RADR 2209 Chapter 4

The ohm defined as

"the electrical resistance equal to the resistance between two points along a conductor that produces a current of 1 ampere when a potential difference of 1 volt is applied."

A volt defined as

"the potential difference that will maintain a current of 1 ampere in a circuit with a resistance of 1 ohm". - express of the difference in electric potential between 2 points. - A volt is the ratio of joules to coulombs (volt=joules/coulombs)

At the console in secondary circuit

- A large positive charge now resides with the anode and a large negative charge with the cathode within the x-ray tube - has sent a voltage through the step-up transformer, increasing it to the kilovoltage => a series of rectifiers => x-ray tube to produce x-ray

Magnetic Flux

- A magnetic field consists of lines of force in space called flux.

Electricity and magnetism are 2 parts of the same basic force

- A moving magnetic field can create an electric current. - Any flow of electrons (in space or in conductor) will be surrounded by a magnetic field.

Electronic devices

- Battery - Capacitor - Diode - Protective devices (fuses and circuit breakers) - Resistor or rheostat - Switch - Transformer

General X-ray circuit

- Divide into 3 main sections: the primary circuit, the secondary circuit, and the filament circuit.

3 laws of magnetism

- Every magnet has a north to south pole - Like poles repel each other and opposite poles attract each other. - The force of attraction or repulsion varies directly with the strength of the poles and inversely with the square of the distance between them.

Flux has 3 basic characteristics:

- Lines of flux travel from south pole to north pole inside the magnet and from north pole to south pole outside the magnet - Lines of flux in the same direction repel each other and lines of flux in the opposite direction attract each other. - Magnetic fields are distorted by magnetic materials and are unaffected by nonmagnetic materials.

Autotransformer

- Operates on the principle of self-induction - Has only one coil of wire around a central magnetic core serving as both the primary and secondary coil - The outside wires are attached at different points along the coil, and the induced voltage varies depending on where the connections are made.

Self-induction

- Requires an understanding of Lenz's law, which states that an induced current flows in a direction that opposes the action (the changing magnetic field) that induced it. - A magnetic field is created in a coil carrying electrical current and expands outward from the center of the coil - As it does so, it "cuts" through the turns of the coil and this act of "cutting" creates a current within the same conductor that opposes the original

The filament current is controlled by the:

- Technologist - mA selector - Variable resistor (rheostat)

X-ray Production Process

- The filament circuit draws electricity from the autotransformer, which then travels to the rheostat - rheostat is a variable resistor controlled by the mA selector. - From the rheostat, electricity then travels to the step-down transformer. - The adjusted current from the step-down transformer travel to filament located within the focusing cup of the x-ray tube. - current heats the filament to the point where electrons are literally boiled off -have a group, or cloud, of electrons created by the filament circuit (heating of the filament). -The kilovoltage applied to the x-ray tube created a large positive charge on the anode and a large negative charge on the cathode (focusing cup). -The large positive charge attracts the electrons => giving them tremendous kinetic energy - The large negative charge on the cathode keeps the electrons crowded together; - electrons travel across to the anode and interact there to produce x-rays until the timer circuit terminates the process

Mutual induction

- The induction of electricity in a secondary coil by a moving magnetic field - When a moving magnetic field is placed near a secondary coil, electricity is induced to flow in that coil

At the console in the primary circuit

- The radiographer has selected the exposure factors (kVp and mAs) among other parameters - a voltage is determined by the autotransformer => step-up transformer => filament circuit. - the voltage and current selected, the stage is set for the rest of the circuit.

Filaments coils

- Tiny coils of wire housed in the cathode of the x-ray tube - Represented as "large focal spot" and "small focal spot"

Automatic Exposure Control (AEC)

- Uses a device called an ionization chamber - Serves the same role as timer - device placed between the patient and the image receptor.

at the console in filament circuit

- a voltage and current is sent through the filament circuit. - The quantity is precisely controlled by a rheostat to reflect the mA selected and indicated - A step-down transformer is used to reduce this voltage and increase current. - This current heats the filament and begins boiling off electrons.

