Download Chapter 4 - RadTherapy

Chapter 4
Electromagnetism
Magnetism:
Magnetism is one of the fundamental forces- like gravity and electricity.
Materials that have the ability to attract iron are classified as having a strong magnetic
force.
When a charged particle is in motion, a magnetic force field perpendicular to the motion
will be created.
Orbital spin moment: the magnetic effect created by electrons orbiting around a nucleus
in an atom.
The closed loop of the orbit cancels out all but the field that is perpendicular to
the plane of the motion.
Spin magnetic moment: the magnetic effect created by electrons spinning on their axes.
The disruption of this axial spinning and the energy released as it reorients itself
are the physical basis that permits magnetic resonance imaging.
Atoms having a significant number of electrons with their magnetic moments in the same
direction (magnetic dipole/magnetic domain), especially when the outer shells are
involved, will exhibit a net magnetic field in a distinct direction.
If an external force has the time or strength to orient enough of the dipoles in the same
direction and/or cause those dipoles to grow in size, the object exhibits a uniformly
strong magnetic field and referred as a magnet.
The force fields created when magnetic dipoles orient to create a magnet are called lines
of force, lined of flux, or the magnetic field.
The stronger the magnetic field, the more lines of flux.
The ends of a magnet are defined as the north and south poles: lines of force always flow
from north to south outside a magnet and from south to north within a magnet; these lines
of force never intersect.
The stronger the magnetic field, the greater the number of lines of flux or the greater the
flux density.
Flux density is determined by field strength and by the area in which the lined of flux are
located.
Magnetic flux = field strength / area
The SI unit for magnetic flux is the Weber. 1 Weber = 108 lines of flux.
The units for flux density are the tesla (1 T = 1 Wb/m2) and the gauss (1 tesla = 10,000
gauss)
Earth: 0.0001 T
Household magnet: 0.1 T
MRI equipment: 0.3 – 2.0 T
Magnets: classification by production
Natural magnets are created when iron oxide (magnetite) remains in the earth’s
magnetic field for ages, slowly orienting the magnetic dipoles in the same direction
(called lodestones).
Artificial permanent magnets are manufactured from a steel alloy called alnico,
composed of aluminum, nickel and cobalt. While it is hot, alnico is subjected to the field
of a strong commercial magnet to permit easier orientation of the magnetic dipoles.
Upon cooling, becomes permanent. Can be destroyed by heat or abrupt physical injurycauses the magnetic alignment to become random again, losing the magnetic fields.
Electromagnets are temporary magnets produced by moving electric current.
Any flow of current produces a magnetic field. When the current ceases flowing,
the magnetic field collapses, the magnetic field is proportional to the electric
current: more current = stronger magnetic field.
Laws governing magnetism:
1. Repulsion / attraction: like poles repel; unlike poles attract, like lines of force
repel; and unlike lines of force attract.
2. Inverse square law: the force between two magnetic fields is directly
proportional to the product of their magnitudes and inversely proportional to the
square of the distance between them.
3. Magnetic poles: every magnet has two poles, a north and a south, dividing the
magnet just make smaller magnets, and it does not affect its strength.
Magnetic induction: when a nonmagnetized iron bar is brought within the lines of force
of a strong magnet, the dipoles will temporarily align themselves with the lines of force
passing through the iron bar (soft iron works best).
Temporary- only magnetic during magnetic induction
Magnetic sinks: shielding is accomplished by providing a highly ferromagnetic material,
such as iron, through which stray magnetic fields can be directed.
Permeability: the ease with which a material can be magnetized.
Retentivity: is the ability of a material to stay magnetized.
Magnets- classification by reaction to a magnetic field:
1. Ferromagnetic: highly permeable and greatly susceptible to induction, majority of
dipoles in same direction, the material must permit the atoms to be oriented on the
same direction. (iron, cobalt and nickel)
2. Paramagnetic: have low permeability and weak attraction to magnetic fields,
slight majority of dipoles in same direction (platinum, aluminum, MRI contrast
material)
3. Diamagnetic: are weakly repelled by all magnetic fields (beryllium, bismuth, lead
and water)
4. Nonmagnetic: not affected by magnetic fields and cannot be magnetized, most
often composed of atoms locked into crystalline or molecular patterns,
elimination the ability of electrons to freely orient themselves to external
magnetic lines of force (wood, glass, rubber, plastic, most materials).
Electromagnetism:
Electricity and magnetism are actually different aspects of the same force,
electromagnetism.
