Basic Electricity and Magnetism Presentation PDF

electricity a phenomenon associated with the presence and motion of electrons and other charged particles electric current the directional motion of electrons electrostatics deals wit...

electricity a phenomenon associated with the presence and motion of electrons and other charged particles electric current the directional motion of electrons electrostatics deals with stationary charged particles magnetism the effect of moving electrons electromagnetism magnetism due to electric current atomic structure Electrons are present in every material and its motions are usually illustrated together with protons and neutrons within an atomic structure. John Joseph Thomson (1856-1940) discovered the electron in 1897, which he initially called corpuscles, meaning a living cell elementary particles Electrons – negatively charged particles Protons – positively charged particles Neutrons – electrically neutral (no charge) Mass of proton – 1836 times the mass of electron elementary particles Particle Charge, C Mass, kg Charge to mass ratio, C/kg Electron Proton Neutron none N/A structure of matter The elementary particles are basic form of matter and as they combine they form another matter, the atom; and as atoms combine forms yet another different matter. structure of matter Matter – anything in the universe that has mass, occupies space, and is convertible to energy Atom – a substance consisting of the basic particles, electrons, protons, and neutrons. As atoms combine they form either an element or a compound structure of matter Element – substance consisting of atoms of only one kind. This is considered as the elementary (irreducible) chemical identity of materials Compound – a combination of two or more different atoms or elements. Most of the insulators are compound structure of matter Molecule – the smallest part of a compound or material that retains all the properties of the compound Atomic number – represents the number of protons in the nucleus of an atom which in a neutral atom equals the number of electrons outside the nucleus. This number determines the place of the element in the periodic table of elements Niel Henrik David Bohr (1885-1962) the Danish physicist who developed a new model of atomic structure called the Bohr Atomic Model in 1913. Bohr atomic model The maximum number of electrons (e-) that can occupy a given shell or the nth shell can be approximated by: where: n is the nth shell Bohr atomic model Energy level – the farther the electron from the nucleus, the higher its energy level Valence shell – the outermost shell or the last shell. This shell or orbit is filled with the remaining electrons. Bohr atomic model Valence electrons – electrons that occupies the valence shell or the last shell Free electrons – originally valence electrons. As they gain enough energy they escape from the valence shell and become free. electrical classifications of material The number of valence electrons is a common indication that tells us the electrical characteristics of a material conductor – material with less than four valence electrons; allows electrical current to flow easily because they have more free electrons insulator – material with more than four valence electrons; will not allow electrical current to flow easily because they have very few or even no free electrons semiconductor – with exactly four valence electrons; have electrical characteristics in between conductors and insulators energy bands Before a valence electron can escape from its shell and becomes free, it must gain energy of at least equal to the energy gap. energy gap – the energy difference between the valence band and conduction band. Its unit is the electron volt (eV) valence band – the region where the valence shell and valence electrons are occupying. It is the highest energy level before conduction band conduction band – the region where free electrons are said to be present. Electrons at this band have a higher energy level than those electrons at the valence band. forbidden band – the region in an atom where no electrons exist. It is in between two allowed bands, such as between valence and conduction bands. electron volt – a unit of energy equal to the energy gained by an electron in passing from a point of low potential to a point one volt higher in potential energy gap of different materials Means that the valence electrons can easily become free. This explains why conductors have the most number of free electrons and can easily support electric current flow electric charge (Q) – a fundamental property of matter and is influenced by elementary particles such as electrons and protons kinds of electric charges Positive charge – carried by protons Negative charge – carried by electrons coulomb (C) – unit of electric charge; named after the French physicist, Charles Augustin de Coulomb (1736- 1806) conservation of charge – the total or net