Electricity, Magnetism, and Electromagnetism Lecture Notes PDF

Electricity, Magnetism, and Electromagnetism Dr. Nahla Atallah  THIS CHAPTER on electricity, magnetism, and electromagnetism briefly introduces the basic concepts needed for further study of the x-ray imaging system and its various components. Because the primary function...

Electricity, Magnetism, and Electromagnetism Dr. Nahla Atallah  THIS CHAPTER on electricity, magnetism, and electromagnetism briefly introduces the basic concepts needed for further study of the x-ray imaging system and its various components. Because the primary function of the x-ray imaging system is to convert electric energy into electromagnetic energy-x-rays-the study of electricity, magnetism, and electromagnetism is particularly important.  Electrostatics is the science of stationary electric charges. Electrodynamics is the science of electric charges in motion. Electromagnetism describes how electrons are given electric potential energy (voltage) and how electrons in motion create magnetism. Magnetism has become increasingly important in diagnostic imaging with the application of magnetic resonance imaging (MRI) as a medical diagnostic tool.  Electrostatics is the science of stationary electric charges. Electrodynamics is the science of electric charges in motion. Electromagnetism describes how electrons are given electric potential energy (voltage) and how electrons in motion create magnetism. Magnetism has become increasingly important in diagnostic imaging with the application of magnetic resonance imaging (MRI) as a medical diagnostic tool.  The primary function of an x-ray imaging system (Figure 4-1 ) is to convert electric energy into electromagnetic energy  ELECTROSTATICS Electric charge comes in discrete units that are positive or negative. Electrons and protons are the smallest units of electric charge. The electron has one unit of negative charge; the proton has one unit of positive charge. Thus, the electric charges associated with an electron and a proton have the same magnitude but opposite signs.  Because of the way atoms are constructed, electrons often are free to travel from the outermost shell of one atom to another atom. Protons, on the other hand, are fixed inside the nucleus of an atom and are not free to move. An object is said to be electrified if it has too few or too many electrons. The smallest unit of electric charge is the electron. This charge is much too small to be useful, so the fundamental unit of electric charge is the coulomb (C): 1 C = 6.3 x 1018 electron charges.  Electrostatic Laws Associated with each electric charge is an electric field. The electric field points outward from a positive charge and toward a negative charge. Uncharged particles do not have an electric field. In Figure 4-6, lines associated with each charged particle illustrate the intensity of the electric field. When two similar electric charges-negative and negative or positive and positive---are brought close together, their electric fields are in opposite directions, which cause the electric charges to repel each other.  When unlike charges-one negative and one positive are close to each other, the electric fields radiate in the same direction and cause the two charges to attract each other. The force of attraction between unlike charges or repulsion between like charges is attributable to the electric field. It is called an electrostatic force.  Electrostatic Laws The force of attraction between unlike charges or repulsion between like charges is attributable to the electric field. It is called an electrostatic force. Coulomb's Law. The magnitude of the electrostatic force is given by Coulomb's law as follows:  Electric Potential A system that possesses potential energy is a system with stored energy. Such a system has the ability to do work when this energy is released. Electrons bunched up at one end of a wire create an electric potential because the electrostatic repulsive force causes some electrons to move along the wire so that work can be done. Electric potential is sometimes called voltage; the higher the voltage, the greater is the potential to do work. The volt is potential energy/unit charge, or joule/coulomb ( 1 V = 1 j/C).  ELECTRODYNAMICS Electrodynamics is the study of electric charges in motion. We recognized electrodynamic phenomena as electricity. If an electric potential is applied to objects such as copper wire, then electrons move along the wire. This is called an electric current, or electricity.  A conductor is any substance through which electrons flow easily. Most metals are good electric conductors; copper is one of the best. Water is also a good electric conductor because of the salts and other impurities it contains. An insulator is any material that does not allow electron flow. Glass, clay, and other earthlike materials are usually good electric insulators  MAGNETISM The word magnetism comes from the name of that ancient village Magnesia. Magnetism is a fundamental property of some forms of matter. Magnetism is perhaps more difficult to understand than other characteristic properties of matter, such as mass, energy, and electric charge, because magnetism is difficult to detect and measure.  