A resistor

- designed to inhibit the flow of electrons. - precisely regulating the flow of electricity through that part of the circuit where it is placed

Hans Oersted

- first identified the principle of electromagnetism. - discovered the magnetic field surrounding a conductor could be intensified by fashioning it into a coil (called solenoid) - Could be intensified further by adding an iron core to the coil (called an electromagnet)

Grounding

- is a process of neutralizing a charges=d object by placing it in contact with the earth. - Positively charged objects take on electrons from the earth, and negatively charged objects give up electrons to the earth until neutral.

2 forms of electromagnetic induction

- mutual induction - self induction

Electric potential (V)

- the ability to do work due to a separation of charges. Unit of measure in Volt (V) - flow from abundance to deficiency.

Principles of Circuit Operation

- the radiographer selects a technique that specifies kVp, mA, exposure time, and focal spot. - the unit is equipped with AEC or anatomic programming, those options are displayed. - The kVp selected adjusts the autotransformer and determines the number of turns => through self-induction => sent to the step-up transformer. -The step-up transformer increases this voltage -Solid-state rectifiers are used to route electricity through the x-ray tube - the electricity creates a large positive charge on the anode of the x-ray tube and a large negative charge on the cathode focusing cup.

The ampere defined as

-" 1 coulomb flowing by a given point in 1 second." - the amount of current flowing with an electric potential of 1 volt in a circuit with a resistance of 1 ohm. - Electricity behaves differently depending on the medium through which it travels.

Magnetic resonance imaging (MRI) units referred to by their magnetic field strength an operate with fields from

0.5 to 5T

What is the amperage for a circuit that has 5 volts and 5 ohms?

1

5 laws of electrostatics

1. Like charges repel & unlike charges attract 2. The electrostatic force between 2 charges is directly proportional to the product of their quantities. And inversely proportional to the square of the distance (also know as Coulomb's law) 3. Electric charges reside only on the external surface of conductors. 4. The concentration of charges on a curved surface of a conductor is greatest where the curvature is greatest. 5. Only negative charges (electrons) are free to move in solid conductors.

The amount of Resistance of particular conductor depends on 4 things

1. Materials: allow a free flow of current 2. Length: long conductor has more resistance than a short one (directly proportional to the length) 3. Cross-sectional area: conductor with large cross-sectional has a lower resistance bc there is greater external surface area that electrons can travel. 4. Temperature: become greater as the temperature rises

Rules for series circuits

1. Total voltage is equal to total current x total resistance: V=IR 2. Resistance is equal to the sum of the individual resistances R=R1+R2+R3 3. Current is equal throughout the circuit I1=I2=I3 4. Voltage is equal to the sum of the individual voltages V=V1+V2+V3

An object may be electrified in 3 ways:

1. friction 2. contact 3. induction

Single-phase, full-wave power (4 rectifiers)

100% ripple

Three-phase, full-wave rectification

13% to 3.5% ripple

What is the voltage in a circuit with 5 amps and 5 ohms?

25

High-frequency, full-wave rectification

< 1% ripple

Diode

A "one-way valve" device, allows electrons to flow in only one direction.

Current (I)

An expression of the flow of electrons in a conductor. Unit of measure in Ampere

Filament circuit

Consists of a rheostat, a step-down transformer, and the filaments

Primary circuit

Consists of the main power switch, circuit breakers, the autotransformer, the timer circuit, and the primary side of the step-up transformer

Secondary circuit

Consists of the secondary side of the step-up transformer, the mA meter, a rectifier bank, and the x-ray tube (except for the filaments)

Electric generators

Devices that convert some form of mechanical energy into electrical energy

What is the name of the device that only allows electrons to flow one direction?

Diode and Rectifier

Moving a conductor through a magnetic field induces ____________ in that conductor

Electric Current

Protective devices (fuses, circuit breakers)

Emergency devices that break or open the circuit if there is a sudden surge of electricity to the circuit or device.

Michael Faraday

Found that moving a conductor through a magnetic field induces an electric current in that conductor (called electromagnetic induction)

The timer circuit is located in the low voltage section because:

It is easier to control with low voltage

When dipoles or "atomic magnets" form groups of similarly aligned atoms, they create:

Magnetic domains

Half-wave rectification(2 rectifiers)

One half of the AC cycle flows through the x-ray tube and the other half is suppressed (100% ripple)

Battery

Produces electrons through a chemical reaction, stores an electric charge long term, and provides an electric potential

Which of the following is a variable resistor?

