Principles of Imaging Science II (120)
Magnetism & Electromagnetism
MAGNETISM


Magnetism is a
property in nature that
is present when
charged particles are in
motion.
Any charged particle in
motion creates a
magnetic field
MAGNETISM

When a charged particle moves, a magnetic
filed is produced around the moving charge.
This magnetic field exerts a magnetic force
on certain kinds of particles that are within
the field


Moving charge produces a magnetic field
Magnetic field of a charged particle is
perpendicular to the motion of the particle

Orbital magnetic moment
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MAGNETISM

When a charged particle moves in a circular or
elliptical path, the perpendicular magnetic field
moves with the charged particle
MOVING CHARGES PRODUCE MAGNETIC FIELD

Every electron has a
charge. Everyone of
these charges is in
motion

Spin magnetic moment:
Electron spin on axis

Dipole: Tiny magnetic
field created by a single
spinning electron.
Magnetic domain: Many
atoms aligned to produce a
larger magnetic field.


Many domains exist in a
magnet
MAGNETISM LAWS

Magnetic Poles




North & South Poles
Iron filings will
concentrate at ends
“Flux” lines extending
from N – S
The greater the
concentration of flux lines
per unit of measure (m2)
the greater the strength
of the magnetic field
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MAGNETISM LAWS

Attraction & Repulsion


Similar charges repel, unlike charges attract
Imaginary lines of the magnetic field leave the North Pole
and enter the South Pole
MAGNETIC CLASSIFICATION
(Susceptibility)

Ferromagnetic (Iron, Cobalt, Nickel)

High Permeability

Ability of a material to be magnetized either by the
application of electric current or exposure to a magnetic
field
High Retentivity


Ability of a magnetized material to remain magnetized
once the magnetizing source (electric current or magnet)
is withdrawn
MAGNETIC CLASSIFICATION

Diamagnetic (wood, glass, plastic)


No Permeability (non-magnetic)
No Retentivity


Cannot be artificially magnetized and are not attracted to
a magnet
Repel magnetic fields
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MAGNETIC CLASSIFICATION

Paramagnetic (Aluminum)


Low Permeability
Low Retentivity

Categorized between ferromagnetic and diamagnetic
Oersted’s Experiment

Discovered that a compass needle is
attracted to a wire that carries a current.
When the current is OFF, the needle points
North, to the earth’s magnetic pole.

Result: Any moving charge produces a magnetic
field. However, it is the movement of electrons in
the electric current that makes the magnetic field
Oersted’s Experiment

Proved that an
electrical current can
be used to produce
magnetic fields
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SOLENOID




Coil of wire with current
flowing through it
Magnetic field lines form
circles around each
section of the wire
Used for detent locks on
x-ray tube
Magnetic field in center
can be intensified by
placing iron in coils
ELECTROMAGNET

Consists of a loop of wire wrapped around a
soft iron core. When electrical current
passes through the wire, the iron core
becomes a magnet. The strength of the
electromagnet is proportional to the


Strength of the current
Number of loops surrounding the core
Magnetic Field Lines
SOLENOID
ELECTROMAGNET
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ELECTROMAGNETIC INDUCTION


Definition: The result of two coils being
placed in close proximity. A varying current is
supplied to the first coil, which then induces a
similar flow in the second coil.
Relies on the principle of interacting electric
and magnetic fields



A changing magnetic field produces an electric
field
The magnetic field must be changing or
fluctuating in order for mutual induction to occur
Purpose: To induce an EMF (electromotive force)
ELECTROMAGNETIC
INDUCTION

Moving a wire through
a magnetic field
induces current to flow
in the wire

Faraday’s experiment
proved that a magnetic
field can generate
electricity (Opposite of
Oersted’s Law)
ELECTROMAGNETIC INDUCTION
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ELECTROMAGNETIC
INDUCTION

Strength of Current Depends on:

