Distance 1. Two charges of the same sign, either positive or negative, always have a repulsive force between them. Two charges of opposite signs always have an attractive force between them.
The force between two charges increases as the Distance 2 strength of the charge increases. Likewise, the force between the charges increases as the distance between them decreases Fig.
If enough negative charges where F is the force, k is a proportionality constant, and d collect, they can cause ionization in the air and discharge is the distance between charges Q1 and Q2. The positive charges or protons are uncharged Fig. The electrons, which orbit the atom, can be easily possible. In a solid conductor, this occurs on the surface removed with the appropriate amount of force. Conductor Figure 2. This is also caused by the law of Electrons repulsion-attraction. Each negative charge is repelled Figure 2.
Electrostatic law of distribution. Negative by other negative charges, which causes the charges to charges will reside on the surface of a solid conductor. These concepts are further explored in Chapter Figure 2. Electrostatic law of concentration. Due to 3: Electric Currents and Electricity. These laws are used in radiographic imaging equipment, including image intensifier tubes, x-ray tubes, and image Case Study display systems.
These systems can only operate through John is being escorted into the MRI department the use of electricity, and electricity cannot exist without for an exam.
Prior to taking John into the exam utilizing the principles of electrostatics and moving nega- room, Elizabeth, the MRI technologist, asked tive charges. Radiographic equipment uses electricity to John to place his wrist watch, belt, keys, and any power equipment and to create, drive, and focus elec- metal object he had into a locker in the dressing trons within the x-ray tube and image intensifier tube.
This chapter has continued to present Which law is Elizabeth thinking about when foundational information that is essential for you she makes this request? Electromagnetic radiation ranges from low- MRI magnets are extremely powerful and energy, low-frequency, long-wavelength radio have the ability to attract ferromagnetic objects waves to high-energy, high-frequency, short- that are brought into the MRI exam room. If a wavelength gamma rays, all of which form an metallic object is brought into proximity of the electromagnetic spectrum.
Waves are character- MRI magnet, the powerful magnetic field lines ized by velocity, frequency, period, wavelength, of the magnet will attract the object and it will and amplitude.
The inverse square law describes become a flying projectile! This poses a lethal how electromagnetic radiation intensity changes risk to any person standing between the magnet with distance. As the distance increases, the and the metal object. You materials are allowed in the exam room. Spe- have learned about four types of materials cial wheelchairs, IV poles, and crash carts have ferromagnetic, paramagnetic, diamagnetic, been developed for safe usage in the MRI room.
What is the basic charged particle? Electron D. Neutron C. Volt 7. A spin magnetic moment is best described as D. Coulomb A. The first electrostatic law states that B.
Which statement is not one of the laws of magne- tism? Like poles repel A. Unlike poles attract B. Stronger magnets have larger poles C. The force between magnets decreases with the D. The frequency of an electromagnet wave is mea- A. The electric charges on a curved surface will concentrate 5. Which type of magnetic material is weakly repelled A. Diamagnetic C.
Ferromagnetic D. Paramagnetic D. Give an example of electrification by contact and Short Answer explain the principles of this type of electrifica- tion. Which type of electromagnetic radiation has the lowest energy and frequency but the longest wavelength? What properties of x-rays are suitable for produc- ing an image? Describe the change in intensity if the distance between two magnified objects is doubled?
If the frequency of a sine wave is decreased, what happens to the wavelength? How is the amplitude of a sine wave related to the energy of the wave? Which electrostatic law deals with the force between two objects? Briefly describe this law. Which magnetic material is not affected by a magnetic field? Identify the four types of electrical materials.
Describe the direction and movement of current flow. Define current, voltage, and electric power and identify their units. Distinguish between alternating and direct induction current. Describe current induction. Distinguish between electric generators and EMF motors. Semiconductors Introduction Semiconductors can act as either conductors or insulators, In Chapter 2, you learned about electrostatic depending on how they are made and their environment.
Rectifiers in an x-ray circuit are made of semiconduct- In this chapter, we will build on these prin- ing material. They conduct electrons in one direction but ciples as we study the movement of electric not in the other direction. Some semiconductors conduct charges or electricity. An understanding of the or insulate, depending on surrounding conditions.
A pho- underlying principles of electricity and electric todiode is a semiconductor that is an insulator in the dark current aids in understanding radiologic equip- but becomes a conductor when exposed to light. In this chapter, we identify different types of electrical materi- als and define current, voltage, resistance, and Superconductors electric power. We also discuss the difference Superconductors are materials that conduct electrons with between alternating current AC and direct zero resistance when they are cooled to very low tempera- current DC , and induction.
