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Lecture 5: Kirchhoff's Laws in Circuits

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Lecture 5: Kirchhoff's Laws in Circuits 8/22/2019 Recommended learning opportunities • Workshops (as announced each week) • Office Hours Room 1005 (near lab), Monday-Friday • CARE Grainger Library • Honors projects targeting James Scholars, ECE110+ECE120 Encountering various difficulties? Contact your Instructor, lab TA, or the advising office on the second floor (2120 ECEB)! 1 Lecture 1: ECE110 Introduction to Electronics: Key Intro Concepts Charge Current Voltage 2 8/22/2019 The Field of Study Defined “Engineers use the knowledge of mathematics and natural sciences gained by study, experience, and practice, applied with judgment, to develop ways to economically utilize the materials and forces of nature for the benefit of mankind. “ - ABET (Accreditation Board for Engineering and Technology) Electrical engineering (EE) is a field of engineering that generally deals with the study and application of electricity, electronics, and electromagnetism - WikiPedia 3 A short history of Electrical Engineering 4 8/22/2019 Charge • Charge is measured in coulombs (퐶) • Capital or lowercase “Q” is the variable typically used to represent charge • an electron is a charged subatomic particle • the coulomb is extremely large compared to the charge of a single electron −19 −1.6 × 10 퐶 (푛표푡푎푡표푛 푐ℎ푎푛푔푒) −1.6 e − 19 퐶 푒푙푒푐푡푟표푛 = 푒푙푒푐푡푟표푛 • Electronics is much more than just movement of electrons 5 Current: the rate at which Charge moves • Current is measured in units of amps (퐴) • Capital or lowercase “I” is the variable typically used to represent current…it means intensity. • Electric current is the flow of electric charge in time (퐶/푠) 푑푞 푡 푡 = 푑푡 Image is public domain. • The ampere is the unit of electric current 1 퐴 = 1 퐶/푠 • Current is measured by an ammeter 6 8/22/2019 “DC” Current For constant rates called “Direct Current” or “DC”, we typically use capitalized variables and can replace the differential with observations in some time, Δ푡. 퐼 = Δ푄/Δ푡 the Δ means "the change in" 7 Help Sheet: Charge and Current −1.6 e − 19 퐶 Question: What is the charge of 1 billion electrons? A. 160 e-12 C 푒푙푒푐푡푟표푛 B. 16 e-12 C C. 1.6 e-12 C D. 1.6 C E. 160 C Δ푄 퐼 = Δ푡 Q: A “typical” electronics circuit might have 1 billion A. 0.00000016 A 1 퐶 electrons pass a cross section of a wire every B. 0.160 A 1 퐴 = nanosecond, what is the electric current in amps? C. 1 A 1 푠 D. 1e-9 A E. 160e-12 A 8 8/22/2019 The Ammeter To use an ammeter to measure current, the circuit must first be “broken” and the ammeter inserted between the detached wires. The ammeter repairs the circuit and the current being measured is forced to flow through the ammeter. insert 9 The Ammeter We say the ammeter is connected in series. Any devices connected in a way to force them to share the same current are said to be connected in series. insert 10 8/22/2019 Voltage • Voltage across two points in space is the energy it requires to move each “unit” of charge between those two points. Alternately, it is the energy released when one unit of charge is allowed to move between two points in space (moving from a higher potential to a lower potential). • As an example, it should take no energy (0 volts) to move charge through an ideal (zero- resistance) conductor connected in a loop. As a second example, a 9-volt battery delivers 9 Joules of energy to each Coulomb of charge it moves. • Voltage, as seen by the description above, is differential (measured between two points) and not absolute (cannot be measured at a single point without a reference). • In many circuits, voltage potential is provided by a battery. Think of a battery that pushes electrons through a circuit (perhaps a light bulb). ΔE • Voltage is measured with a voltmeter in units of volts [푉]. 푉 = Δ푄 11 The Voltmeter To use voltmeter, the meter’s probes are placed across the device whose voltage value is desired. The circuit is not broken-and-repaired when using the voltmeter. The meter is merely placed between two circuit locations. probe 12 8/22/2019 The Voltmeter We say the voltmeter is connected in parallel. Any devices connected in a way to force them to share the same voltage are said to be connected in parallel. Red probe Black probe 13 (A) Series, (B) Parallel, (C) Neither, or (D) Both? Q: Q: 14 8/22/2019 (A) Series, (B) Parallel, (C) Neither, or (D) Both? Q: Q: 15 ECE110 Laboratory • Measure device data • Model behavior • Make interesting circuits • Master design of your own circuits Photo by C. Schmitz, 2016 The laboratory provides a hands-on opportunity to both learn and to showcase your skills! 