Magnetism notes

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Oct 18, 2013 (3 years and 9 months ago)

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Magnetism Notes


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1


MAGNETISM


Since ancient times, certain materials, called magnets, have been known to have the property of attracting
tiny pieces of
____________________
. This attractive property is called magnetism
.

A
_____________________

magnet is a material that keeps

its magnetic properties even when it is NOT
close to other magnets.

Materials that make good permanent magnets are called
_______________

magnets.

Materials that lose their magnetism quickly are called soft magnets.

___
_________________

materials are meta
ls which are attracted by magnets.

Examples
include iron
, cobalt,
___________________

and alloys of these metals.
The sources of nearly all magnetic effects in matter are the electrons in atoms.

All metals have
___________________

electrons that are free t
o move
randomly throughout the metal.
Each electron occupies a
____________________
.

There are two ways in which electrons create
magnetism:

(1)
Electrons around the nucleus and their
____________________

makes the entire atom a
small magnet.


(2)
Electr
ons themselves act as though they were magnets.

Electrons have
___________
.

The classical model is to consider the electrons to spin like tops.

Electrons usually pair up with their spins
opposite each other, so their fields
___________________

each other.

That is why most materials are not
naturally magnetic.

Magnetic domains in a ferromagnetic material will always orient themselves to
__________________

a permanent magnet.
In a magnet the e
lectrons in each domain line up.







Ma
gnets have two opposite poles, a

north and a

__________________
.

Magnetic poles cannot be isolated.

If a permanent magnetic is cut in half repeatedly, you will still have a north and a south pole.

If a material
is magnetic, it has the ability to exert
____
_______________

on magnets or other magnetic materials.

The
forces depend on the alignment of the poles.

Like poles _________________. Unlike poles attract.
The
strength of the force between magnets depends on the
__________________

between them. The magne
tic
force decreases with distance much faster than does either
_______________

or the electric force.

As early
as 500 B.C. people discovered that some naturally occurring materials


such as
lodestone

and
_________________

have magnetic properties
.

By 1200,

explorers from Italy were using a
________________

to guide ocean voyages beyond the sight of land.

When you use a compass, the north
-
pointing end of the needle points toward a spot near (but not exactly at) the Earth’s geographic
__________

pole.

The Ea
rth’s magnetic poles are defined by the planet’s magnetic field.

That means the

domains of an unmagnetized material


domains of a magnetized material


Magnetism Notes


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________________
magnetic pole

of the planet is near the north geographic pole.

All magnets create a
magnetic
________________

in the space around them, and the magnetic field
creates forces on other
magnets.

All magnets create a magnetic

field in the space around them
,

and the magnetic field
creates
forces on other magnets.
Magnetic field lines always p
oint
___________

from a magnet’s north
pole and
toward its south pole.

The a
rrows on the field lines indicate the direction of
the force. The closer the lines







are together, the
______
_______

the field. A

greater the
number of field lines in a certain area indicates
a
__________
______

magnetic field in that area.
Iron filings are used to show the pattern of the magnetic
field lines.


Electromagnetism

_________________
fields are generated around particles that bear electric charge. Electric fields AND
magnetic fields are produced by
_______________

electric charge
s. The magnetic field created is due to
the “distortions” in the electric field caused by motion.

An electric
________________

also has a magnetic
effect. Winding a nail with a wire and then connecting the wire to a cell produces a magnet (an
_____________
___
).

An electromagnet is a magnet created when
______________


flows through a wire.
If a long straight wire is bent into a coil of several closely

spaced loops, the resulting device is called a
__________________
.

It is also

known as an
_______________
_______

since it acts like a magnet only when it

carries a current.

The field lines of the solenoid resemble those of a bar magnet.


The field lines inside the solenoid are nearly parallel, uniformly
____________
____
,


and close together.

This indicates t
hat the field inside the solenoid is nearly uniform and strong.

The
exterior field is non
-
uniform, much
____________________
, and in the opposite direction to the field
inside the solenoid.

With a metal
____________

in a loop, the magnetic field is enhance
d since the
domains in the core material align, increasing the magnetic field.

The strength of the magnetic field of a
solenoid increases as the number of
_______________

in the solenoid increases.

The strength of the
magnetic field of a solenoid also incr
eases as the
____________________

applied to the solenoid
increases.


Electromagnetic induction is the phenomenon of inducing
_________________

in a conductor by
changing the magnetic field near the conductor

(moving a magnet inside a coil of wire

or movin
g the coil
N

S

N

N

magnetic field lines unlike poles

magnetic field lines like poles


Magnetism Notes


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around the magnet
)
. Faraday’s law states that the induced voltage in a coil is proportional to the product of
the
________________

of loops and the
____________

at which the magnetic field changes within these
loops.

Rather than moving the magne
t, it is often more practical to move the
____________
.

This is best
accomplished by rotating the coil in a stationary magnetic field.

This arrangement is called a
_______________
.

A generator converts mechanical energy into
____________________

energy.

Wh
en
the loop of wire is rotated in the magnetic field, there is a change in the number of magnetic field lines
within the loop.

As rotation continues, the magnetic field inside the loop changes in a
_____________

fashion.

The voltage induced by the generato
r alternates and the current produced is alternating current
(
______
).

A useful device that makes use of electromagnetic induction is the ac transformer.

The
transformer transfers electric energy from one circuit to another usually with a change in
______
______
.

An AC transformer consists of two coils of wire wound around a core of soft

___________
.

The side
connected to the input AC voltage source is called the
_______________

and has N
1

turns.

Current in the
primary coil creates a magnetic field through
the secondary coil having N
2

turns.

The core is used to
increase the magnetic
_________

and to provide a medium for the flux to pass from one coil to the other.

When ac voltage is applied to the primary coil, an ac voltage of the same
_________________

is
induced
in the secondary coil.

This frequency in

North America is _____

Hz.

When N
s

> N
p
, the transformer is
referred to as a
________________

transformer.

When N
s

< N
p
, the transformer is referred to as a
step
down

transformer.

The voltages are related by



Energy is conserved in a transformer so…



If a transformer was 100% efficient, the power produced in the secondary coil should equal the power
output of the primary coil. In practice, this efficiency is closer to
____
% because of eddy currents
(es
sentially friction).


Example 1:

Calculate the voltage output by the secondary winding of a transformer if the primary voltage
is 35 volts, the secondary winding has 4500 turns, and the primary winding has 355 turns.

Is this a step
-
up
or step
-
down transfor
mer?


Example 2:
Calculate the number of turns needed in the secondary winding of a transformer to transform a
primary voltage of 300
V

down to a secondary voltage of 180
V
, if the primary has 1150 turns of wire. Is
this a step
-
up or step
-
down transformer?


Example 3:

A transformer transforms a primary voltage of 12 volts to a secondary voltage of 150 volts. If
the primary current is 18 A, what is the secondary current?

Is this a step
-
up or step
-
down transformer?