Strength of magnetic field



speed with which conducting material cuts or is cut by
magnetic lines of force
Angle of conductor to magnetic field


Larger magnet yields greater strength
Velocity of magnetic field

Perpendicular better than oblique
Number of turns of wire coil

Greater number of turns produces greater current
TYPES OF CURRENT
ALTERNATING CURRENT
(AC)
DIRECT CURRENT (DC)

Electrons flow in only one
direction

Waveform begins at zero
and moves to its
maximum potential at its
peak



Electrons flow first in one
direction (the first half of
the cycle), and then in the
other direction (the
second half of the cycle)
U.S. current: 60 Hz AC
Waveform represented
using a sinusoidal or sine
wave
TYPES OF CURRENT
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GENERATORS

Definition: A electromagnetic device that
converts mechanical energy to electrical
energy

Produces alternating current (AC) with the
use of slip rings and an armature

Produces direct current (DC) with the use of
a commutator ring in place of the slip rings
GENERATORS
GENERATORS
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ELECTRIC MOTOR


Converts electrical energy to mechanical energy
Induction motor to turn the anode at a very high
speed to dissipate heat during x-ray production

Consists of rotor and stator
Simple DC Motor
MOTORS
INDUCTION MOTOR
 The stator is made of stationary
electromagnets located around the
outside. The rotor, located with the stator,
is made of an iron core surrounded by
coils.
 The magnetic field of the stator around
the rotor is created by a series of
electromagnets. These magnets are
turned on and off in a sequence, such
that the outside magnetic field itself
rotates. The inner coils around the central
rotor of the motor are not connected to a
current source. Instead, a current is
induced in them by the magnetic field of
the stator, and this induced current
creates the inner magnetic field that
attempts to align itself with the stronger
outside magnetic field. This force is what
turns the rotor.
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Transformers



Designed to change the voltage and current
in agreement with Ohm’s Law
There is an inverse relationship between
voltage and current
Control of voltage and current is achieved by
a process of Mutual Induction
Transformer Types

Closed Core



Square core of
ferromagnetic material
Primary coil & Secondary
coil at opposite ends
Shell Type


More efficient
Center cores with
separate primary &
secondary windings
Transformer Types

Autotransformer


Single ferromagnetic column core with single coil wrapped
around
Smaller design
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Transformer


Operate on mutual or
self- induction
Mutual induction
requires alternating
current (AC) and 2
coils of electrically
conductive material


Generates AC in a 20
coil when AC is applied
to the 10 coil
Step-Up, Step-Down
Transformers
Transformer Law


Designed to alter voltage and current in an
AC circuit
The ability to control current and voltage is
dependent upon:


# windings (turns) on the primary and secondary
sides
Voltage & Current on the primary side
Transformer Video
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Transformer Law

There is a direct relationship between
transformer voltage and the # of primary &
secondary turns:
Vs = Ns
Vp
Np
Law Applied


A transformer has 100 primary and 400
secondary turns of wire. What is the
secondary voltage if 220 volts are applied to
the primary coil?
Vs = Ns
Vp
Np
880 Volts
Transformer Law
The inverse relationship between transformer
voltage and current is expressed as:
Vs =
Ip
Vp
Is
Vs = Voltage secondary side
Vp = Voltage primary side
Ip = Current primary side
Is = Current primary side

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Law Applied


A step-down transformer is delivered a total
of 220 volts and 3 amps on the primary side.
Output voltage is 110 volts what is the output
(secondary) current?
Vs = Ip
Vp
Is
6 amps
Transformer Law
There is an inverse relationship between
transformer current and the # of primary and
secondary turns
Is
Ip
=
Np
Ns
Law Applied
A transformer has 3,000 turns on the
secondary side and 600 windings on the
primary side. If 0.5 amps flow through the
primary windings, what is the output current
on the secondary side?
Amps? mA?
Is = Np
Ip
Ns
 0.1 amps
 100 mA
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