Electrostatics. Electricity and Electromagnetism. Atomic Nature of Electricity. Electrostatics is the study of electric charges in stationary form.

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1 Electricity and Electromagnetism This unit will explain the basic concepts and properties of electricity and electromagnetism. Atomic Nature of Electricity Electricity can be converted to a variety of other useful forms of energy, such as chemical, mechanical, & thermal. Electricity is made up of positive & negative charges. The electron has one unit of negative charge The proton has one unit of positive charge Since electrons orbit the nucleus & are held at different binding energies, the outer shell electrons are often free to travel from one atom s s outer shell to the next. Electrostatics Electrostatics is the study of electric charges in stationary form. 1

2 Five Laws of Electrostatics Unlike charges attract; like charges repel Faraday.jar Charges reside on the external surface of conductors and equally throughout nonconductors. Electric charges are concentrated along the sharpest curvature of an object s s surface. Only negative charges are free to move in solid conductors. Coulomb s s Law (inverse square law) Electrostatic force is directly proportional to the product of the charges & inversely proportional to the square of the 2 I1 D2 distance between them. = 2 I D 2 1 Three Methods of Electrification Friction Buildup of electrons caused by rubbing two objects together (sliding feet on carpet in winter) The electrons rapidly transfer from one object to the other causing a buildup Balloons.jar Three Methods of Electrification Contact Connection between two objects causing electron flow travoltage.jar One object has an abundance of electrons and the other a deficiency. Through contact there is an equalization of charges. (touching sibling on the ear after sliding feet on carpet in winter!) Induction Using the electromagnetic field of a charged object to induce a charge in a neutral object This is how we rotate the anode of the x-ray x tube (discussed later) 2

3 How Electrostatics Work In te ra c tio n s W h e n y o u w a lk a c ro s s a c a rp e t in w in te r, y o u p ic k u p e le c tro n s o n y o u r fe e t. W h e n y o u ru b a b a llo o n o n y o u r h a ir, y o u tra n s fe r e le c tro n s fro m y o u r h a ir to th e b a llo o n. C lo u d s in a th u n d e rs to rm tra n s fe r e le c tro n s b e tw e e n o n e a n o th e r & s o o n a s ig n ific a n t c h a rg e is b u ilt. R e s u lts o f in te ra c tio n s W h e n y o u to u c h a d o o rk n o b, y o u g e t a s h o c k a s a re s u lt. T h e b a llo o n b u ild s u p a c h a rg e & th e s u rfa c e o f th e b a llo o n c a n th e n p ic k u p s m a ll o b je c ts (y o u r h a ir, th in p a p e r, e tc ). T h e re s u lt L ig h tn in g! T h e c h a rg e h a s to m o v e fro m c lo u d to c lo u d o r fro m a c lo u d to th e e a rth. Electrodynamics The study of electric charges in motion. Electric current moves along a wire creating a flow of electrons along the wire Electric current & electron flow always move in opposite directions Units of Measure for Electrodynamics Ampere (A) measures the number of electrons flowing in the electric circuit. Equal to 6.24 x e/sec (or 1 coulomb/sec) Volt (V) measures electric potential. Equal to the potential difference that will maintain a current of one amp in a circuit with a resistance of one Ohm. Ohm (Ώ) measures electric resistance. Equal to the resistance of a standard volume of mercury under standard conditions (106.3 cm column, 1mm 0º 0 C) 3

4 Electrical Properties of Materials State Material Characteristics Insulator Semiconductor Conductor plastic, rubber, glass, wood Silicon, germanium Copper, aluminum High resistance to electron flow Conducts or resists depending on conditions Little resistance to electron flow, but can vary with conditions Ohm s s Law Conductors obey Ohm s s Law: The total voltage in a circuit, or any portion of that circuit, is equal to the current times the resistance. V=IR (voltage=current x resistance) Ohm-1d.jar Types of Circuits Series circuit elements are wired in a series along a single conductor. Parallel circuit elements bridge or branch across a conductor. Cck-ac.jar 4