Hans Oersted (1777 – 1851)
 Observed the deflection of a compass during a classroom experiment
 When there is no current flowing in the wire, the compass needle aligns itself with
the earth’s magnetic field
 When current is flowing, the needle is deflected toward the wire
 Any moving current produces a magnetic fields, a charge at rest has no magnetic
field
John Fleming (1849 – 1945)
 Hand rules to describe the relationships between electricity and magnetism
 Right hand thumb rule: if the right hand is used to grasp a conducting wire with
the thumb in the direction of the current flow, the fingers will indicate the
direction of the magnetic field lines of force surrounding the conductor.
Solenoids and electromagnets:
Solenoid; a coil consisting of a series of loops, which serve to increase the flux density;
inside the loop the magnetic fields from both sides join to double the magnetic flux
density.
Electromagnet: temporary magnet by moving electric current, the flux density is
increased further by adding a ferromagnetic core to a solenoid.
The strength is determined by:
 The number of loops or turns of wire
 The current strength
 The permeability of the core
Both the solenoid and the electromagnet demonstrate magnetic properties only while
electric current is flowing.
The effectiveness of solenoids and electromagnets:
 Diameter of the coil
 Its length
 The current passing along the coil
Electromagnetic induction:
Michael Faraday (1791 – 1867) discovered that the simple presence of a magnetic field
is not sufficient to cause electrons to move along a wire, the magnetic lines of force and
the wire must have a motion relative to each other to induce electric current.
1. Move the conductor through a stationary unchanging magnetic field
2. Move the magnetic lines of force through a stationary conductor with an
unchanging strength magnetic field
3. Vary the magnetic flux strength from a stationary magnet through a stationary
conductor, lines of force will contract and expand causing the motion necessary to
induce current.
The First law of Electromagnetics (Faraday’s Law): states that four factors regulate the
strength of induced current when magnetic lines of force and a conductor are in motion
relative to one another
1. The strength of the magnetic field
2. The speed of the motion between the lines of force and the conductor
3. The angle between the magnetic lines of force and the conductor and
4. The number of turns in the conduction coil
The Second law of Electromagnetics (Lenz’s Law): states that induced current flow sets
up a magnetic field opposing the action that produced the original current, that induced
current opposes any flux change. Determines the direction of the induced current.
The First and Second Laws of induction of current by magnetic fields apply to both forms
of induction: mutual and self induction.
Mutual induction: occurs when two coils are placed in proximity and a carrying current
supplied to the first coil induces a similar flow in the second coil, the flow occurs in the
secondary coil because the primary current alternates (moving lines of force from a
varying intensity current will induce electron flow in the wire through which it passes).
Self-induction: the ability of an alternating current to switch directions, causing an
opposing potential difference to induce against the incoming supply of electrons; allows
direct current to flow while at the same time hindering alternating current.
 A coil supplied with alternating current permits a steady flow of electrons and
establishes an electromagnetic effect for half the cycle, when current supply
reverses, the previously established electromagnetic north and south poles will
induce an opposing potential difference, attempting to induce against the
incoming supply of electrons (inductive reactance)
Components of the x-ray tube
Electric generator – mechanical motion produces electricity
Electric motor – electricity produces mechanical motion
Transformer – alternates current and voltage from one side to the other
Generators
Generator (dynamo) is a device that converts mechanical energy to electrical energy
(moving lines of flux in relation to a conductor to induce current)
A simple generator is composed of a conductor and magnets. The conductor is a coil of
wire (armature), set between opposing magnetic poles so that it encounters the strongest
lines of force. Because the armature is moving in the magnetic field, a current is
produced.
A set of slip rings and brushes permits the circuit to remain stationary while the
armature rotates without breaking the electrical contact between them. Each slip ring
connects to one end of the armature wire. This allows the electrons to flow without
interruption.
Alternating current: produced when the wires motion relative to the lines of force is
reversed.
A direct current generator- made by exchanging the slip rings for a commutator ring, a
single ring that is divided in half, with each half connected to one end of the armature
wire.
The commutator ring changes the brushes with which it is in contact with, reversing the
exiting connections, thus keeping the current flow in the circuit flowing in the same
direction. It is at this point that direct current is produced.
Motors
Motors: a device supplied with electrical current to produce mechanical motion.
The motor principle is a result of the interaction of magnetic fields when an electric
current is sent along a conductor that is residing in a magnetic field. As current flows
through the conducting coil, a magnetic field is established, because the conducting coil
lies within the lined of force from the stationary magnets, the induced lines of force will
be attracted in one direction while at the same time being repelled in the other direction.
