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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is withdrawn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

Larger magnet yields greater strength

Velocity of magnetic field

speed with which conducting material cuts or is cut by

magnetic lines of force

Angle of conductor to magnetic field

Perpendicular better than oblique

Number of turns of wire coil

Greater number of turns produces greater current

TYPES OF CURRENT

DIRECT CURRENT (DC)

Electrons flow in only one

direction

Waveform begins at zero

and moves to its

maximum potential at its

peak

ALTERNATING CURRENT

(AC)

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

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.

INDUCTION MOTOR

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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 = Np

Ip 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