5 Rules for Calculating in a Series Circuit Total voltage is equal to total current x total resistance. Resistance is equal to the sum of the individual resistances. Current is equal throughout the circuit. Voltage is equal to the sum of the individual voltages. V T = I T R T R T = R 1 + R 2 + R 3 I T = I 1 = I 2 = I 3 V T = V 1 + V 2 + V 3 Rules for Calculating in a Parallel Circuit Total voltage is equal to the total current x the total resistance Total current is equal to the sum of the individual currents. Voltage is equal throughout the circuit. Total resistance is inversely proportional to the sum of the reciprocals of each individual resistance. V T = I T R T I T = I 1 + I 2 + I 3 V T = V 1 = V 2 = V 3 1/R T = 1/R 1 + 1/R 2 + 1/R 3 The Nature of Magnetism Magnetism the ability of certain materials to attract iron, cobalt, or nickel Any charged particle in motion creates a magnetic field. In magnetic materials, the orbital electrons of its atoms orbit in predominately one direction creating magnetic poles. (each atom becomes a tiny magnet) Groups of atoms with similar orientation hang out together in magnetic domains. 5

6 The Nature of Magnetism cont d Magnetic domains are generally disorganized allowing a magnetic material to exist in a non-magnetized state. When the magnetic material is magnetized,, the domains are organized creating a net north and south pole. Laws of Magnetism These laws are similar to laws of electrostatics. Every magnet has 2 poles Like poles repel, unlike poles attract. The force of attraction (or repulsion) between 2 magnetic poles varies directly with the strength of the poles or inversely with the square of the distance between them Classifying Magnetic Materials Classified according to the origin of their magnetic properties. Naturally occurring magnets found in nature & are substances that are magnetized by sitting in the earth s s magnetic field over long periods of time. Permanent magnets (artificial) substances that are given a magnetic charge usually by exposing them to an electromagnet must be able to hold a magnetic charge on their own for an extended period of time after being magnetized Electromagnets simply an electric wire wrapped around an iron core when the wire is charged, a magnetic field is created flow of the electric current is directly proportional to the strength of the magnetic field 6

7 Types of Magnetic Materials Diamagnetic weakly repelled by magnetic fields. Beryllium, lead. Non-magnetic not affected by magnetic fields. Wood, glass, plastic, most forms of clay, rubber, etc. Ferromagnetic strongly attracted by magnetic fields iron, cobalt, nickel Paramagnetic only slightly influenced by external magnetic fields Gadolinium Solenoids Solenoid A simple coil of wire When the coiled wire is charged with an electron flow A magnetic field forms & moves through the center of the coil The more turns the coil has in it, the stronger the magnetic field will be. Electromagnets Electromagnet A ferromagnetic material wrapped in a coil of wire strengthens the magnetic field that passes through it Magnetic field lines of a solenoid coil & a electromagnet coil run in the same direction & pattern; however, the electromagnet is considerably stronger. Faraday.jar 7

8 Fleming s s Hand Rules for Electromagnetic Relationships (refer to text for illustrations) Environment Along a conductor Thumb=current/electron flow Fingers=magnetic field Solenoid & Electromagnet Poles Thumb=direction of N. pole Fingers=current/electron flow Generator Effect Thumb=conductor movement Index finger=magnetic field Middle finger=current/elect. flow Motor Principle Thumb=conductor movement Index finger=magnetic field Middle finger=current/elect. flow Current Flow Right-hand hand thumb rule Right-hand hand thumb rule Right-hand hand generator rule Left-hand motor rule Electron Flow Left-hand thumb rule Left-hand thumb rule Left-hand generator rule Right-hand hand motor rule Fleming s s Hand Rules - cont d Example of right-hand hand thumb rule. Using left hand will demonstrate the same for electron flow. Magnetic and Electromagnetic Induction Magnetic induction - occurs when a ferromagnetic material is placed near a strong magnetic field. The magnetic field will partially alter its normal north to south flow so that it may pass through the ferromagnetic material. The ferromagnetic material picks up the magnetic charge but the magnetic lines must pass through it. Electromagnetic induction occurs when a moving magnetic field causes electron current to flow through a wire As the magnet is passed near the wire magnetic field moves electrons along the wire induces a current flow Faraday.jar 8

9 What causes Electromagnetic Induction? Magnet may be moved back & forth near a coil of wire Coil of wire may be moved back & forth in front of a magnet Electromagnet can be fixed near a coil of wire Current is applied to the electromagnet Current will be induced into the coil of wire The stronger the electromagnetic current, the stronger the induction of current into the coiled wire. Self-Induction Self-induction is the induction of an opposing EMF in a single coil by its own changing magnetic field Alternating current example: AC flowing through a coil of wire, the current flows first in one direction and a magnetic field is created (Fleming s s right- hand thumb rule). When the current changes direction (negative half of the cycle), the old magnetic field dies and a new one is created with the new direction of current flow (again Fleming s right-hand hand thumb rule). The process repeats itself. The effect is a magnetic field constantly cutting back and forth through the same coil creating a back EMF *Electromotive force (EMF) potential difference measured in volts. Mutual Induction The induction of an EMF in a secondary coil by placing it near an electromagnet and varying the current (therefore magnetic field) of the electromagnet. By varying the current we cause the associated magnetic field to fluctuate and cut back and forth through the secondary coil. This cutting action induces a current in the secondary coil 9

