets On and Off Electromagnets are very useful because they can be turned on and off as needed. The solenoid has a field only when there is electric current in it.
Chapter 18
Applications of Electromagnetism
Doorbells Two solenoids in a doorbell allow the doorbell to work.
Magnetic Force and Electric Current An electric current can cause a compass needle to move. The needle is a small magnet. This property is useful in electric motors.
Chapter 18
Applications of Electromagnetism, continued
An electric motor is a device that changes electrical energy into mechanical energy. All electric motors have an armature—a loop or coil of wire that can rotate.
Galvanometers A galvanometer measures current. A galvanometer has an electromagnet placed between the poles of a permanent magnet.
Chapter 18
Chapter 18
Bellringer
Have you ever discovered something by accident? Maybe you looked in a dictionary for the definition of an unknown word, only to find the definition of another word you didn’t know.
Write a short paragraph in your science journal describing how you have discovered something by accident.
Chapter 18
Objectives
Explain how a magnetic field can make an electric current.
Explain how electromagnetic induction is used in a generator.
Compare step-up and step-down transformers.
Chapter 18
Electric Current from a Changing Magnetic Field
Faraday’s Experiment Michael Faraday conducted an experiment trying to get the magnetic field of the electromagnet to make an electric current in a second wire.
Chapter 18
Electric Current from a Changing Magnetic Field, continued
Success for an Instant Faraday realized that electric current in the second wire was made only when the magnetic field was changing. The process by which an electric current is made by changing a magnetic field is called electromagnetic induction.
Inducing Electric Current Look at the next slide to see electromagnetic induction.
Chapter 18
Chapter 18
Electric Generators
Alternating Current The electric current produced by the generator shown on the next slide changes direction each time the coil makes a half turn. Because the electric current changes direction, it is an alternating current.
Generating Electrical Energy The energy that generators convert into electrical energy comes from different sources such as fossil fuels and nuclear energy.
Chapter 18
Chapter 18
Transformers
A transformer increases or decreases the voltage of alternating current.
Step Up, Step Down The number of loops in the primary and secondary coils of a transformer determines whether it increases or decreases the voltage.
Electrical Energy for Your Home The electric current that brings electrical energy to your home is usually transformed three times.
Chapter 18
Transformers, continued
Chapter 18
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Use the terms below to complete the Concept Mapping on the next slide.
Chapter 18
Concept Mapping
electrical energy electric motor