electric charge in an isolated system always remains constant conservation of charge-energy – electric charge is neither created nor destroyed but is transferred from one body to another charged atom and charged body Basically, atoms are electrically neutral (balance) which means the number of negatively charged electrons and the number of positively charged protons are equal. This is uncharged atom and the material whose atoms are uncharged is called uncharged body. ions Anion – negatively charged ion Cation – positively charged ion cation Atom that loses electron lacks negative charge and the atom becomes positively charged ion, cation. Electropositive elements – elements that give up electrons in chemical reactions to produce positive ions. These elements are metallic in nature. anion Atom that gains electron will have more negative charge and the atom becomes negatively charge ion, anion. Electronegative elements – elements that accept electrons in chemical reactions to produce negative ions. These elements are nonmetallic in nature. electric field and electric force When the body is electrically charged, it is said to have electric field in its surroundings. This field interacts with other charged bodies and will produce an electric force that may cause them to move. electric field and electric force Electric field – the area or region surrounding an electrically charged particle or body Electric force – the force produced due to the electric field of a charged particle or body electrical potential – the ability of a charged body to do work on charged particles such as electrons. The fact that charged bodies tend to move charged particles, it is said to have a capacity to do work or it has a potential to do work. This is also called electrical potential energy. electrical potential Electrical potential difference – the difference between the capacities (potentials) of two charges to do work Volt – the unit of potential difference. A potential of one volt has the capacity to do one joule of work in moving one coulomb of charge; named after the Italian physicist, Alessandro Volta (1745- 1827) in 1881. electrical potential Voltage – another name of potential difference expressed in volts Electromotive force (EMF) – the electrical force that moves the charged particles such as electrons (electron moving force). The term emf is used interchangeably with potential difference and voltage Count Volta Count Alessandro Giuseppe Antonio Anastasio Volta invented the voltaic pile, the first electric cell, in 1796. Using his voltaic pile, he produced a continuous electric current for the first time on earth. electric current – any directional movement of electric charges such as electrons electric current Current in gases and liquids – generally consists of a flow of positive ions in one direction together with a flow of negative ions in the opposite direction electric current Current in solids – consists of the flow of electrons and is a measure of the quantity of charge passing any point of the wire per unit of time Example: 1. When one coulomb charge continuously passes a given point every second, the electric current is said to be? 2. Solve for the current when a charge of 5 coulomb travel per minute. Example: 1. When one coulomb charge continuously passes a given point every second, the electric current is said to be? Ans : 1 A 2.. Solve for the current when a charge of 5 coulomb travel per minute. Ans : 83.33mA ampere – the unit of electric current. Current of one ampere is equal to one coulomb of charge flows a given point in one second; named after French physicist and mathematician Andre M. Ampere (1775-1836) current density – the current per unit cross- sectional area Example 1. A total of 72 Coulombs of  Answer charge is transmitted by a silver wire 1.3 mm in  I= 17.143mA diameter over a time of 1  J=12,915.47 A/m2 hour and 10 minutes. What is the current in the wire and the current density in the wire? current Direct current – charges flow in one direction only Alternating current – the motion of electric charges is periodically reversed Conventional current – the assumption which considered the flow of charge from positive to negative. This is opposite to the actual charge flow negative to positive material resistance – the ability of a material to oppose or block the flow of charge or current material resistance Basically, the resistance of a material depends on its dimensions and type. where: R – resistance ρ – resistivity in Ω-cm or Ω-m L – length in cm or m A – cross-sectional area in cm2 or m2 Examples: 1. A 100 m long wire with a 2. A 50 cm piece of cross-sectional area A=10 nichrome wire is used in exp -3 m2 has a resistance a home heating element. of 10 ohms. Determine the The cross sectional area resistivity of the wire. of the wire is 0.05 mm2. What is the resistance of the heating element ( Nichrome = 100 x 10-8 - m) Examples: 1. A 100 m long wire with a cross- 