Superconductors are used to produce the magnetic fields in magnetic resonance imaging units. Table 3. Types of Electrical Materials Electrodynamics The study of moving electric charges is called electrody- There are four types of electrical materials: conductors, namics. Moving electric charges or electric current can insulators, semiconductors, and superconductors. Each occur in a variety of conditions. Electrons can move in a is discussed in detail below.
Of these, we will focus on the principles necessary for Conductors electrons to flow in a wire or metal conductor. Electrons move freely through a conductor. Tap water containing impurities and most metals are good electrical Movement of Electric Charges conductors. Copper and silver are very good conductors. Electric current will flow easily through conductors. Electric charges will move when an electrical poten- tial energy difference exists along a conductor. An electrical potential energy difference occurs when one Insulators end of a conductor has an excess of electrons while The electrons in an insulator are held tightly in place the other end has a deficiency of electrons.
When this and are not free to move. Rubber, wood, glass, and many occurs electrons will move from the area of excess to plastics are good insulators.
Electric current will not flow the area of deficiency, which causes an electric current in insulators. Type of Material Characteristics Examples. TABLE 3. A voltage increase X-ray filament 2—5 A results in an increase in current flow, just as higher water X-ray tube 50— mA pressure increases the amount of water flow. Electrons flow in response to the difference in pressure or potential difference PD in the circuit. Unit of Current Unit of Voltage An electric current is a flow of electrons over a set amount of time.
The milliampere mA is a smaller EMF. Therefore, the force with which electrons Diagnostic radiographic equipment uses a variety of mA move can be described by the terms PD, EMF, or volt- units to regulate the number of electrons needed to pro- age V. Higher voltages give electrons higher energies. Different current values are used in Voltages of 20, to , V are used in x-ray circuits different parts of x-ray circuits Table 3. One kilovolt kV is equal The filament of an x-ray tube is supplied with a high to 1, volts.
X-ray tubes are discussed more to exist, just as there can be water pressure in a pipe but completely in Chapter 6. Figure 3. Higher water pressure causes more water to flow, and higher volt- Direction of Current Flow age produces more current flow. When Ben Franklin was working with electricity, he spec- ulated about whether positive or negative charges come Resistance out of the battery. Unfortunately, he guessed wrong. He thought that positive charges flow from the positive termi- Resistance is the opposition to current flow in a circuit.
What really happens is that negative charges omega W. The composition of the circuit will determine electrons flow from the negative cathode to the posi- the amount of resistance that is present. There are four tive anode. We assume that cur- rent is flowing from positive to negative, even though we 1. Conductive material know that electrons are actually flowing in the opposite 2. Length of conductor direction. Cross-sectional diameter and electron flow in a wire.
Conductive Material Current direction As previously discussed, the conductive ability of the — — — — — — — — material will have a direct effect on the flow of electrons. Electron direction Electron Length Figure 3. In an electric circuit, electrons flow in one The length of the conductor is directly proportional to direction and positive current flows in the opposite direction.
Voltage is like water pressure in a hose. If the length doubles, the resistance will be halved. A wire with a large of a water pipe doubles, the resistance to the flow of water diameter has more area for electron flow and therefore will also double Fig.
This principle is utilized when it is desirable to Cross-sectional Diameter decrease the overall resistance in a wire while maintain- The cross-sectional diameter of the wire is inversely pro- ing the length of the wire Fig.
As the cross-sectional diameter Temperature When electrons flow along a conductor, heat is pro- duced. As the heat builds on the conductor the electrons Greater resistance due to length of tube, electrons slow down — — — — — — — — — — — — — — — — — — — — — — — — — — — — Greater resistance — — — — —— — — — — — — — — — — in the narrower tube.
Electron direction Electron direction. Illustrates the effect of cross-sectional area on resistance. Therefore, higher resistance leads to less cur- rent flow Fig. What is the resistance of a circuit if the voltage is German physicist Georg Ohm studied the relationships V and the current is 5 A? Higher voltage and lower resistance result in higher current flow.
As you can see, when one factor is changed it affects the remaining factors. What is the voltage in a circuit if current is A and I R resistance is 4 W? Shows the circle of Ohm. Remem- ber that V is always on top. Chapter 3: Electric Currents and Electricity One cycle per second is also known as one hertz Hz.
Currents Current flow is demonstrated by using a sinusoidal or sine Electric Power waveform. The vertical axis represents the amplitude of the current while the horizontal axis represents time. The Electric power is measured in watts W. Power is the rate sinusoidal wave is used to demonstrate current flow as either of energy used and describes the amount of work done or positive flow or negative flow.
