Lecture 1 - Fundamentals of Electricity PDF

IM115 Basic Electricity Fundamentals of Electricity  Bohr’s Model The atom is the smallest particle of an element An atom consists of: protons neutrons electrons Number of protons distinguish each element  Figure 2-1 The Bohr model...

IM115 Basic Electricity Fundamentals of Electricity  Bohr’s Model The atom is the smallest particle of an element An atom consists of: protons neutrons electrons Number of protons distinguish each element  Figure 2-1 The Bohr model of an atom showing electrons in orbits around the nucleus. The “tails” on the electrons indicate they are moving. Thomas L. Floyd Copyright ©2003 by Pearson Education, Inc. Principles of Electric Circuits, 7ed. Upper Saddle River, New Jersey 07458 All rights reserved.  Figure 2-3 Energy levels increase as the distance from the nucleus increases. Thomas L. Floyd Copyright ©2003 by Pearson Education, Inc. Principles of Electric Circuits, 7ed. Upper Saddle River, New Jersey 07458 All rights reserved.  The Copper Atom Copper is the most common metal in electrical work Copper has 29 electrons arranged in shells around the nucleus Outermost shell (valence shell) has only one electron (valence electron)  Figure 2-4 The copper atom. Thomas L. Floyd Copyright ©2003 by Pearson Education, Inc. Principles of Electric Circuits, 7ed. Upper Saddle River, New Jersey 07458 All rights reserved.  Free Electrons The electron in the outer shell of a conductor is “loosely held” and can easily break away and move from atom to atom Once broken away, these electrons are called free electrons as they are free to move through the material The movement of these electrons is what we call current  Conductors Conductors are materials that allow current (electrons) to flow easily Silver is the best conductor, followed by copper copper is more widely used as it is much less expensive Gold is sometimes used for contacts – not an exceptional conductor but malleable  Wire Sizes In the US, wire is sized according to a standard called American Wire Gage (AWG) Gage size is determined by the wire cross-sectional area The larger the gage number the smaller the cross-sectional area of the wire The larger the wire, the less resistance it has and the more current it can carry Gage size varies from 0000 to 40  Semiconductors Semiconductors are materials that have four valence electrons in their valence shells With appropriate impurities added – semiconductors have electrical properties that allow fabrication of transistors, diodes.. Silicon and Germanium are the two most common semiconductors  Insulators Insulators are poor conductors of electricity they have very few free electrons Insulators have more than four electrons in their outer shell Insulators are used to stop current from going to undesired locations  Voltage Voltage is the electrical version of potential energy voltage is electrical pressure One volt (V) is the potential difference (voltage) between two points when one joule of energy is used to move one coulomb of charge from one point to the other after – Alessandro Volta (1745-1827) – Professor of Physics, Pavia Voltage can be produced by different methods 1. Electromagnetism: Generator 2. Light: Photocell 3. Pressure: Piezoelectric 4. Heat: Thermocouple 5. Chemical Reaction: Battery 6. Friction: Static Electricity Voltage is the pressure that causes electrons to flow  Current Voltage is only potential energy – it cannot do any work Converting voltage (potential energy) into actual work requires current current is the movement of charge – electrons – through a conductor  Current Electrical current ( I ) is the rate of flow of charge historically known as the Intensity of the flow As a rate – current is measured in charge per time Q I= t  Figure 2-13 Random motion of free electrons in a material. Thomas L. Floyd Copyright ©2003 by Pearson Education, Inc. Principles of Electric Circuits, 7ed. Upper Saddle River, New Jersey 07458 All rights reserved.  Figure 2-14 Electrons flow from negative to positive when a voltage is applied across a conductive or semiconductive material. Thomas L. Floyd Copyright ©2003 by Pearson Education, Inc. Principles of Electric Circuits, 7ed. Upper Saddle River, New Jersey 07458 All rights reserved.  Ampere: The Unit of Current One ampere is the amount of current that exists when a total charge of one coulomb moves through a given cross-sectional area in one second after – André Marie Ampère (1775-1836), Professor of Mathematics, Paris One coulomb of electrons = 6.24 x 1018  Figure 2-15 Illustration of 1 A of current (1 C/s) in a material. Thomas L. Floyd Copyright ©2003 by Pearson Education, Inc. Principles of Electric Circuits, 7ed. Upper Saddle River, New Jersey 07458 All rights reserved.  Resistance Resistance (R) is the opposition to current all materials have some resistance however the resistance of conductors is often small enough it is ignored the exception is superconductors R Resistance / resistor schematic symbol  Ohm: The Unit of Resistance One ohm of resistance exists if there is one ampere of current in a material when one volt is applied across the material after – George Simon Ohm (1789-1854) Teacher, Koln  Resistors Resistors are circuit components that are specifically designed to have a certain amount of resistance Uses in a circuit can be reducing voltage, limiting current, generating heat, etc. Resistors come in two broad categories: fixed resistors variable resistors Electrical Safety  Safety is the most important topic! Electricity can kill without warning. In most cases it has no odor, it makes no sound and it cannot be seen. However, Many people have worked all their lives around electricity and have had no problems. Do not be scared, but be respectful, smart and alert.  Physiological Effects of Electric Current Safe Current Values 1 mA or less No sensation, not felt 1 mA to 8 mA Sensation of shock, musclular control is not lost, the individual can let go  Physiological Effects of Electric Current Unsafe Values 8 to 15 mA Painful shock, however, muscle control is not lost and the individual can let go. 15 to 20 mA Painful shock, victim cannot let go.  Physiological Effects of Electric Current Unsafe Current Values 20 to 50 mA Painful, severe muscle contractions, difficult breathing 50 to 100 mA Ventricular fibrillation, instant death is possible 100-200 mA Ventricular fib., death 200+ mA Burns, heart stops, death  Shock severity depends on these factors: Becomes more severe with increased voltages Increases with the amount of moisture on contact surfaces Increases with an increase of area of body contact Resistance of body portions  Human resistance to Electrical Current External Resistance Dry Skin………………….100,000-600,000  Wet Skin…………………………………….1,000  Internal Resistance Hand to foot…………………………400 – 600  Ear to Ear……………………………………….100   How about this scenario? A worker with wet clothes or wet with perspiration, comes in contact with a defective 120V light cord and establishes a good ground. Wet skin resistance = 1000 ohms Body resistance = 500 ohms Total resistance = 1500 ohms 120V ÷ 1500 = 80 mA (TOAST)