Lesson IV “Magnets & Magnetic Fields”

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