# Chapter 17 Notes - John Jaros

Electronics - Devices

Oct 18, 2013 (4 years and 8 months ago)

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Chapter 17

Section 1:

Magnets and Magnetic Fields

A remarkable fact about electric charges is that if charge
q
1

is moving with respect to charge
q
2

at velocity
v
, charge
q
1

will exert a force on charge
q
2

that is different from the electric force betwe
en. This
magnetic force
.

The magnetic

force

arises in this way:

1)

A moving charge
(such as electric current) creates a
magnetic field

(similar to an electric field).

2)

A different charge that is moving with respect to the

magnetic field is acted on by the magnetic force.

?

A
magnet

is something that produces a magnetic field. But most magnets don’t have any electric current
going through them. It turns out that the atoms of three elements,
iron, coba
lt, and nickel

can
produce a small magnetic field due to the configuration of the electrons in their atoms.

In a “plain” piece of these elements, the magnetic fields from the various atoms are randomly oriented
collect together in
magnetic domains

con
taining billions of atoms. Each magnetic domain acts as a
magnet but the domains are randomly “aligned” and their magnetic fields
cancel each other out. Thus,
an iron nail is not
magnetic
. However, a piece of iron, cobalt, or nickel can be made into

a magnet by
lining up the magnetic fields of the
magnetic domains

so that they don’t cancel out.

1)

Stroking a piece of iron, cobalt, or nickel (in the same direction) with a magnet will line up the
magnetic domains

in the iron, cobalt, or nickel and

make the piece of iron, cobalt, or nickel a magnet
(
make it
magnetic).

2)

Touching a piece of iron, cobalt, or nickel w
ith a magnet

will line up the atoms in the iron, cobalt, or
nickel and make the piece of iron, cobalt, or nickel a magnet (magn
etic) BUT the magnetized iron,
cobalt, or nickel will gradually “lose” its magnetism as the atoms un
-
line up randomly.

A
permanent magnet

is
a magnet that remains a magnet after the “magnetizing source” has been
removed; a
temporary magnet

is a magnet
that loses its magnetic quality when the “magnetizing
source” has been removed.

Magnetic Poles
:

Just like there are two kinds of electric charge (positive and negative), there are two kinds of magnetic
poles

(
south and north). But there is a big differe
nce: isolated magnetic charges are very common but
as yet no one has found an isolated magnetic pole. In other words, magnetic poles are always found in
pairs. There are some similarities with electric charge: Like magnetic poles repel (just like char
ges) and
un
-
like magnetic poles attract (just like charges).

The consequence of not having isolated magnetic poles means that if you cut a magnet, you get two
magnets each one with a north and south pole!!

N

S

†††

††††
S†
†††

NOTE how the “new” poles are situated

Magnetic Fields
:

All magnets produce a
magnetic field

which can be modeled like an electric field with field lines.
Whereas electric field lines start or end on charges and are N
OT closed loops, magnetic field lines
ARE

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closed loops that pass through the poles of a magnet.
The closeness of magnetic field lines

indicates
the strength of a magnetic field just as the closeness of electric field lines indicates the strength of an
el
ectric field. As shown in the diagram below, a bar magnet is strongest at its “ends” and weakest “in the
middle.”

Magnetic Field Strength of a Bar Magnet

NOTE

that the
white

north
-
seeking

pole of the compass always points toward the north pole of the
m
agnet.
NOTE

also that the magnetic field lines are closest at the “ends” of the magnet (indicating a
strong magnetic field) and far apart “in the middle” of the magnet, indicating a weak magnetic field.

The

magnetic

field

lines

continue

inside

the

magne
t

from

the

S

pole

to

the

N

pole,

completing

the

loop.

Earth’s Magnetic Field
:

The Earth has a magnetic field probably created by the motion of ions in the molten iron in its outer core.
The exact cause of Earth’s magnetic field is still a subject of sci
entific debate!!

The direction of Earth’s magnetic field is NOT constant and the field has actually
reversed

its direction
several times in Earth’s past. Currently, Earth’s magnetic north pole is near to Earth’s geographic North
Pole (the point where Ea
rth’s axis “exits” the Earth in the Northern Hemisphere).

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Notice that there is a magnetic
south

pole indicated for the North magnetic pole. This is because like
poles repel. If the North magnetic pole is really a “north pole”, then the north pole of
a magnet would not
be attracted to it. To avoid all of these messy details, the poles on a magnet should really be called
north
-
seeking poles

and
south
-
seeking poles
.

