Magnetism Review
Magnetism Review
1. Lodestone- are magnetic rocks that are rich in
(3.)magnetite, a iron oxide mineral
2. William Gilbert- made observations and proved in
the 1600s that the earth is a great magnet or giant
lodestone
4. Andre Marie Ampere- proposed the theory of
magnetism in Fe to explain electromagnets. It is a
result of electric current through its loops. The
force between 2 long straight parallel currents was
proportional to the distance between them and
proportional the intensity of the current flowing in
each.
Magnetism Review
4. Magnet Domains- each electron in an atom acts like a
tiny electromagnet. When the magnetic fields of the
electrons align in the same direction, the group is called
a magnetic field.
Draw Picture
6. Magnetic Field- (symbol B) (unit T(tesla) or G(gauss))are the forces around magnets, they are vector
quantities. When drawing the magnetic field lines they
leave the North pole and enter the South Pole
Draw Picture
7. Electromagnet- a magnet created when current flows
through a wire coil.
Magnetism Review
8. Hans-Christian Oersted- in 1820 laid a wire
across the top of a small compass and
connected the ends of the wire to an
electrical circuit. He expected needle to
point in the same direction as current wire
but it pointed perpendicular.
***So the electromagnetic force is
perpendicular to the direction of current. ***
Show Phet
Magnetism Review
9. Michael Faraday- given credit for creating
the first motor, generator, transformer, and
capacitor. Mainly in this topic we discuss the
3 right-hand rules that he came up with
about Magnetic Fields.
1st Right-Hand Rule
• It is a method used to determine the
direction of a magnetic field relative to the
current (+ current)(from the positive side of
the terminal)
– Your thumb is the current in the wire and your
folded fingers are the magnetic field. Draw
picture.
2nd Right-Hand Rule
• Used to determine the direction of the
magnetic field produced by an electromagnet
relative to the current.
– The thumb is the north pole. Draw a picture
3rd Right Hand Rule
• Used to determine the direction of the magnetic
force on a current- carrying wire in a magnetic
field.
– Draw a picture
– Formulas:
• F = ILB
• F = qvB
• Faraday’s Law:
– Emf(electromotive force)=
– (anything that causes voltage) = N(∆BA)/t
– ΔBA stands for flux
Magnetism Review
10. Changing Magnetic Field- Can cause
current in a wire.
11. Electromagnetic Induction – the process of
generating a current through a circuit due to
the relative motion between a wire and a
magnetic field when the wire is moved
through the magnetic field or the magnetic
field moves past the wire.
Draw a picture. Use Phet.(use ac current)
Magnetic Review
12. James Maxwell- postulated that a changing electric field
produces a changing magnetic field, which was later
proven by Hertz. This lead to the discovery of
electromagnetic waves(19)- which are a combination of
electric and magnetic fields that travel through space.
13. Generator and motors- converts mechanical energy to
electrical energy . It has a number of wire loops that
placed in a strong magnetic field. The wire is wound
around an iron core to increase the strength of the
magnetic field. The wires and the iron are called the
armature, which is similar to that of an electric motor.
http://physci.kennesaw.edu/mzoughi/Simulations.shtm
Magnetism Review
13. The armature is mounted so that it can
rotate freely in the magnetic field. As the
armature turns, the wire loops cut through the
magnetic field lines and induce an EMF.
Commonly called voltage, the EMF developed
by the generator depends on the length of the
wire rotating in the field. Only the portion
within the magnetic field induces an EMF. In a
closed circuit an electric current is created.
Phet.
To keep the torque on a DC motor from
reversing every time the coil moves through
the plane perpendicular to the magnetic field, a
(16)split-ring commutator is used to reverse the
current at that point. The electrical contacts to
the rotating ring are called "brushes" since
copper brush contacts were used in early
motors. Modern motors normally use springloaded carbon contacts, but the historical name
for the contacts has persisted.
(17) Slip ring commutator- are electrical connections
that are used to transfer power to and from the rotor
of an AC generator . The slip ring consists of a circular
conducting material that is connected to the rotor
windings and insulated from the shaft. Brushes ride
on the slip ring as the rotor rotates. The electrical
connection to the rotor is made by connections to the
brushes. Slip rings are used in AC generators because
the desired output of the generator is a sine wave. In
a DC generator, a commutator was used to provide an
output whose current always flowed
in the positive direction. This is not necessary for
an AC generator. Therefore, an AC generator may
use slip rings, which will allow the output current and
voltage to oscillate through positive and negative
values.
Magnetism Review
14. Transformer- are used to increase or
decrease AC voltage. Uses of transformers is
common because they change voltage with
relatively little loss of energy. Many printers,
game systems, and computers have
transformers inside their casings or part of
their cords.
Magnetism Review
• 14.transformer- How they work: A
transformer has 2 coils, electrically insulated
from each other, but wound around the same
iron core. One coil is called the primary coil,
the other coil is called the secondary coil.
When the primary coil is connected to a
source of AC voltage, the changing current
creates a changing magnetic , which is carried
through the core to the secondary field. This
changing field induces a varying EMF.
Transformers
• Formula :
Vs/Vp= Ns/Np= Is/Ip
V stands for voltage
N stands for number of turns of coil
Draw picture
• Voltage increase corresponds to current
decrease
Draw Magnetic Fields coming in and
out of board
Magnetism Review
17.Mass spectrometer- an instruments that
can distinguish between isotopes. It can
measure charge-mass ratios of positive ions.
A magnet causes the positive ions in a
vacuum chamber to be deflected according
to their mass and it is recorded on a
photographic piece of paper.
Draw picture.
Magnetism Random Facts
• Cathode Ray tube computer minitors and TV screens
use magnets to focus and direct particles on a screen
which causes the colors you see.
• Electromagnets are in the heads of audiocassette and
video recorders. The recorder creates the electrical
signal and the recorder head creates the magnetic
field. The recording tape has tiny bits of magnetic
material bonded to thin plastic . This tape passes over
the recording head and the domains align according to
the direction of the current. These stay until they are
erased and the domains become scattered again.