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Lesson IV “Magnets & Magnetic Fields”
Matter & Energy
S.W.B.A.T.
• Identify the effects of magnetic forces and how magnetic poles interact
• Describe magnetic field lines around a magnet
• Explain how & why materials become magnetized
Magnetic Forces
• Ancient cultures recognized that some rocks contained “mysterious” particles that attracted iron
• The word “magnet” originates from the ancient Greek city of Magnesia
Magnetic Forces
• Magnesia contained mineral deposits later called magnetite or lodestone
• The Chinese were the first to use magnetite to eventually make a compass needle for navigation
Magnetic Forces
• Magnetic force is an area of influence that attracts or repels other magnets, iron and other metals, or electric charges
• Magnetic force strength decreases with distance like the electric force
Magnetic Forces
• Magnetic poles include a “north” and “south” point at the end of the magnet
• Opposite poles (N)(S) attract
• Like poles (N)(N) & (S)(S) repel
Like Poles Repel – Unlike Poles Attract
Magnetic Forces
• A magnetic field describes the area around the poles
• Lines are used showing field direction and strength as measured by a compass
Magnetic Forces
• Magnetic field lines start from the north and are strongest at the poles
• Iron filings can also be used to “map out” magnetic field lines
• Iron filings show where magnetic forces are strongest & weakest
Magnetic Forces
• Earth’s magnetosphere is generated from the iron core rotating slightly faster than the mantle
• The magnetic field extends (on average) about ½ way to the Moon and protects Earth from harmful cosmic radiation
Magnetic Forces
• Geographic north is located 90 degrees north of the equator
• Magnetic north is located about 81 degrees north of the equator
• Earth’s magnetic field “wanders” or moves due to changes in iron core currents and mantle interactions
Video (1:58) “Magnetic North Pole Moves to Siberia”
Magnetic Forces
• Magnetic north is actually magnetic south - since the north tip of a magnet is attracted to it and opposites attract!
• Magnetic poles on Earth have switched places many times in the past
Magnetic Materials
• Atoms have electrons that “spin” – which gives the electrons magnetic properties
• “Paired” electrons spin opposite to each other which cancels out their magnetic properties
Magnetic Materials
• Unpaired electrons in metals such as iron, nickel & cobalt can form a strong magnetic field
• Magnetic domain refers to a region with a very large number of unpaired electrons
Magnetic Materials
• Ferromagnetic metals like iron can become magnetized only because of its magnetic domains
• Ferromagnetic materials can be temporarily magnetized if placed in a magnetic field
• The magnetic domains “line up” or align to create a temporary magnet
Magnetic Materials
• Temporary magnets quickly lose their magnetism after leaving a magnetic field
• A permanent magnet holds on to its magnetic properties after leaving a magnetic field
Magnetic Materials
• Cutting a magnet in half will create two separate magnets, each with a north & south pole
• A magnet cannot have just one pole – no matter how many times you cut it in half!
Magnetic Materials
• Super magnets are the strongest type of permanent magnets
• Super magnets are used in many applications such as MRI’s and particle accelerators
Video (3:32) “Largest Neodymium Magnet in the World?”
Class Activity
• A magnet is placed on a refrigerator door to hold up your A+ assignment given in science class
• Explain which material is the permanent magnet and which is the temporary magnet and why