We can feel mass, visualize energy, and be shocked by electricity, but we cannot sense magnetism. Any charged particle in motion creates a magnetic field. The magnetic field of a charged particle such as an electron in motion is perpendicular to the motion of that particle. The intensity of the magnetic field is represented by imaginary lines (Figure 4-15).  If the electrons motion is a closed loop, as with an electron circling a nucleus, magnetic field lines will be perpendicular to the plane of motion (Figure 4-16).  Spinning electric charges also induce a magnetic field (Figure 4-17). The proton in a hydrogen nucleus spins on its axis and creates a nuclear magnetic dipole called a magnetic moment. This forms the basis of MRI. Artificially produced permanent magnets are available in many sizes and shapes but principally as bar or horse shoe-shaped magnets, usually made of iron.  Electrons behave as if they rotate on an axis clockwise or counterclockwise. This rotation creates a property called electron spin.  Electromagnets consist of wire wrapped around an iron core. When an electric current is conducted through the wire, a magnetic field is created. The intensity of the magnetic field is proportional to the electric current.  Nonmagnetic materials: Many materials are unaffected when brought into a magnetic field. Such materials are nonmagnetic and include substances such as wood and glass  Diamagnetic materials are weakly repelled by either magnetic pole. They cannot be artificially magnetized, and they are not attracted to a magnet. Examples of such diamagnetic materials are water and plastic. Ferromagnetic materials include iron, cobalt, and nickel. These are strongly attracted by a magnet and usually can be permanently magnetized by exposure to a magnetic field.  An alloy of aluminum, nickel, and cobalt called alnico is one of the more useful magnets produced from ferromagnetic material Paramagnetic materials lie somewhere between ferromagnetic and nonmagnetic. They are very slightly attracted to a magnet and are loosely influenced by an external magnetic field. Contrast agents used in MRI are paramagnetic  When wood is placed in a strong magnetic field, it does not increase the strength of the field: Wood has low magnetic susceptibility. On the other hand, when iron is placed in a magnetic field, it greatly increases the strength of the field: Iron has high magnetic susceptibility.  Magnetic Induction Just as an electrostatic charge can be induced from one material to another, so too some materials can be made magnetic by induction. The imaginary magnetic field lines just described are called magnetic lines of induction, and the density of these lines is proportional to the intensity of the magnetic field.  When ferromagnetic material, such as a piece of soft iron, is brought into the vicinity of an intense magnetic field, the lines of induction are altered by attraction to the soft iron, and the iron is made temporarily magnetic (Figure 4-25). If copper, a diamagnetic material, were to replace the soft iron, there would be no such effect.  This principle is used with many MRI systems that use an iron magnetic shield to reduce the level of the fringe magnetic field. Ferromagnetic material acts as a magnetic sink by drawing the lines of the magnetic field into it. When ferromagnetic material is removed from the magnetic field, it usually does not retain its strong magnetic property. Soft iron, therefore, makes an excellent temporary magnet. It is a magnet only while its magnetism is being induced.  If properly tempered by heat or exposed to an external field for a long period, however, some ferromagnetic materials retain their magnetism when removed from the external magnetic field and become permanent magnets. The electric and magnetic forces were joined by Maxwell’s field theory of electromagnetic radiation. The force created by a magnetic field and the force of the electric field behave similarly.  This magnetic force is similar to electrostatic and gravitational forces that also are inversely proportional to the square of the distance between the objects under consideration. If the distance between two bar magnets is halved, the magnetic force increases by four times.  ELECTROMAGNETISM A charge at rest produces no magnetic field. Electrons that flow through a wire produce a magnetic field about that wire. The magnetic field is represented by imaginary lines that form concentric circles centered on the wire (Figure 4- 29).  Magnetic field lines form concentric circles around each tiny section of a loop of the wire. Because the wire is curved, however, these magnetic field lines overlap inside the loop. In particular, at the very center of the loop, all of the field lines come together, making the magnetic field strong (Figure 4-30). Stacking more loops on top of each other increases the intensity of the magnetic field running through the center or axis o the stack of loops. The magnetic field of a solenoid is concentrated through the center of the coil (Figure 4-31).  The magnetic field can be intensified further by wrapping the coil of wire around ferromagnetic material, such as iron. The iron core intensifies the magnetic field. In this case, almost all of the magnetic field lines are concentrated inside the iron core, escaping only near the ends of the coil. This type of device is called an electromagnet (Figure 4-32).