Rheostat

Capacitor

Temporarily stores an electric charges

The strength of the magnetic field is measured in the SI unit

Tesla (T)

Resistance (R)

The property of an element in a circuit that resists or impedes the flow of electricity. Unit of measure in Ohm

Alternating 3 phase currents

Three waveforms can be phased or synchronized

The mA meter in the secondary circuit monitors:

Tube current

Relationship of Voltage, Current, and Resistance (Ohm's Law)

V=IR The potential difference (voltage) across the total circuit or any part of that circuit is equal to the current (amperes) multiplied by the resistance (ohms)

Closed circuit

a closed pathway for electricity is required for electricity to flow (switch on)

Line compensator

a device usually wired to the autotransformer that automatically adjusts the power supplied to the x-ray machine to 220 volts

The rectifier commonly used in today's x-ray circuit

a solid-state rectifier (One is a p-type crystal and the other is an n-type crystal.)

the autotransformer

an adjustable transformer controlled by the kilovolt peak (kVp) selector on the operating console

Open circuit

an open pathway occurs when a switch is tuned off

Parallel circuit

circuit elements "bridge" or branch across a conductor.

Series circuit

circuit elements are wired along a single conductor

Electric circuit

closed pathway composed of wires and circuit elements through which electricity may flow

Semiconductors

conduct electricity but not well as conductors, and will insulate but not well as insulators.

rectifiers

convert AC to DC

mA meter

device placed in the secondary circuit that monitors x-ray tube current

Electric motors

devices that convert electrical energy to mechanical energy through electromagnetic induction

Transformers

devices use to increase or decrease voltage (or current) through electromagnetic induction. - a step-up transformer increase/decrease voltage.

Relationship between current and resistance

direct relationship

Which type of charge can move in solid conductors?

electrons

Nonmagnetic materials

glass, wood, plastic (not attracted to magnetic fields at all)

Good conductor (gold,copper)

have a large number of free electrons

Good insulators (wood, plastic, glass)

have few free electrons

the advantage of closed core and shell type transformer configurations?

incorporate a ferromagnetic core to maximize efficiency.

Step-down transformer

increase the current by reducing the voltage that is applied to the filament.

Relationship between voltage and current

inverse relationship

Ferromagnetic materials

iron, cobalt, nickel (strongly attracted to magnetic materials)

An X-ray circuit

is a complex circuit that has different voltages and current flowing through different sections.

A transformer

is a device that can increase or decrease voltage by a predetermined amount.

A switch

is a device that opens a circuit

mA

is an abbreviation for "milliampere" and is called "tube current"

AC (Alternating Current)

is current that changes direction in cycles as the electric potential of the source changes.

A rheostat

is simply an adjustable or variable form of resistor.

the time circuit (exposure timer)

located in this section because it is easier to control (turn on and off) a low voltage than a very high one - Electronic - mAs - Automatic Exposure Control (AEC)

Faraday's Law

magnitude of induced current depends on 4 factors: 1. strength of magnetic field 2. velocity of magnetic field as it moves past the conductor 3. angle of the conductor to the magnetic field 4. number of turns in the conductor

Electromagnetic induction

moving a conductor (such as copper wire) through a magnetic field induces an electric current in that conductor.

Paramagnetic materials

platinum, gadolinium, aluminum (weakly attracted to magnetic fields)

Tube circuit

primary and secondary circuit

circuit breakers

protect against short circuits and electric shock

The smallest unit of charge that exists is:

protons and electrons

Power switch

simply an on-off switch for the unit, connected to power supply of the facility

Magnetism

the ability of a material to attract iron, cobalt, or nickel.

Electrodynamics

the study of electric charges in motion

Electrostatics

the study of stationary electric charges. Measured in coulombs

In electrostatics, electrification occurs when...

they gain either a net positive or negative charge.

DC (Direct Current)

type of current that flow in one direction (Battery)

step-up transformer

used to increase the voltage from the autotransformer to the kilovoltage necessary for x-ray production

Rheostat

variable resistor controlled by the mA selector on the operating console

Diamagnetic materials

water, mercury, gold (weakly repelled by magnetic fields)