10 Factors of Induction Strength The strength of the magnetic field. The velocity of the magnetic field as it moves past the conductor. The angle of the conductor to the magnetic field. The number of turns in the conductor. Example of an AC Generator (see text for more examples and illustrations) Faraday.jar A loop of wire is rotated manually and cuts the lines of magnetic flux (force) The waveform has a plus & a minus side when the wave is near zero, the current loop (wire shown by A & B) is farthest from the magnetic poles When it is positive or negative, the wire is parallel with either the north or south pole of the magnet. Direct Current (DC) Generator (see text for examples and illustrations) A DC waveform looks like this: The waveform does not have a negative charge due to the placement of a commutator ring. This ring changes the polarity by breaking contact just as the wire loop is about to change from positive to negative. 10

11 Direct Current (DC) Generator AC Motor (see text for examples and illustrations) An AC motor utilizes the same principle in a different way. With the motor, AC current is applied to the wire loop. Remember with AC current the associated field is constantly changing. The changing field causes the wire loop to flip continuously in an effort to orient itself with the external magnetic field Transformers Used to increase or decrease electric potential (voltage) Step-up transformer if the voltage is increased Step-down transformer if the voltage is decreased Faraday.jar 11

12 Transformers Utilize the mutual induction principle Require AC to function. Calculating Voltage and Amperage in Transformers V = voltage; N = number of turns in the coil; s = secondary coil; p = primary coil. Transformer Law for Voltage is: V s / V p = N s / N p This is a direct relationship Transformer Law for Current is: I s / I p = N p / N s This is an inverse relationship The Transformer Law for Voltage and Current is: I s / I p = V p / V s This is an inverse relationship Transformer Efficiency and Construction Closed-core transformer a square donut of magnetic material two separate windings on opposite sides of the square Autotransformer a single rod of magnetic material with a single winding of wire used only for self-induction allows only small steps in voltage change, thus not suitable for the high voltage changes of the x-ray machine (used as kv selector) 12

13 Transformer Efficiency and Construction Shell-type transformer Looks like a cinder block (tile brick) square shape with a rod in the center (all one piece of material) windings attach to the center rod the square shell strengthens the magnetic field of the primary winding most useful & efficient type of transformer for use in x-ray x equipment Capacitors Capacitor A device used to temporarily hold a charge of electricity until a certain level is reached, at which point the charge is then released A TV is an example of the use of a capacitor When you turn your TV on, you hear a click This click is the release of the capacitor A charge builds until a certain voltage is obtained & releases the charge to complete the circuit Rectification The process of changing AC into DC Like a one-way routing system Remember transformers need AC to work, but the x-ray x tube needs DC. Rectifiers are used to route each half of an AC cycle through the x-x ray tube correctly. 13

14 Solid-State State Rectifiers A solid-state state rectifier is formed by the union of two silicon-based semiconductors The two crystals are called p-type p and n-typen p-type has open spaces (called holes) that allow the electrons to fit in them Acts as the anode or positive end n-type has loosely-bound electrons inside it Acts as the cathode or negative end *this is the modern (new) rectifier Solid-State State Rectifiers Cont d The p-n p n junction is where these two types of semi-conductors meet current is applied to the p-type p side electrons migrate across the junction into the holes and current flows when positive current is used on the n-n type side attracts the electrons to it no current flow across the junction 14

15 Solid-State State Rectifiers Cont d Current flow is from positive to negative and with the arrow of a solid-state state rectifier symbol Electron flow is from negative to positive and against the arrow of a solid-state state rectifier symbol Rectifier symbol Current flow Electron flow Waveforms of Rectification Half-wave (self) rectification Waveform resembles evenly spaced humps There is a positive wave The negative wave is simply cancelled out leaving an evenly proportioned blank space between each positive wave Two rectifiers are used to protect the x-rayx tube. Waveforms of Rectification Full-wave rectification A full, never-ending ending set of positive waves When the positive charge reaches zero, the negative side is converted to a positive charge filling the gap Resemble a line of continuously connected humps 4 rectifiers are used to route current & electrons thru x-rayx tube the same way every time 15