Section
2
:

Magnetism from Electric Currents

Electricity and magnetism are closely re
lated:

1)

A moving electric charge (that is, electric current) produces a magnetic field

2)

A changing magnetic field produces an electric current in a conductor

The direction of the magnetic field produced by a current in a (thin) wire is given by the
r
ight
-
hand rule

as shown below:

Never Grasp or Touch an UN
-
insulated Wire

The magnetic field produced by a current in a wire
is fairly weak. This can be changed by: (
1
) increasing
the current or (
2
) by wrapping the wire into (essentially) concentric c
ircles. This produces a
solenoid
: a
coil of current
-
carrying wire. The magnetic field of a solenoid looks like the magnetic field of a bar
magnet.
Note that a solenoid has a north and south magnetic “pole”/”end”
.

The strength of the magnetic field o
f a solenoid depends on:
(
1
)
the amount of current in the wire of the
solenoid and (
2
) the
loop
-
density

of the solenoid (the number of loops of the wire per each cm of the
solenoid). The higher the current or the greater the loop
-
density, the larger the

magnetic field.

An
electromagnet

is a solenoid with a magnetizable material (iron or steel) inserted inside its coils.

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The Origin of Magnetic Fields
:

ALL magnetic fields (even the magnetic field in a bar magnet) are due to moving electric charges.

The
moving charges in a bar magnet are the electrons in the atoms of the material composing the bar magnet.
In most elements, the tiny magnetic fields of its atoms cancel out and these atoms are not even
magnetizable. But in certain elements (especiall
y iron, cobalt, and nickel) the tiny magnetic fields act
differently. In iron, cobalt, and nickel, the atoms are organized into domains where each domain
contains about a billion atoms. The tiny magnetic fields of the atoms in a single domain align
themselves
in a particular direction and add up to give a meaningful magnet field. However, any macroscopic/visible
piece of iron, cobalt, or nickel has its domains randomly arranged so that
the overall effects of the
domains cancel out. This is why a

piece of iron, cobalt, or nickel is not magnetic.

BUT iron, cobalt, and nickel are magnetizable by placing either of them in an
external magnetic field
.
The external magnetic field aligns the domains so that they don’t cancel out. As a result, the

piece of
iron, cobalt, or nickel becomes magnetic.

Electromagnetic Devices
:

Almost all electric devices make use of both electric and magnetic fields. Electric light bulbs
(incandescent and fluorescent) are two notable exceptions.

A
galvanometer
is
an electric device that can be used to measure the current in a circuit (it is then called
an
ammeter
) or the voltage drop across a circuit element (it is then called a
voltmeter
). The basis of the
operation of a galvanometer is a rotatable electromagnet
set between the poles of a permanent magnet.
The current going through the electromagnet determines the size of the electromagnet’s magnetic field.
The electromagnet’s magnetic field interacts with the magnetic field of the permanent magnet causing the
e
lectromagnet to rotate. The amount of rotation can be used to determine the current in a circuit or the
voltage drop across a circuit element.

A
Direct Current (DC) electric motor

uses a rotatable coil between the poles of a permanent magnet.
But the co
il is attached to a
commutator

which effectively reverses the direction of the current going
through the coil (at each half turn of the coil)
so the coil will continually turn

between the poles of the
permanent magnet. The turning motion of the coil can b
e used to do mechanical work.

The key part of a DC electric motor is its split
-
ring commutator
. The split in the commutator enables
the current in the coil to periodically reverse, enabling the magnetic force of the permanent magnet to act
in the same d
irection and keep the coil spinning. If the current in the coil did not change direction after
each half
-
turn of the coil, the coil would just oscillate back and forth after each half
-
continually turning.

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An Electromagnetic Speak
er uses a coil whose magnetic field can cause a diaphragm to vibrate back and
forth based on changing the direction of the current in the coil. The vibrating diaphragm produces
longitudinal vibrations in the air (that is, sound waves).

diaphragm

Section
3
:

Electric Currents from Magnetism

An electric current produces a magnetic field, Can a magnetic field produce an electric current?? YES,
if the magnetic field is time
-
varying (changing as time passes). When a (time
-
varying) magnetic fie
ld
produces an electric current,
the phenomenon is called
electromagnetic induction
.

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There are 3 ways to change the amount of current that is induced in a closed loop of conductive wire:

1)

Move the loop faster/slower in the direction shown in the di
agram

2)

Change the strength of the permanent magnet

3)
Spin the loop
on an axis shown by the dashed line in the diagram

The faster the “number of lines of
magnetic field

that pass through the loop changes”, the greater the
current induced in the loop
.

Electromagnetic induction does NOT create electric energy from nothing. The Principle of Conservation
of Energy is NOT violated. The energy in the electric current comes from the work an outside/external
force did when it moved the loop. In fact, some
of the energy used to move the loop is “lost” as thermal
energy so the conversion is not even 100% efficient.

