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ECE 101 Exploring Electrical Engineering Chapter 5 Circuit Elements Herbert G. Mayer, PSU Status 1/30/2016 Derived with permission from PSU Prof. Phillip Wong 1 Syllabus Electric Charge Electric Field What is Conductance References 2 Intro Many SI units you saw earlier Now we discuss detail of a few key SI units With their: name, symbol, equation, unit etc. 3 Electric Charge Electric charge (q or Q) is an intrinsic property of certain subatomic particles A particle’s charge affects its motion in the presence of electric and also in magnetic fields The SI unit for electric charge is the coulomb [C] 1 [C] = 1 [A] * 1 [s] The charge of 1 electron is ≈ 1.602×10-19 C Can be positive or negative 4 Electric Field Lorentz Force Law → The force F, acting on a charged particle in the presence of an electric field and a magnetic field is: F, E, B, & v are vectors q = electric charge of the particle v = velocity of the particle E = electric field, B = magnetic field F qE qv B If the magnetic field is zero, then the electric field that pervades the space around the charge is: F E= q 5 What Is? An electron? Subatomic particle with electric charge; we call that charge negative Electron is part of the lepton family Called an elementary particle, since it seems to have no sub-particles Has mass of approx. 1/1836 of a proton Yet electrons have some properties of particles AND waves 6 What Is? Coulomb is the fundamental unit of electrical charge And identifies the SI derived unit of electric charge Symbol for Coulomb is C; the symbol for charge flowing, creating a current, is: Q or q A coulomb is equal to a charge of 6.24150934 × 1018 electrons (or protons) What exactly a charge really is, we don’t understand, but we do know some key properties, and we can measure such properties quite accurately Similar to gravity: we can measure and use it, even fly to the moon with rockets overcoming gravity, but don’t fundamentally understand how gravity works; theories exist 7 7 SI Base Units: No Coulomb, No Volt 8 7 SI Base Units 9 Units Derived from 7 SI 10 What was Ampere Again? Unit of current. One of the base units of the SI Named after André Marie Ampère, French physicist 1775 – 1836 When about 6.24150934 × 1018 electrons stream though a conductor in 1 second, the charge moved is 1 C and the current is 1 A; ACA “amp”. i = dq / dt 1A=1C/s assumes even flow of current during one second s 11 AC DC Current Carriers of charge may be: Electrons Ions + Holes + André Ampère In direct current (DC), charge carriers move in one direction, viewed macroscopically In alternating current (AC), charge carriers alternate direction periodically; frequency in Hertz [Hz] SI unit for current is ampere is A = C / s Picture of physicist André-Marie Ampère 12 What is? We discussed definition of Volt, Power, Resistance Discuss inverse of resistance: Conductance In later presentation we discuss Ohm’s Law in detail Here highlight: 13 Ohm’s Law At constant temperature, the current I through some material is directly proportional to the potential difference ΔV between its ends; Δ is generally omitted! I ≈ ΔV The resistance R is defined as: V R I The general form of Ohm’s Law is: V IR V R I 14 V I R Georg Ohm Application of Ohm’s Law Given: Material of known resistance R Voltage V is applied across the material Result: Current I = V / R will flow through it. Given: Material of known resistance R Known current I flowing through it Result: Voltage V = I · R exists across the material (known as a “voltage drop”). Given: Known voltage V across the material Known current I through the material Result: Resistance of the material is R = V / I. 15 What is? Conductance G is a measure of the ease with which a current flows through a material; i.e. it is the reciprocal of resistance: 1 G R Ohm’s Law in terms of conductance (I below, not 1): I G V I I VG V G SI unit for conductance is Siemens [S], the inverse of resistance 16 References 1. Ohm’s law: https://en.wikipedia.org/wiki/Ohm%27s_law 2. https://en.wikipedia.org/wiki/André-Marie_Ampère 17