RESISTANCE - XRAY PRODUCTION

Resistance

The amount of opposition to the current in the circuit

Ohm

Unit of resistance

Admittance

the measure of resistance in an AC

Conductance

the measure of resistance in a DC

The ability to conduct electrons, The length of the conductors, The cross-sectional diameter, The temperature

FACTORS AFFECTING RESISTANCE

Conductor

Allows the electrons to flow easily

Lower resistance

Insulator

Does not allow the electrons to flow

Higher resistance

Length of the Conductors

Directly proportional to the resistance

Length of Conductors

Increased Length of Wire: increases resistance

Decreased Length of Wire: decreases resistance

Cross-sectional Diameter

Inversely proportional to the resistance

Cross-sectional diameter

Large Diameter: small amount of resistance

Small Diameter: large amount of resistance

Temperature

Directly proportional to the resistance

Temperature

Higher Temperature: more resistance

Lower Temperature: less resistance

Rheostat

Potentiometer or choke coil

A variable or precision resistor

Used to vary voltage or current

Potentiometer or choke coil

other term for rheostat

To control current

main purpose of rheostat

Filament circuit

Location of rheostat

Electric Power

1 W: 1 A (current) x 1 V (voltage)

Formulas:

P = IV –

P = I2R (for power losses)

20-150 kW

Xray Imaging System

Watts

Electric Power is measured in?

Kilowatts

Power rating

/ 1,000

to get kW

Ohm’s Law

The voltage across the total circuit or any portion of the circuit is equal to the current times the resistance

Formula for Ohm’s Law

V = IR (for voltage)

R = V/I (for resistance)

I = V/R (for current)

Magnetism

One of the fundamental forces

Difficult to understand, detect & measure

No smallest units

Magnet

any material that produce magnetic field

Bipolar/ Dipolar

magnets that have two poles

Poles

ends of two poles

Orbital Magnetic Moment

Magnetic effect that is established by charged electrons orbiting the nucleus

A moving charged particle induces a magnetic field in a plane perpendicular to its motion

Spin Magnetic Moment

Magnetic effect that is established by movement of protons on their axes

A spinning charged particle will induce a magnetic field along the axis of spin

Basis for MRI

Magnetic Dipole

Groups of atoms with most of the magnetic moment force in a single or same direction

Single Dipole

10^15 atoms

Randomly oriented

Normal Orientation

Magnetic Domain

An accumulation of many atomic magnets with their dipoles aligned

Magnetic Lines of Flux

Lines of force, lines of flux or magnetic field

Imaginary lines of force of the magnetic field

The force field that are created when magnetic dipoles orient to create a magnet

Never intersect

Weber (Wb)

SI Unit of magnetic flux

10^8 lines of flux

1 Wb

Tesla (T) and Gauss (G)

the units of magnetic flux density

Tesla

considers area coverage

unit of flux densities

no. of lines x area

Gauss

scattered MRI

Fringe magnetic field

1T

10,000 G or 1 Wb/m2

Stronger MF

More lines of flux

Greater flux density

Outside a magnet

always flow from N to S

Within a magnet

always flow from S to N

0.0001 T, 1 G

Earth

0.1 T, 1000 G

Household Magnet

0.2-2.0 T, 3000-20000 G

Modern MRI

Natural Magnet, Artificial Permanent, Electromagnets

CLASSIFICATION OF MAGNETS BY TYPE OF PRODUCTION

Natural Magnet

A magnet (iron oxide or magnetite) that gets its magnetism from the Earth

Lodestone, Earth

Natural magnet

Artificial Permanent Magnet

A magnet whose magnetism is induced artificially

Manufactured from steel alloy (alnico) or rare earth ceramics

Compass

ARTIFICIAL PERMANENT MAGNET

Electromagnets

Consist of wire wrapped around with an iron core

Temporary magnet produced by moving electric current

Any flow of electric current produces a MF

Repulsion-Attraction, Inverse Square Law, Magnetic Poles

MAGNETIC LAWS

Repulsion-Attraction

Like magnetic poles repel

Unlike magnetic poles attract

South pole

Imaginary lines of magnetic field enter the

South to North

Lines of force flow from ____ pole within a magnet

North Pole

Imaginary lines of magnetic field leave the __

North to South

Lines of force flow from ___ pole outside the magnet

Inverse Square Law

The force between two magnetic fields

– directly proportional to the product of their magnitudes

– Inversely proportional to the square of distance between them

Magnetic Poles

Every magnet has two poles – North pole – South pole

• No matter how much a magnet is divided, both poles continue to exist

Permeability, Susceptibility, Retentivity

MAGNETIC CLASSIFICATIONS OF MATERIALS

Magnetic Permeability

The ability of a material to attract the lines of magnetic field intensity

The ease with which a material can be magnetized

Iron

deviates the MF

Wood

MF not disturbed

Magnetic Susceptibility

The degree to which a material can be magnetized

Iron

increase the strength of MF

Wood

does not increase the strength of MF

Magnetic Retentivity

The ability of a material to stay magnetized

Inversely proportional to magnetic permeability

High Retentivity

difficult to disorient the dipoles

Low Permeability

difficult to orient the dipoles

Nonmagnetic, Diamagnetic, Paramagnetic, Ferromagnetic

Four Magnetic States of Matter

Nonmagnetic

Unaffected by magnetic field

Wood, glass

Nonmagnetic

Diamagnetic

Weakly repelled from both poles of a magnetic field

Water, plastic

Diamagnetic

Paramagnetic

Weakly attracted to both poles of a magnetic field

Gadolinium

Paramagnetic

Ferromagnetic

can be strongly magnetized

Iron, nickel, cobalt

Ferromagnetic

Electromagnetic Induction

The production of a current in a conductor by a changing magnetic field near the conductor

Faraday and Oersted Experiments

The basis of transformer operation

Michael Faraday

He described the first law of electromagnetic induction

He observed the current in a changing magnetic field

Faraday Experiment

Changing magnetic field induces current

Magnetic field generates electricity

Ammeter

measures current

First Law of Electromagnetic Induction

It states that an electric current is induced in a circuit if some part of that circuit is in a changing magnetic field

Strength of MF, Velocity of MF, Angle of Conductor to the MF, Number of turns in conductor

FARADAY’S LAW

Strength of Magnetic Field

Directly proportional to the magnitude of the induced current

Strength of MF

Increased MF Strength: increases the magnitude of the induced current

Decreased MF Strength: decreases the magnitude of the induced current

Velocity of Magnetic Field

Directly proportional to the magnitude of the induced current

Velocity of MF

Increased speed: increases the magnitude of the induced current

Decreased speed: decreases the magnitude of the induced current

Angle of Condutor

MF 90o to the Conductor: increases the magnitude of the induced current

the conductor is crossing through the maximum number of field lines possible per second

Number of Turns

Directly proportional to the magnitude of the induced current

Number of Turns

More Turns: increases the magnitude of the induced current

Less Turns: decreases the magnitude of the induced current

Move the Conductor, Move Magnetic Lines of Force, Vary the MF Strength

THREE WAYS TO INDUCED ELECTRIC CURRENT IN THE CONDUCTOR

Move the Conductor

Through a stationary unchanging strength MF

Move Magnetic Lines of Force

Through a stationary conductor with an unchanging strength MF

Vary the MF Strength

causes relative motion to induced current

Hans Oersted

He demonstrated that electricity can be used to generate magnetic fields

Oersted Experiment

Moving or spinning charges induces magnetic field

Electricity generates magnetic field

Source of Electron, Appropriate Target Material, High Voltage, Vacuum

CONDITIONS NEEDED FOR X-RAY PRODUCTION