Download Magnetic field

Chapter 8
Magnetism & Its Uses
Sec 1: Magnetism
► Over
2000 years ago Greeks first discovered
magnetism in a mineral they called magnetic.
► Magnetism—properties and interactions with
magnets
► Magnets attract metal objects.
Magnetic Force & Magnetic Fields
► Magnetic
magnets.
force—the interaction between
 The strength of this force increases as magnets
move closer together.
► Magnetic
field—the area surrounding a
magnet that exerts the magnetic force.
 Iron and other magnets react to the magnetic
force when placed in the magnetic field.
 Magnetic field can be represented by lines of
force, or magnetic field lines.
Magnetic field lines of a bar magnet
Magnetic Poles
► Magnetic
field lines are closest to each
other at the ends of the bar magnet
► These are magnetic poles—where the
magnetic force exerted by the magnet is
the strongest.
► All magnets have a north and south pole.
 Bar magnet—poles are at opposite ends
 Horseshoe magnet—the poles are the two ends
(like a bent bar magnet)
Interacting Magnets
► Magnets
either attract or repel each other.
► When 2 north poles or 2 south poles come
close, you can feel a force preventing the
magnets from touching.
► North poles always attract south poles.
► When 2 magnets come close, their magnetic
fields combine.
A Compass Needle
►A
compass contains a needle (a small bar
magnet) that is free to rotate.
► When placed near a bar magnet, the north
pole of the needle will turn towards the
south pole of the magnet.
► A compass needle will point towards the
North Pole of Earth.
► Earth acts as like a gigantic bar magnet,
and its magnetic field extends into space.
Earth’s Magnetic Poles
► The
north pole of a magnet is the end of
the magnet that points towards geographic
north.
► The north pole of a magnet attracts a south
magnetic pole.
► Earth’s south magnetic pole is near the
geographic north pole.
 It is located in Canada about 1500km from the
geographic north pole.
Magnetic Materials
►A
magnet will not attract all metal objects.
► Only a few elements can be made into
permanent magnets
 Iron, cobalt, & nickel
► Objects
made from these metals are not
always magnetic.
 Put an iron nail next to a refrigerator, let go,
and it falls to the floor.
► You
can make these metals behave like
magnets temporarily.
Permanent Magnets
► Iron,
cobalt, or nickel can be made into permanent
magnets by placing them in a strong magnetic
field.
► This creates a magnetic field inside the material
► It can retain magnetic properties for a long time.
► Permanent magnets can lose their magnetism if
heated or dropped.
► When a magnet is broken, each piece still has a
north and south pole!
 Even the smallest pieces of a magnet have a north and
south pole.
Sec 2: Electricity & Magnetism
► Electric
currents cause magnetic fields.
► The magnetic field around a currentcarrying wire forms a circular pattern
around the wire.
► The strength of the field depends on the
amount of current flowing in the wire.
Electromagnets
► Electromagnet—a
temporary magnet made by
placing a piece of iron inside
a current-carrying coil of
wire.
 When current flows through
the loop of wire, magnetic
field lines form around the
wire.
 The more loops of wire, the
stronger the magnetic field.
Properties of Electromagnets
►Electromagnets are temporary
because they are only magnetic
when current is flowing through
the wire.
►Electromagnets convert
electrical energy into
mechanical energy to do work.
Stereo Speakers
► The
mechanical energy produced from an
electromagnet vibrates parts of a speaker to
produce sound.
Electric Motors
► Electric
motor—a device that
changes electrical energy into
mechanical energy.
 Contains an electromagnet that is free
to rotate between poles of a
permanent magnet connected to the
battery.
 When current flows through the
electromagnet, a magnetic field is
produced in the coil.