Download Magnetism PPT

P.Sci.
Unit 6 cont.
Ch. 21
Magnetism
Magnets
• Permanent magnets – are
magnetic all the time
(lodestones)
• Other substances can be made
into permanent magnets by
placing a strong permanent
magnet near them or by stroking
them with a permanent magnet.
• Materials are classified as either
magnetically hard or soft.
–Hard – easily magnetized but
looses magnetic properties
easily.
–Soft – difficult to
magnetize but
does not loose its
magnetic properties.
Magnetic Force
• the invisible force exerted by
magnets.
• It depends on:
– the strength of the magnet
–the distance an
object is from
the magnet
Magnetic Poles
• Magnets have a pair of poles, a
north pole and a south pole.
–Like poles repel (S/S or N/N)
–Unlike poles attract (N/S)
Just like with
electrical
charges
North Pole
South Pole
Magnetic Fields
• The region where a
magnetic force can be
detected.
• Magnetic Field lines
are used to show the
magnetic field.
A compass needle will always point along a
field line. The lines are close together
where the magnetic force is strong, and
spread out where it is weak
Earth as a Magnet
• Earth acts like a giant bar magnet. It
has a North and South pole. These
magnetic poles are not the same as
the geographic poles. The pole that
is above Canada is actually a
magnetic South pole and the one in
Antarctica is a North pole – to
reduce confusion they are named for
the geographic pole
. Earth’s magnetic field has
changed direction more than 20
times in the last 5 million years.
Magnetism and Electric
Currents
• In 1820, Hans Oersted found that
magnetism is produced by moving
electric charges by experimenting
with the effects of an
electric current on
the needle of a
compass.
• A current-carrying wire creates a
magnetic field that forms in
concentric circles around the wire.
• Right Hand Rule – if you hold a wire
in your right hand with your thumb
pointing in the direction of
the + current, your fingers
would curl in the direction
of the magnetic field.
• The magnetic field created by the
current-carrying wire is very weak.
To create a stronger magnetic
field:
–Increase the current in the wire
–Wrap the wire in a coil
(solenoid)
Solenoids
• Each loop of wire adds to the
strength of the magnetic field of
the loop next to it.
• The strength of the magnetic
field depends on the number of
loops of wire and the amount of
current in the wire.
Electromagnet
• If you add an iron rod in the center
of the coil (in a solenoid) it will
create an electromagnet. This
adds to the magnetism of the coil
creating a stronger magnet.
Electromagnet
Electromagnetic Devices
• Galvanometers – used to
measure current in ammeters and
voltage in voltmeters.
• Electric motors – convert
electrical energy to mechanical
energy to cause motion.
• Stereo speakers – magnetic
forces cause motion that produce
sound waves
Faraday’s Law
• An electric current can be
produced in a circuit by a
changing magnetic field.
• Electromagnetic induction -moving
a magnet in and out of a coil of
wire causes charges in the wire to
move.
Electric Generator
• converts mechanical energy to
electric energy by electromagnetic
induction
Alternating Current (AC)
• For each ½ rotation of the wire
loop, the current produced by
the generator reverses direction
or alternate.
See table on page 477
Electricity in Your Home
• The electricity you use in your
home is created by large
generators at the power plants.
• The mechanical energy used to
power the generators can come
from: Running water (dams),
burning coal, nuclear fission,
wind, hot water (geothermal)
• The electricity coming from a
power plant is too strong for you to
use in your home.
Transformers
• increase or decrease
the voltage so that it can
be used.
–Step up – increases voltage
primary coil < secondary coil
–Step down – decreases voltage
primary coil > secondary coil