2. Answer : sectional area A=10 exp -3 m2 has a resistance of 10 ohms. Determine the resistivity of the R = 10 ohms. wire. Ans: 10 exp -4 ohm-m temperature effect Change in resistance due to change in temperature or temperature effect Resistance at new temperature where: – the change in resistance due to temperature change or – the initial resistance at temperature – the final resistance at temperature – the initial temperature – the final temperature – the temperature-resistance coefficient or temperature coefficient of resistance at Examples: A wire has a resistance of 5 ohms at room temperature and a temperature coefficient α = 4 x 10 exp -3 / C, calculate the wire resistance at 75 degrees Celsius. Examples: A wire has a resistance of 5 ohms at room temperature and a temperature coefficient α = 4 x 10 exp -3 / C, calculate the wire resistance at 75 degrees Celsius. Ans: 5.96 ohms electrostatics – deals with phenomena due to attractions or repulsions of electric charges that are not moving. properties of electric force According to Charles Augustin de Coulomb (1736-1806) French physicist, the electric force for charges at rest has the following properties: 1. The size of the force of attraction or repulsion between two charges is directly proportional to the value of each charge (Coulomb’s first law of electrostatics) properties of electric force 2. The size of the force varies inversely as the square of the distance between the two charges (Coulomb’s second law of electrostatics) 3. The attraction or repulsion acts along the line between the two charges 4. Like charges repel each other, unlike charges attract. Thus, two negative charges repel one another, while a positive charge attracts a negative attracts a negative charge Coulomb’s law Coulomb’s law or Law of Electrostatics The force F between two electrical charges is directly proportional to the product of the charges and inversely proportional to the square of the distance between them. This is the sum of Coulomb’s first and second laws. Examples: Determine the force in Newton between 4 micro Coulomb charge s separated by 0.1 meter in air. Examples: Determine the force in Newton between 4 micro Coulomb charge s separated by 0.1 meter in air. Ans: 14.4 N Coulomb’s law The unit of force F depends on the units of other factors. The table below gives the common units used. Force, F Charge, Q Distance, r Constant, k Newton Coulomb, C Meter (SI) Dyne Electrostatic cm 1 (in vacuum) (cgs) unit, esu or statcoulomb Coulomb’s law – permittivity – free space permittivity – relative permittivity (permittivity of materials) Coulomb’s law Other expression of constant k, (SI) where: (speed of light) quick facts When two or more charges exert forces simultaneously on another charge, the total force acting on that charge is the vector sum of the individual forces exerted by each charge. This is known as the principle of superposition. example: Three charges of +5 C, -6 C and +7 C are inside a sphere, what is the total electric flux passing through the surface of the sphere? example: Three charges of +5 C, -6 C and +7 C are inside a sphere, what is the total electric flux passing through the surface of the sphere? Ans +6 C quick facts Electric charges should be at rest during the calculation of forces. When the charges are in motion the forces are different. quick facts In using Coulomb’s law, there should be no matter in between charges, the matter will cause an erroneous result. quick facts The force F will cause another charged particle to move and is therefore can be considered as electromotive force. The force F is a vector quantity. electric field and electric force Electric field – the region of space around an electrically charged body; exerts a force on another charged body and is usually illustrated by imaginary lines called electric field lines of force or simply lines of force. electric field and electric force The strength of an electric field E at any point may be defined as the electric force F exerted per unit positive electric charge Q at that point. Example: 1. Calculate E @ M (3,-4,2) in free space caused by (a) a charge Q1 = 2 μC @ P1 (0,0,0);(b) a charge Q2 = 3 μC at p2 (-1,2,3); (c) a charge Q1= 2μC at p1 (0,0,0) and a charge Q2 = 3μC at p2 (-1,2,3).  