16 8/22/2019 Required Recommended • ECE Supply Center • ECE Supply Center – Voltmeter – ECE110 Electronics Kit – Multipurpose wire stripper – i>clicker/app – Arduino (or RedBoard) + cable • Online (courses.engr.Illinois.edu/ece110) – ECE110 Lecture Slides (IUB bookstore) – ECE110 Lab Procedures (IUB bookstore) 17 Schedule • Homework – First assignment due on Wednesday, September 4 – Online via PrairieLearn – Discussion of problems and course announcements on Piazza – Due Wednesdays at 11:59 pm. Get it done early! – Office Hours…To be posted soon 18 8/22/2019 Schedule • Lab – Labs start on Monday, September 9 – Purchase Lab kit in ECE Supply Center – Purchase Lab Procedures at IUB – Prelab assignments due at the beginning of each meeting 19 L1 Learning Objectives a. (L1a) Compute relationships between charge, time, and current. b. (L1b) Define voltage. c. (L1c) Identify series and parallel elements in a circuit. d. (L1d) Describe how to insert an ammeter and a voltmeter into a circuit. Δ푄 ΔE 퐼 = 푉 = Δ푡 Δ푄 20 8/22/2019 Lecture 2: Current and Voltage Measurements • Measuring current: galvanometer • Measuring voltage: Comparators 퐼 • Current-vs-Voltage plots • IV characteristics • Ohm’s Law 푉 • Cylindrical Conductors • IV-based modeling 21 Electric current deflects a compass needle In History… Hans Christian Oersted’s observation of this effect in 1820 may have surprised him during his lecture demonstration to advanced students. Detailed experiments followed later. Image in Public Domain 22 8/22/2019 Galvanometer measures current • Each winding in coil adds to magnetic field, B • B counteracts Earth’s magnetic field • More current – bigger angle of needle • More sophisticated galvanometers came later Image from book: Electrical Measurement and the Galvanometer: Its Construction and Uses, by T. D. Lockwood, New York: J. H. Bunnell and Co., 1890 Image in Public Domain. 23 Inside a Voltmeter • Compares the input voltage to known voltages. • Uses “voltage dividers” and “comparators” • This is stuff we will understand through ECE110! Image from https://www.nutsvolts.com/questions-and-answers/led-voltmeter 24 8/22/2019 Current vs Voltage Measurements • Current-vs-Voltage plots • IV characteristics 퐼 • Ohm’s Law 푉 25 Ohm’s law models the current and voltage relationship in conductors Motivated by applications of long-distance telegraphy, Georg Ohm (~1825) conducted careful experimentation to find this widely-used approximate mathematical model: 푉 퐼 = 푅 푙 where 푅 = 휌 is resistance of a conductor (e.g. wire) 퐴 with length, 푙, and area 퐴, and where 휌 is resistivity - a material parameter 26 8/22/2019 Resistors also known as Conductors 푉 푙 퐼 = 푅 = 휌 푅 퐴 Question: Find the diameter of one mile of Cu A. 1.7 휇푚 (휌 = 1.7 × 10−8 Ω 푚) wire when 푅 = 10 Ω. B. 1.9 푚푚 C. 1 푐푚 Our ohmmeter uses the same D. 19 푐푚 connections but different settings E. 1.7 푚 than the voltmeter! Polarity doesn’t matter for Ohms. Why? Q: If the resistance of one wire is 10 Ω, what A. 2.5 Ω is the resistance of two such wires in parallel? B. 5 Ω C. 10 Ω D. 20 Ω E. 40 Ω 27 The Relationship between Current and Voltage is very revealing for many devices Devices composed of voltage sources, current sources, and resistors have “IV” relationships described by a simple line: 퐼 ≈ 푚푉 + 푏 where 푚 is the slope and 푏 is the intercept of this line on the I (current) axis. 퐼 푉 28 8/22/2019 Linear IV Characteristics 퐼 ≈ 푚푉 + 푏 Example: For a “resistor”, zero voltage means zero current and the intercept is at the origin (푏 = 0). Physical Circuit schematic 29 Linear IV Characteristics 퐼 ≈ 푚푉 + 푏 Example: For an ideal current source, 푚 = 0 such that 퐼 = 푏 independent of 푉 (the voltage across the current source). Physical Circuit schematic ? (later…) 30 8/22/2019 Linear IV Characteristics 퐼 ≈ 푚푉 + 푏 “Banana” cables and “alligator” clips are used to make connections to the Example: For an ideal voltage source, 푉 = 푉푆 and sources and meters in the lab. the current through the source is unconstrained (the limit as 푚 → ∞, 푏 → −∞). Physical Circuit schematic ? 31 Linear IV Characteristics 퐼 ≈ 푚푉 + 푏 Example: What happens with a non-ideal voltage source, for example, a battery? Physical Circuit schematic 32 8/22/2019 Resistances are used to model devices • Lengths of wire • Incandescent bulbs • Heating elements models • Battery terminals • Stalled motors • Fuses, etc. Q: If a 9 V battery provides (at maximum) a A. 0 Ω current of 2 A, what is its modelled “internal” B. 2 Ω C. 4.5 Ω resistance, 푅푇? D. 18 Ω E. ∞ Ω 33 Linear IV Characteristics Physical Circuit schematic IV plot IV equation 퐼 = 푚푉 + 푏 Q: For what region of the empirical data might we want the model to best fit? A. Near the intersection with the I-axis.
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