Magnetism and its Uses

Chapter 8

Magnets

• Greek discovery of magnets (mineral in Magnesia)

• Magnetism—refers to the properties and interactions of magnets

• Strength of magnetism increases as magnets move closer together

Magnetic Field

• A magnet is surrounded by a magnetic field that exerts the magnetic force.

• The magnetic field is strongest close to the magnet and weakest far away

• The magnetic field can be represented by lines of force or magnetic field lines– Fig. 2 pg. 227

Magnetic poles

• Magnetic poles—regions where magnetic force exerted by a magnet is strongest

• All magnets have a north and south pole• Bar magnets —north and south poles are at

opposite ends• Horseshoe magnets —two ends are north and

south poles• Magnetic field lines always connect north and

south poles of a magnet

How magnets interact

• Two magnets can either attract or repel ea/other

• North poles always attract south poles

• When two magnets are brought close to ea/other, their magnetic fields can combine to produce a new magnetic field– Ex: fig. 4-pg. 228

A Compass Needle

• A magnet that is free to rotate can turn when it is placed in a magnetic field

• Compass—contains a needle, & small bar magnet– Can freely rotate– When placed near a bar magnet, the needle

will turn so that the north pole of the needle points toward the south pole of the bar magnet

• Pg. 228 Fig 5

Earth’s Magnetic Field

• A compass helps determine your direction because the north pole of the compass needle always points north

• The north pole of a magnet is defined as the end of the magnet that points toward the geographic north.

• Sometimes the north pole and south pole of magnets are called the north-seeking pole and south-seeking pole

Earth’s Magnetic Field cont’d.

• Earth is like a bar magnet w/its south magnetic pole near its geographic north pole

• Earth’s south magnetic pole—in northern Canada about 1500 km from the geographic north pole

• A compass needle points to magnetic north south pole (geograhic north)

• The source of Earth’s magnetic core is unknown???

Magnetic Materials

• Not all metals are attracted to magnets• What makes elements magnetic?

– Atoms contain electrons– Electrons have magnetic properties– Atoms of magnetic metals have electron

charges that don’t cancel out– Ea/atom in these elements then acts like a

magnet/creates its own magnetic field– However: objects can behave like magnets

» temporarily

Magnetic Domains

• Magnetic Domains—the groups of atoms w/aligned magnetic poles

• Each domain contains billions of atoms

• Magnetic poles of atoms in a domain are aligned

• Magnetic domains behave like magnets with a north pole and a south pole

MORE ON DOMAINS

• Although ea/domain acts like a magnet, poles of the domains are randomly arranged

• Magnetic fields from all the domains cancel ea/other out– Ex: pg. 231 (figure 7)

Permanent Magnets

• Placing a piece of magnetic material in a strong magnetic field creates a—Permanent Magnet– Ex: iron, nickel, cobalt, etc• Even permanent magnets can lose their

magnetic behavior if heated or dropped

• Also—A magnetic pole cannot be isolated• (ex: if cut in two, ea/piece has a N & S pole)