z E a.) M (3, -4, 2) y P(0,0,0) Q1 = 2 μC x E z b.) M (3, -4, 2) P(-1,2,3) Q2 = 3 μC y x c.) E z M (3, -4, 2) P(-1,2,3) Q2 = 3 μC y P(0,0,0) Q1 = 2 μC x SOLUTION: electric field and electric force The concept of representing the electric field with lines was introduced by Michael Faraday (1791-1867), an English chemist and physicist. determination of electric field To detect the presence of electric field at a particular region, another charged body, usually a test charge (q) is placed and if the test charge experiences a force of electrical origin, then an electric field is present at that region. quick facts Every electrically charged body will produce an electric field. quick facts Electric field at any point in the region is directly proportional to the charge and inversely to the square of the distance. When charge is static, the field produced is known as electrostatic field. quick facts Electric field is a vector quantity but not a single vector, it is an infinite set of vector quantities called vector field. quick facts The direction of field lines depends on the charge, and it is directed outward from a positive charge and inward to a negative charge. quick facts The number of field lines or line density is directly proportional to the electric field; more lines should be drawn for strong fields and less for weak fields. quick facts Field lines never intersect and in a uniform field lines are straight, parallel, and uniformly or equally spaced. quick facts Electric field will bring about a force to any other charged body or particle within its vicinity. The force direction is the same as with the direction of field lines. quick facts The equations above are derived from Coulomb’s law and are therefore applicable only directly to point charges. For distributed charges such as charges in a long wire or a plane, Gauss’s law is more appropriate. magnetism The property of a device or material to attract bodies of iron and other magnetic materials or magnet. electromagnetism Magnetism due to electric charges that are moving such as the flow of electric current. electromagnetic induction The production of electric current, potential or voltage due to magnetism. atomic theory of magnetism Magnetism is the effect of moving charged particles such as motion in an atom. In atoms of most elements, the magnetic forces produced by its charged particles, electrons and protons cancel each other and produce a very small or zero net magnetic force. They are called nonmagnetic materials. atomic theory of magnetism The common elements whose magnetic forces do not cancel completely and is externally significant are iron, nickel and cobalt and are called magnetic materials. atomic theory of magnetism In iron, nickel and cobalt, the molecules arrange themselves into magnetic entities called domains. Domains are completely magnetized. three domain directions of magnetization Where iron, nickel or cobalt is exposed to a magnetic field of force, or magnetizing force (H), its domains will align in three possible directions. three domain directions of magnetization Easy – the domains direction of alignment when exposed to a weak magnetic field of force. Semi-hard – the domains direction of alignment when exposed to a stronger magnetic field of force. Hard – the domains direction of alignment when exposed to a very strong magnetic field of force which causes saturation. saturation the situation where any increase in the amount of the magnetizing force will have very little magnetic effect of the material magnetic field (B) the space around a magnetic pole or magnetized body. This field causes other materials to become magnetized or at least exerts a force on moving electric charge Magnetic flux density( B) The magnetic flux density is the magnetic flux per unit area of a section perpenpendicular to the direction of flux.  The equation of flux density is:  B Where: A B= magnetic flux density in Tesla Φ= mgnetic flux, Wb A= area in square m Example What is the flux density in tesla when there exist a flux of 600 microWeber through an area of 0.0003 square meter.  Example What is the flux density in tesla Answer : 2T when there exist a flux of 600 microWeber through an area of 0.0003 square meter.  magnetizing force (H) the intensity of the magnetic field that causes a material to become magnetized or that causes the magnetic domains in a material to align and become magnetized also called magnetic field intensity, field intensity, magnetic intensity or magnetic field strength magnetizing force (H) If a coil with a certain number of Ampere-Turns (NI) ampere-turn s is stretched out to twice its original length, the Is known a the mmf. intensity of the magnetic field , The more the current the that is the concentration of lines of force , will be half as great. Thus stronger the magnetic field. for magnetic field intensity H NI l Where: H= field intensity, At/m NI= Ampere-turns. At l= length between poles of the coil in meter. Example Calculate the ampere-turn for a A. Find the field intensity of coil with 1500 turns and a 4 mA current. a 40 turn, 10 cm long coil with 3 A flowing in it. B. If the same coil is stretched to 20 cm with the wire length and current remaining the same, what is the new value of field intensity? Example Calculate the ampere-turn for a A. Find the field intensity of coil with 1500 turns and a 4 mA current. a 40 turn, 10 cm long coil with 3 A flowing in it. ANSWER: 6 At Answer:1200At/m B. If the same coil is stretched to 20 cm with the wire length and current remaining the same, what is the new value of field intensity? Answer: 600 