In DC circuits, electrons flow the amount of energy used per second. This is true for both only in one direction. The waveform begins on the line or AC and DC circuits. One watt is produced by one ampere at zero amplitude when the electrons are at rest. As the elec- of current flowing with an electrical pressure of one volt. For DC the electron flow will where P, power in watts; I, current in amperes; and V, continue in this manner until the electricity is turned off.
PD in volts. AC flows half of the time in one direction, and the other half of the time in the other direction. Once the positive direction flow has stopped and the electrons are at rest, the electron flow then reverses and flows in the negative direc- The power expended in a light bulb in a V circuit tion with increasing potential until maximum is reached. In the United States, standard AC electric current has a frequency of 60 cycles per second. How much power does it use per second? Alternating and direct currents.
Single turn on coil Current. Electrons Many turns on coil. A magnetic field is always present around a wire that is carrying a current. Electromagnetism deals with the relationship between electricity and magnetism.
A current flowing in a conduc- B tor creates a magnetic field around the conductor. The accompanying magnetic field is a funda- core added mental property of electric currents. The strength of the magnetic field increases with an increase in the current.
The magnetic fields from different parts of the coil Electromagnet add together and increase the magnetic field in the center of the coil. Increasing the current or the number of turns — terminal in the coil produces a stronger magnetic field in the cen- ter of the coil.
This occurs because the magnetic fields will interact with each other, which creates a stronger cur- rent flow. Adding a ferromagnetic material such as iron C in the center of the coil also increases the magnetic field Figure 3. A Helix. The ferromagnetic iron becomes magnetized and its magnetic field lines interact with the electric field of the solenoid. This produces a powerful electromagnet, induction and is the basis of transformer operation. A which is utilized in radiographic equipment.
The electro- changing magnetic field produces an electric field. Electromagnetic induction is the production of the electromagnet will no longer produce current. There- a current in a conductor by a changing magnetic field fore, an electromagnet is a temporary magnet. The magnetic field must be chang- shows how the magnetic field in the center of a coil is ing. A steady magnetic field does not induce an electric increased by adding turns to the coil and adding ferromag- current.
The magnitude of the induced EMF depends on the number of magnetic field lines crossed or cut per sec- ond. There are four factors that regulate the strength of Electromagnetic Induction induced current. Maximizing these variables will pro- The relationship between current, a ferromagnetic core, duce the highest possible voltage for the materials used and changing magnetic fields is called electromagnetic in the conductor.
The strength of the magnetic field. Increasing the Current magnetic field strength will produce stronger field lines, which will then produce a more powerful EMF in the conductor.
The speed of the motion between field lines and the conductor. As the rate or speed at which the con- ductor crosses the magnetic field lines increases, a higher EMF will be produced because more field lines will be cut per second. Conductor 3. The angle between the magnetic field lines and the conductor. Magnetic field lines that are at a Magnetic field.
The number of turns in the conducting coil. Con- Figure 3. Demonstrates the three ways in which current can be induced in a conductor. The induced EMF is directly proportional to the number of turns in the coil. Mutual Induction An electric current can be induced in a conductor in Mutual induction occurs when two coils are placed close three ways. As the magnet approaches the conductor, the ond or secondary coil.
AC in an electromagnet produces magnetic field around the conductor increases. As the a changing magnetic field. As the magnetic field lines magnet moves away from the conductor, the magnetic expand and contract in the primary coil, they are provid- field decreases.
The change in the magnetic field as the ing the relative motion necessary to induce AC flow in magnet approaches and recedes induces a current to flow the secondary wire. This is the principle of transformer in the conductor.
A second way to induce a current is by moving a con- ductor near a stationary magnet. In this case, the magnetic field is stationary and the conductor moves. The relative Electric Generators motion between the magnet and the conductor is the same regardless of which moves and which is stationary.
In either case, a current is induced to flow in the conductor. The magnetic field from the electromagnet expands and contracts. This changing magnetic field induces a current to flow in the conductor. An electric generator converts mechanical energy into AC flowing in a conducting coil will produce a changing electrical energy. A simple generator is made of a con- magnetic field in the center of the coil.
In every case, the change in the magnetic field induces The conductor is a coil of wire called an armature, and the current.
The amount of induced current depends the armature is set between the opposing magnetic poles. Alternately expanding Mechanical crank and contracting magnetic field Magnetic field. Secondary circuit Primary circuit. Motion of wire. Current direction —. Mutual induction. When the primary coil is Figure 3. AC generator. Mechanical energy rotates supplied with AC, lines of force are induced in the primary the shaft that is attached to the armature.
