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Table of Contents Chapter: Magnetism Section 1: Magnetism Section 2: Electricity and Magnetism Magnetism 1 Magnets • More than 2,000 years ago Greeks discovered deposits of a mineral that was a natural magnet. • The mineral is now called magnetite. Magnetism 1 Magnets • In the twelfth century Chinese sailors used magnetite to make compasses that improved navigation. • Today, the word magnetism refers to the properties and interactions of magnets. Magnetism 1 Magnetic Field • A magnet is surrounded by a magnetic field. A 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 far away. Magnetism 1 Magnetic Poles • Magnetic poles are where the magnetic force exerted by the magnet is strongest. • All magnets have a north pole and a south pole. Magnetism 1 Magnetic Poles • Magnetic field lines always connect the north pole and the south pole of a magnet. Magnetism 1 How Magnets Interact • Two magnets can either attract or repel each other. • Two north poles or two south poles of two magnets repel each other. However, north poles and south poles always attract each other. Magnetism 1 Magnetic Field Direction • The north pole of a compass points in the direction of the magnetic field. • This direction is always away from a north magnetic pole and toward a south magnetic pole. Magnetism 1 Earth’s Magnetic Field • A compass can help determine direction because the north pole of the compass needle points north. • This is because Earth acts like a giant bar magnet and is surrounded by a magnetic field that extends into space. Magnetism 1 Earth’s Magnetic Field • Just as with a bar magnet, the compass needle aligns with Earth’s magnetic field lines. Magnetism 1 Earth’s Magnetic Poles • Currently, Earth’s south magnetic pole is located in northern Canada about 1,500 km from the geographic north pole. • Earth’s magnetic poles move slowly with time. • Sometimes Earth’s magnetic poles switch places so that Earth’s south magnetic pole is the southern hemisphere near the geographic south pole. Magnetism 1 Magnetic Materials • You might have noticed that a magnet will not attract all metal objects. • Only a few metals, such as iron, cobalt, or nickel, are attracted to magnets or can be made into permanent magnets. Magnetism 1 Magnetic DomainsA Model for Magnetism • Groups of atoms with aligned magnetic poles are called magnetic domains. Magnetism 1 Magnetic DomainsA Model for Magnetism • Each domain contains an enormous number of atoms, yet the domains are too small to be seen with the unaided eye. Magnetism 1 Can a pole be isolated? • Because every magnet is made of many aligned smaller magnets, even the smallest pieces have both a north pole and a south pole. Magnetism 1 Lining Up Domains • If you place a magnet against the same nail, the atoms in the domains orient themselves in the direction of the nearby magnetic field. • The nail itself now acts as a magnet. • The nail is only a temporary magnet. Magnetism 1 Permanent Magnets • 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. • The magnetic fields of these aligned domains add together and create a strong magnetic field inside the material. Section Check 1 Question 1 What is the difference between a magnetic field and a magnetic pole? Section Check 1 Answer A magnetic field is the area surrounding a magnet that exerts a force on other magnets and magnetic materials. A magnetic pole is the region on a magnet where the magnetic force is strongest. Section Check 1 Question 2 How do unlike magnetic poles interact? Answer Two magnets can either attract or repel each other. Like magnetic poles repel each other and unlike poles attract each other. Section Check 1 Question 3 Groups of atoms with aligned magnetic poles are called __________. A. B. C. D. magnetic charges magnetic domains magnetic fields magnetic materials Section Check 1 Answer The answer is B, magnetic domains. Magnetic materials contain magnetic domains. Electricity and Magnetism 2 Electric Current and Magnetism • In 1820, Han Christian Oersted, a Danish physics teacher, found that electricity and magnetism are related. • Oersted hypothesized that the electric current must produce a magnetic field around the wire, and the direction of the field changes with the direction of the current. Electricity and Magnetism 2 Moving Charges and Magnetic Fields • It is now known that moving charges, like those in an electric current, produce magnetic fields. • Around a currentcarrying wire the magnetic field lines form circles. Electricity and Magnetism 2 Moving Charges and Magnetic Fields • The direction of the magnetic field around the wire reverses when the direction of the current in the wire reverses. • As the current in the wire increases the strength of the magnetic field increases. Electricity and Magnetism 2 Electromagnets • An electromagnet is a temporary magnet made by wrapping a wire coil carrying a current around an iron core. • When a current flows through a wire loop, the magnetic field inside the loop is stronger than the field around a straight wire. Electricity and Magnetism 2 Electromagnets • A single wire wrapped into a cylindrical wire coil is called a solenoid. • The magnetic field inside a solenoid is stronger than the field in a single loop. Electricity and Magnetism 2 Electromagnets • If the solenoid is wrapped around an iron core, an electromagnet is formed. Electricity and Magnetism 2 Electromagnets • The solenoid’s magnetic field magnetizes the iron core. As a result, the field inside the solenoid with the iron core can be more than 1,000 times greater than the field inside the solenoid without the iron core. Electricity and Magnetism 2 Properties of Electromagnets • Electromagnets are temporary magnets because the magnetic field is present only when current is flowing in the solenoid. • The strength of the magnetic field can be increased by adding more turns of wire to the solenoid or by increasing the current passing through the wire. Section Check 2 Question 1 Who correctly hypothesized that electric current produces a magnetic field? A. B. C. D. Neils Bohr Heinrich Hertz Hans Christian Oersted Max Planck Section Check 2 Answer The answer is C. In 1820, Oersted hypothesized that electric current produces a magnetic field and that the direction of the field changes with the direction of the current. Section Check 2 Question 2 How can you make an electromagnet? Answer An electromagnet is a temporary magnet made by wrapping a wire coil carrying a current around an iron core. Section Check 2 Question 3 Which of the following is a device that uses an electromagnet to measure current? A. B. C. D. electric motor galvanometer generator transformer Section Check 2 Answer The answer is B. In a galvanometer, the electromagnet is connected to a small spring. End of Chapter