At/m magnetization curve (B-H curve) The B-H curve depicts the ability of a material to accept, allow or set-up a magnetic field as it is subjected to a magnetizing force. permeability of materials In the absence of a B-H curve of a material, its ability to accept , allow or set up a magnetic field is described by a numerical value called permeability, of a material, which is the ratio of the magnetic field B to the magnetizing force H. relative permeability the ratio of the permeability of material to the permeability of vacuum or air where: – the permeability of vacuum or air, also called free space permeability, equals to magnetic materials compared Ferromagnetic and Paramagnetic Diamagnetic ferromagnetic Very strong attractive Very low attractive effect Very low repellent effect effect (domains easily (domains easily align with (domains turns away with align with the magnetizing the magnetizing force H) the magnetizing force H) force H) With relative permeability With relative permeability With relative permeability very much greater than 1 slightly greater than 1 slightly less than 1 Common materials: iron, Common materials: Common materials: nickel, cobalt, ALNICO, aluminum, chromium, bismuth, antimony, permalloys, ferrites, and manganese, platinum, copper, silver, gold, zinc, magnetic oxides and carbon and mercury ferromagnetic and ferrimagnetic Ferromagnetic Ferrimagnetic In ferromagnetism all domains align in In ferrimagnetism some domain are parallel anti-parallel Common materials are mostly Common materials are mostly conductors: iron, nickel, cobalt, insulators: ferrites and other magnetic ALNICO, permalloys, and steel oxides that are used as core materials including the powered iron core used in coils operating at microwave in some radio frequency coil. frequency. types of magnets Magnets are made of ferromagnetic or ferrimagnetic materials. types of magnets Natural magnets – a natural material that exhibits permanent magnetism such as lodestone or magnetite Artificial magnets – produce by exposing or subjecting a magnetic material into magnetizing force. There are two types of artificial magnets. types of artificial magnets Permanent artificial magnets or permanent magnets Temporary artificial magnets or temporary magnets some properties of artificial magnets Permanent Temporary Made of hard magnetic material Made of soft magnetic material Hard to demagnetized; requires higher Easy to demagnetized; requires lower coercive force coercive force With high retentivity With low retentivity With higher residual magnetism With lower residual magnetism With higher hysteresis With lower hysteresis Magnetic domain alignment is well held Magnetic domains are easily rolled back even if the demagnetizing force is when the magnetizing force is removed. removed. They won’t easily roll back. Other materials: cobalt steel, nickel- Other materials: soft iron, pure iron and aluminum steels, and other special steels, iron oxides such as manganese ferrite. hardened steels, or cast iron. Also ticonal Insulators are used for magnetic cores in (titanium, cobalt, nickel, aluminum) many applications; these are called ferrite cores or ferrites. Hipernik (an alloy of 50% nickel Used in meters, headphones, Used in transformers, chokes, relays, and loudspeakers, radar transmitting tubes circuit breakers magnetic hysteresis The delayed reaction of the magnetization of a ferromagnetic material with the change of the magnetizing force is called hysteresis. quick facts When a ferromagnetic material is completely demagnetized, it means that there is no magnetic field (B=0) within its surroundings. quick facts When magnetizing force H is applied into a demagnetized ferromagnetic material, magnetic field or flux density B rises. As H is continuously increased, B also increases until the material saturates. quick facts When saturation is reached, further increase of H will have very little increase in B. Practically this is the point of maximum flux density or magnetic field. quick facts From saturation, when the magnetizing force is decreased until H=0, flux density B also decreases but it will not drop to zero as H drops to zero. There will be some magnetic field left even if the magnetizing force is zero. quick facts The magnetic field or flux density B left after the removal of the magnetizing force (H=0) is called remanence or residual magnetism. quick facts To completely demagnetize the material, the residual magnetism must be counteracted by opposite magnetizing force. The amount of force that can bring residual magnetism to zero is called the coercive force. quick facts A material with higher residual magnetism is said to have good retentivity or remanence, the ability to retain magnetism when magnetizing force is removed. quick facts Permanent magnets are constructed from materials with good retentivity, while temporary magnets with low retentivity. hysteresis loop When a ferromagnetic