The Magnetic Force on a Moving Charge
:

A charged particle
moving

in a constant magnetic field experiences a force exerted on it by the magnetic
f
ield. The charge can be “totally free/isolated” OR the charge can be an electron in an atom. When a
conducting, metal wire moves through a magnetic field, the electrons in the metal experience a force
due to the magnetic field. This force moves the
electrons, producing the electric current.

NOTE:

That in
A

,

any motion of the wire that is perpendicular to the direction of the magnetic field
produces the maximum force exerted on the charges. The
CHARGES SHOULD BE

-

.

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Electric Generators
:

Elect
ric generators

convert mechanical energy into electric energy. During an electric power blackout,
gasoline
-
driven electric generators can provide electricity to vital places such as hospitals. An electric
generator can produce two types of electric curr
ent: (
1
)
direct current

(
DC
)
in which the direction of
charge motion stays the same (a good example is a battery) or (
2
)
alternating current

(
AC
)
in which the
direction of charge motion changes in a predictable way (the kind of current found in your elect
ric outlet in
which the directions changes 60 times each second).

In an AC generator, the direction of the current changes after each half
-
turn of the wire loop shown in the
diagram below.

NOTE:

that an AC generator has
two

complete
slip rings

(as opp
osed to the
single

split
commutator

in
a DC motor). Also note that each end of the wire loop touches a different slip ring.

AC Generator Current

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Producing Electricity
:

Where does the electricity in your electric outlets come from? It comes from elect
ric generators!!! There
are different kinds of electric generators depending on the
source

of mechanical energy that is used to
generate the electric current.

1)

wind power

wind “turns the loop”

2)

water power

falling water “turns “the loop”

3)

coal,

gas, and oil

burning these fuels boils water that “turns the loop”

4)

nuclear power

nuclear fission boils water that “turns the loop”

5)

solar power

energy from the sun
is used in various ways to create electricity

As a general rule, fossil fuel
sources of electricity are quite inefficient because a great deal of the energy
in the fossil fuels is “lost” as thermal energy during the generating process.

Electromagnetic Waves
:

The following two statements turn out to be true:

A time
-
varying magnet
ic field produces an electric field

A time varying electric field produces a magnetic field

These two ideas show that electricity and magnetism are really two aspects of the same phenomenon.
That’s why the subject is now called
electromagnetism

and pe
ople refer to
electromagnetic

-
rays, and
gamma rays) ALL consist of oscillating (time
-
varying) electric and magnetic fields. The electric an
d
magnetic fields oscillate in perpendicular planes which are both perpendicular to the direction of motion of
the wave/radiation. This is illustrated below:

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Electric Transformers
:

An
electric transformer

is a device used to
change the voltage

in an electric circuit. As a side
-
effect of
changing the voltage, the current is also changed so that their product is constant.

A transformer has two parts (or circuits): a
primary circuit

and a
secondary circuit
. The object of the
transformer is to
make the voltage in the secondary circuit some desired value.

A transformer consists of a permanent magnet wrapped with two coils of wire (or two closely
-
spaced
solenoids). The current in the primary circuit produces a changing magnet field that induces
a current in
the secondary circuit.
The two circuits are
NOT

electrically connected
.

Transformers

require

a

changing

electric

field;

therefore,

transformers

require

AC

current.

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The basis of operation of a transformer is the ratio of the number o
f loops in the primary and secondary
circuits.

Electric Transformer Equation

primary circuit voltage

# of loops in primary circuit

--------------------
---

=
-----------------------------

secondary circuit voltage # of loops in seconda
ry circuit

Due to
frictional and thermal energy “losses”, the voltage in the secondary circuit will be slightly less than
the ideal value calculated from the transformer equation.

Step
-
UP Transformer
:

Voltage in secondary circuit
>

voltage in primary c
ircuit. Secondary circuit has
more

loops

than primary
circuit. Used at or near the electric generator because a high voltage and low current results in less “loss”
of electrical energy. Typically, electricity is transferred over power lines at a voltag
e of 12,000 V.

Step
-
DOWN

Transformer
:

Voltage in secondary circuit
<

voltage in primary circuit. Secondary circuit has
less

loops than primary
circuit. Used at or near the user of the electric current since a lower voltage is safer to use. Most
househo
ld appliances require a voltage of 120 V.

Chapter Review Questions

page 584:

#1
-

#5, #
8

-

#10, #12
-

#13

page 58
5
:

#16
-

#26

(
Table 1

is on page 579)

page 58
6
:

#29

-

#33

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