The net result is that the conductor begins to move.
 The magnetic field of the conductor would be either
o Attracted to the external field if the external lines of force were opposing
directions
o Repelled by the external magnetic field lines of force if the lines of force
were in the same direction
Synchronous alternating current motors have conducting coils that turn at the same
speed as the generator armature supplying the current (used in turning devices).
Alternating current induction motors
 Used in some x-ray tubes with rotating anodes to prevent overheating
 Utilize a rotor coil- consist of bars of copper around an iron core with the exterior
magnetic field supplied by several pairs of electromagnets
o Called a stator which mist be supplied with multiphase current which
activates the next pair of electromagnets causing the rotor to turn to align
itself with the magnetic force
 Thus producing a strong magnetic field, increasing the power of the motor and
permitting it to run at any speed
When a meter is connected in series, it measures current in amperes and called an
ammeter.
When a meter is connected in parallel, it measures voltage in volts and called an
voltmeter.
Controlling electrical current
Transformers transform electrical current and potential (voltage) into either higher or
lower intensities; are composed of two coils placed near one another. If current is
supplied to one coil, the lines of force that are induced will pass through the other coil
and induce a flow of electrons. Transformers are used to change voltage and to control
electrical current.
The coil that is supplied with current is the primary.
The coil in which current is induced is the secondary.
Instead of the electromagnet having ends, it is an enclosed loop, which keeps the
magnetic field confined to the core. The primary core is wrapped around one side, the
secondary coil the opposite side. Alternating current necessary because AC causes the
magnetic fields to change.
When the voltage is increased from primary to secondary, it is called a step up
transformer (more turns in coil of secondary coil). A step-up transformer increases
voltage from primary to secondary while decreasing amperage.
When the voltage is decreased from primary to secondary, it is called a step down
transformer (less turns in coil of secondary coil). A step-down transformer decreases
voltage from primary to secondary while increasing amperage.
All transformers must operate on alternating current to provide the establishing and
collapsing magnetic fields that induce voltage changes in the secondary coil.
The transformer Law:
Vs / Vp = Ns / Np
Where:
V = potential difference in volts
N = number of turns of wire in the coil
P = primary coil
S = secondary coil
Voltage and number of turns are directly proportional, while voltage and amperage are
inversely proportional. The number of turns and amperage is inversely proportional.
Is / Ip = Vp / Vs
Is / Ip = Np / Ns
Air core transformers: arrangement of two coils of wire in proximity to facilitate
induction, no iron core.
Open core transformers: arrangement of two coils of wire each filled with an iron core
in proximity to facilitate induction.
Closed core transformers: arrangement of two coils of wire each filled with an iron core
in proximity to facilitate induction that converges the inside and outside lines of force
through the core.
Autotransformer: a transformer that automatically sets up by adjustments.
The autotransformer is used to vary the incoming line voltage to an appropriate level for
the high voltage step up transformer (small changes in voltage or current).
The high voltage step up transformer is used to raise the incoming line voltage to the 5 –
15 volt range and 3 – 5 ampere range used to heat the x-ray tube filament.
A capacitor is a device capable of accumulating and storing an electrical charge. A
simple capacitor consists of two insulated metal plates with opposite charges. The
repulsion between the charges in the two plates permits a greater number of electrons to
be stored on each.
The capacitor will accept a charge until it equals the DC voltage. When discharged the
capacitor has the ability to deliver the stored charge in short and easily controlled
intervals.
Dielectric: the insulation between the plates.
Rectification
X-ray tubes work best when receiving direct current.
Rectification is the process by which alternating current is changed to pulsating direct
current. Creates electrical one-way streets by permitting electrons to flow easily in one
direction while offering a high resistance to movement in the other direction.
 Electrons flow only in one direction – cathode to anode
 It cant reverse from anode to cathode
 Tubes called diodes or solid-state silicon tubes are used today instead of valve
tubes
Half-wave rectification: suppressed rectification from only half of the incoming
alternating current being converted to pulsating direct current. The supply of alternating
current to a rectifier results in pulsating direct current. Danger of this type of rectification
is over heating and thermionic emission of the anode, which would break the filament.
Full-wave rectification: the conversion of the opposing half of the incoming electron
flow to always move in the same direction, instead of discarding half the cycle.
 Uses four diodes in a high voltage circuit
o Diodes change negative current into positive current
 Allows exposure times to be cut in half
 4 rectifiers used in a full wave rectification circuit