material is subjected to a magnetizing force created by an alternating current, the B-H curve when plotted will form a close loop called hysteresis loop. quick facts All ferromagnetic materials have hysteresis loops, some small and some wide. quick facts Hysteresis loops provide information regarding the materials saturation, coercive force, and residual magnetism or retentivity. But more importantly, the hysteresis loops provide information about hysteresis loss. magnetic hysteresis loop lagging of the magnetization of a ferromagnetic material such as iron, behind variations of the magnetizing field magnetic circuit A closed path to which a magnetic field represented as lines of magnetic flux is confined. magnetic circuit parameters Magnetomotive force, mmf – the magnetic force that tends to set up magnetic flux. This force is produced due to the applied electric current (I) in the coil of N turns. magnetic circuit parameters Reluctance ( ) – the opposition offered in a magnetic circuit to magnetic flux flow. magnetic field strength (H) Reluctance – the opposition offered in a magnetic circuit to the flow of magnetic flux Permeance – the reciprocal of reluctance magnetic field strength (H) Permeability – the ability of a material to allow magnetic flux to flow Reluctivity – reciprocal of permeability reluctance in series reluctance in parallel magnetic circuit parameters Magnetic flux ( ) where: – the lines of force representing – magnetic flux (Weber) magnetic induction. – magnetomotive force, mmf (ampere-turn) – number of turns of the coil (turns) – electrical current flowing in the coil (ampere) – reluctance (1/Henry) EXAMPLE A coil has an emf of 500 At and a reluctance of 2000000 At/Wb. Compute the total flux. EXAMPLE A coil has an emf of 500 At and a reluctance of 2000000 At/Wb. Compute the total flux. Answer: 250 micro Weber magnetic circuit units Weber – SI unit of magnetic flux equal to 108 lines or maxwells. Named after the German physicist Wilhelm Weber (1804- 1891) Maxwell – cgs unit of magnetic flux equal to one line of force. Named after the Scottish physicist, James Clerk Maxwell (1831-1879) magnetic circuit units Gilbert – cgs unit of magnetomotive force. Named after the English physician and physicist, William Gilbert (1540- 1603) where: – mmf (Gilbert) N – number of turns I – electrical current flowing (ampere) magnetic circuit units Tesla – SI unit of magnetic flux density equal to Webers per square meter. Named after the Crotian-American engineer Nikola Tesla (1856-1943) Gauss – cgs unit of magnetic flux density equal to Maxwells per square centimeter. Name after the German mathematician Johann Karl Freidrich Gauss (1777-1855) magnetic circuit units Oersted – cgs unit of magnetic field strength equal to Gilbert per centimeter. Named after the Danish physicist and chemist Hans Christian Oersted (1777- 1851) electromagnetic induction Faraday’s law The magnitude of the electromotive force (emf) induced in a circuit is proportional to the rate of change of the magnetic flux that cuts across the circuit. Faraday’s first law of electromagnetic induction Electromotive force is induced whenever a conductor cuts magnetic flux. Faraday’s second law of electromagnetic induction The magnitude of the induced where: emf is proportional to the relative – induced emf (volt) rate of change of flux. – number of turns of the Mathematically, conductor – rate of change of flux (Wb/sec) Faraday’s second law of electromagnetic induction The magnitude of the induced where: emf is proportional to the relative – induced emf (volt) rate of change of flux. – number of turns of the Mathematically, conductor – rate of change of flux (Wb/sec) Examples The flux of an electromagnet is 6 Wb. The flux increases uniformly to 12 Wb in a period of 2 sec. Calculate the voltage induced in a coil that has 10 turns if the coil is stationary in the magnetic field. Examples The flux of an electromagnet is 6 Answer : Wb. The flux increases uniformly to 12 Wb in a period of 2 sec. E= 30V Calculate the voltage induced in a coil that has 10 turns if the coil is stationary in the magnetic field. Lenz’s law In electromagnetic induction, the current set up by an induced voltage tends to create flux whose direction opposes any change in the existing flux. electric and magnetic circuits Electric circuit Magnetic circuit Symbol Unit Quantity Symbol Unit Quantity V or E Volt Emf Amp-turn Mmf I Amp Current Weber Magnetic flux R Ohm Resistance 1/H Reluctance V/m Field strength H Amp/m Magnetizatio n J A/m Current B Tesla Flux density density Ω-m Resistivity m/H Reluctivity G Siemens Conductance P Henry Permeance S/m Conductivity H/m Permeability magnetic units conversion Quantity SI CGS Relation Magnetomotive Ampere-turn Gilbert force (A-t) (Gb) Magnetic field Ampere-meter Oersted strength H (A/m) (Oe; Gb/cm) Magnetic flux Weber (Wb) Maxwell (Mx) Magnetic flux Tesla (T; Gauss (G; density B Wb/m2) Mx/cm2) The end!!