DC1-1 VOLTAGE CIRC-1005 DC Circuits Department of Electrical Engineering Technology, RRC Polytech © 2022 RRC Polytech This document is protected by Canadian copyright law. Except as otherwise provided for under Canadian copyright law, this document may not be copied, published, distributed, downloaded or otherwise stored in a retrieval system, transmitted or converted, in any form or by any means, electronic or otherwise, without the prior written permission of the copyright owner. DC1-1 VOLTAGE Dr. Norman Huang Instructor of EET BSEng, MSEng, PhDEng Based on the Slides made by Preet Dhingra Why study about Electrical Circuits? Electrical circuits are everywhere in this technologically advanced world: lights, TVs, computers, cars, cell phones, air conditioning, you name it: life would be totally different if we didn't have circuits. But what exactly is a circuit? What is an Electric Circuit? An electrical circuit is an interconnection of various electrical circuit elements like an electrical energy source, a load, a switch and conducting wires. But What is Electricity? Electricity is a form of energy. To understand this, you need to know about atoms and the structure of atoms. Voltage and Current The term voltage is encountered practically everyday. We have all replaced batteries in our flashlights, answering machines, calculators, automobiles and so on, that had specific voltage ratings. The most outlets in our homes are 120 volts. Voltage and Current Although current may be a less familiar term, we know what happened when we place too many appliances on the same outlet- the circuit breaker opens due to the excessive current. Atomic Theory Review: • All elements are arranged in the periodic table according to their atomic number. • An atom consists of a nucleus with electrons orbiting around it. • Atom has equal number of protons in the nucleus which is the same as the number of electrons. Atomic Theory Review: • Protons, have a positive electrical charge. Neutrons have no electrical charge while electrons, have a negative electrical charge. • Neutrons and protons exist inside the nucleus making it positively charged whereas the electrons revolve around the nucleus in orbitals. The outermost shells are called valence shells and electrons revolving in it are called valence electrons. Atomic Theory Review: • An electron has a charge equal to approximately 1.602 x 10-19 Coulombs. • A proton has a charge equal to approximately +1.602 x 10-19 Coulombs Atomic Theory Review: • The coulomb (C) is the SI unit of electric charge. Hydrogen and Helium atoms Copper: Is the most commonly used metal in the electrical /electronics industry. It has 29 electrons in orbits around the nucleus, with the 29th electron appearing all by itself in the 4th shell. Copper: • If 29th electrons gain sufficient energy from the surrounding medium to leave the parent atom, it is called a free electron. • In 1 cubic in. of copper at room temperature, there are approximate 1.4 × 1024 Electrical Charge: • Electrical Charge is an intrinsic property of matter that manifests itself in the form of forces – electrons repel other electrons but attract protons, while protons repel each other but attract electrons. Electrical Charge: • Example: Consider an atom has equal numbers of electrons and protons, and since their charges are equal and opposite they cancel, leaving the atom as a whole uncharged. • However, if the atom acquires additional electrons (leaving it with more electrons than protons), we say that it (the atom) is negatively charged; conversely , if it loses electrons and is left with fewer electrons than protons, we say that it is positively charged. Electrical Charge: • The term “charge” in this sense denotes an imbalance between the number of electrons and protons present in the atom. • The term “charge” is denoted by the letter Q, and its unit of measurement in the SI system is Coulomb. One coulomb of charge = 6.242 x1018 electrons. Coulomb’s Law: According to Coulomb’s law, force between charges is directly proportional to the product of their charges and inversely proportional to the square of the distance between them. 𝑄1 𝑄2 𝐹=𝑘 2 𝑟 where F = Force (Newton), k = constant, Q1and Q2 are the charges in coulombs and r is the distance between the two charges in meters. Voltage: • When charges are detached from one body and transferred to another, a potential difference or voltage results between them. • In general, every source of voltage is established by simply creating a separation of positive and negative charges. Definition of Voltage: the Volt • In electrical terms, a difference in potential energy is defined as voltage. • In general, the amount of energy required to separate charges depends on the voltage developed and the amount of charge moved. Definition of Voltage: the Volt • By definition, if a total of 1 joule (J) of energy is used to move the negative charge of 1 coulomb (C), there is a difference of 1 volt (V) between the two points. • In equation form: 𝑊 𝑉= 𝑄 𝑉𝑜𝑙𝑡𝑠, 𝑉 • Where W is the energy in joules, Q is the charge in Coulombs and V is the voltage in Volts. 𝑉= 1 = 1V 1 1 Note carefully that voltage is defined between points. • For the case of the battery, for example, voltage appears between its terminals. • Thus, voltage does not exist at a point by itself, it is always determined with respect to some other point. • The difference in voltage between any two points, connections or junctions (called nodes) in a circuit is known as the Potential Difference, commonly called the Voltage Drop. • The Potential difference between two points is measured in Volts with the circuit symbol V • A constant voltage source is called a DC Voltage and a voltage that varies periodically with time is called an AC voltage. Electrical Voltage • Voltage, ( V ) is the potential energy between two points on a circuit. One point has more charge than another. This difference in charge between the two points is called voltage. Water = Charge Pressure = Voltage • Voltage between two points is one Volt if it require one Joule of work to move one Coulomb of charge from one point to another in a conducting material. DC Voltage Source: Batteries: a) Primary (non-rechargeable) b) Secondary (chargeable) Primary Cells: a) Alkaline (1.5V nominal Voltage) b) Lithium (3V nominal voltage, good for low temp.) Secondary Cells: a) Lead Acid b) Nickel Metal Hydride c) Nickel-Cadmium DC Voltage Source: Solar Cells Other DC Sources: a) Generators b) Power Supplies (in labs) c) Fuel Cells Slide 28 Reference • Some figures and animations and Table are cited from • Boylestad, R. L., Introductory Circuit Analysis, Pearson, 13th Edition Thank you! See you next time!