As the armature coil. These expanding and contracting lines of force then rotates through the magnetic fields, it crosses through the interact with the nearby secondary coil, thereby inducing magnetic lines of force and produces electric current. AC flow in the secondary coil. As the armature is turned by a mechanical method, a A DC generator utilizes the same design as an AC current is induced in the armature that is moved through generator, except that the slip rings are replaced with a a magnetic field across or perpendicular to the magnetic commutator ring.
A commutator ring is a single ring that field lines. If the conductor or armature moves parallel is divided in half with the halves separated by an insu- to the magnetic field, there is no current induced in the lator. Each half of the commutator ring is connected conductor. The conductor must cut through the mag- to one end of the armature. The armature is turned netic field lines in order to induce a current.
It is crucial mechanically and moves through the magnetic field to note the method by which the induced current flows lines inducing a current. Although the action of the DC in a circuit. Each end of the armature is connected to generator is the same as an AC generator, the result- brushes, which allow constant contact with a set of slip ing sine wave is routed differently. As the commutator rings. The slip rings are stationary and allow the armature ring rotates the armature through the magnetic field, to rotate within the magnetic fields.
The induced cur- the polarity of each half of the ring will alternate. This rent flows from the armature through the slip rings to the allows the current to flow first in one direction and then brushes and finally through the circuit Fig. Thus, in the reverse direction resulting in current flowing out this type of generator produces AC. This is illustrated As the coil rotates in a magnetic field, the induced in Figure 3. No current is induced when the wire is moving parallel to the magnetic field.
The induced current is an AC. Addi- tional loops in the coil increase the voltage induced in Electric Motors the coil. In a generator, the mechan- A motor converts electrical energy into mechanical ical energy of rotation is converted into electric energy. Rotating coil of wire 0. Time produces an AC. Notice how the this direction. If this armature is placed in an be repelled by the external magnetic field if the external external magnetic field, or between the north and south lines of force are in the same direction as the magnetic poles of a horseshoe magnet, the external magnetic field lines of force surrounding the armature or attracted to the and the magnetic field of the coil interact.
The result will which the armature will rotate. The armature will either be that the armature is pulled upward or downward. DC generator. Magnetic motor. The components of an induction motor include field a rotor and a stator. The rotor is in the center of the induction motor and consists of copper bars arranged around a cylindrical iron core. The stator has an even number of stationary electromagnets placed around the rotor.
Both the rotor and the anode are sealed inside an evacuated glass envelope. The stator, located outside Motion the glass tube, is supplied with AC multiphase current of wire which produces changing magnetic fields by switching Current the current in each set of electromagnets. As the magnetic fields of the copper bars reach a point, they are equalized with the electromagnets mag- netic field, the next set of electromagnets is activated by Figure 3.
The simple electric motor has the same basic the multiphase current, thus forcing the rotor to follow construction as a DC generator, except there is an external the external magnetic fields of the stator, thereby pull- power source supplying current to the commutator ring.
The commutator ring directs the flow of current to the armature. This sequence contin- As the magnetic field of the armature is attracted to the ues to occur as long as there is multiphase current being external magnet, the armature will rotate through the exter- supplied to the stator.
At this point the Capacitors magnetic fields of the external magnet and armature are the same, which will cause the armature to be repelled by the external magnet, moving the armature out of the external Capacitors are used to temporarily store electric charge.
The armature will rotate degrees toward A capacitor consists of two conducting plates separated by the other pole of the external magnet, seeking alignment with an insulator. Positive charges flow magnetic field. As the armature moves into the magnetic to one plate; negative charges flow to the other plate.
The field, the commutator will again switch the current flow to capacitor will accept a charge until it equals the DC volt- force the armature to rotate again. This process will continue repeatedly keeping the armature rotating continuously. Covering the big picture, expert authors Arlene M. Adler and Richard R. Carlton provide a complete overview of the radiologic sciences professions and of all aspects of patient care.
More than photos and line drawings clearly demonstrate patient care procedures. Step-by-step procedures make it easy to follow learn skills and prepare for clinicals. Chapter outlines and objectives help you master key concepts. Key Terms with definitions are presented at the beginning of each chapter. Up-to-date references are provided at the end of each chapter. Appendices prepare you for the practice environment by including practice standards, professional organizations, state licensing agencies, the ARRT code of ethics, and patient's rights information.
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Essentials of radiologic science Item Preview. EMBED for wordpress. Want more? Advanced embedding details, examples, and help! The text is a guide to the fundamental principles of medical imaging physics radiation protection and radiation biology with complex topics presented in the clear and concise manner and style for which these authors are known. Coverage includes the production characteristics and interactions of ionizing radiation used in medical imaging and the imaging modalities in which they are used including radiography mammography fluoroscopy computed tomography and nuclear medicine.
Special attention is paid to optimizing patient dose in each of these modalities.
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