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Chapter 14 - Ohm's Law
This document provides an introduction to basic concepts of electricity and electronics, focusing primarily on Ohm's Law, which describes the relationship between current, voltage, and resistance in electric circuits. It outlines different types of circuit configurations (series, parallel, and series-parallel) and presents Kirchhoff's laws, which govern current and voltage in circuit junctions. The document includes examples and practice problems to help students apply these concepts effectively.
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Chapter 14 - Ohm's Law
- 1. Introduction to BasicElectricity and Electronics Section II – DC Circuits Chapter 14 – Ohm’s Law Authored by: Earl D. Gates President, TEK Prep; Retired Electronics Technology Associate Professor, SUNY, USA; Retired Electronics Technician Senior Chief, U.S. Navy © 2014, Delmar, Cengage Learning Library of Congress Control Number: 2012949451 ISBN-13: 978-1-133-94851-3 ISBN-10: 1-133-94851-0 Dex Marco Tiu Guibelondo, BS Pharm, RPh, BS CpE IV ES 121 – Workshop Theory and Practices Saturday, 18 May 2019 Holy Cross of Davao College Guibelondo | Ohm’s Law | 1
- 2. After completing thischapter, the student will be able to: • Identify the three basic parts of a circuit. • Identify three types of circuit configurations. • Describe how current flow can be varied in a circuit. • State Ohm’s law with reference to current, voltage, and resistance. • Solve problems using Ohm’s law for current, resistance, or voltage in series, parallel, and series-parallel circuits. Objectives Guibelondo | Ohm’s Law | 2
- 3. After completing thischapter, the student will be able to: • Describe how the total current flow differs between series and parallel circuits. • Describe how the total voltage drop differs between series and parallel circuits. • Describe how the total resistance differs between series and parallel circuits. • State and apply Kirchhoff’s current and voltage laws. • Verify answers using Ohm’s law with Kirchhoff’s laws. Objectives Guibelondo | Ohm’s Law | 3
- 4. • Ohm’s lawdefines the relationship among three fundamental quantities: current, voltage, and resistance. • It states that current is directly proportional to voltage and inversely proportional to resistance. • This chapter examines Ohm’s law and how it is applied to a circuit. Some of the concepts were introduced in previous chapters. Introduction Guibelondo | Ohm’s Law | 4
- 5. • As statedearlier, current flows from a point with an excess of electrons to a point with a deficiency of electrons. • The path that the current follows is called an electric circuit. All electric circuits consist of a voltage source, a load, and a conductor. • The voltage source establishes a difference of potential that forces the current to flow. The source can be a battery, a generator, or another of the devices described in Chapter 12: Voltage. Electric Circuits Guibelondo | Ohm’s Law | 5
- 6. • The loadconsists of some type of resistance to current flow. The resistance may be high or low, depending on the purpose of the circuit. • The current in the circuit flows through a conductor from the source to the load. The conductor must give up electrons easily. Copper is used for most conductors. Electric Circuits Guibelondo | Ohm’s Law | 6
- 7. • The paththe electric current takes to the load may be through any of three types of circuits: a series circuit, a parallel circuit, or a series-parallel circuit. Electric Circuits Guibelondo | Ohm’s Law | 7
- 8. • A seriescircuit (Figure 14-1) offers a single continuous path for the current flow, going from the source to the load. Electric Circuits Guibelondo | Ohm’s Law | 8
- 9. • A parallelcircuit (Figure 14-2) offers more than one path for current flow. It allows the source to apply voltage to more than one load. Electric Circuits Guibelondo | Ohm’s Law | 9
- 10. • A series-parallelcircuit (Figure 14-3) is a combination of the series and parallel circuits. Electric Circuits Guibelondo | Ohm’s Law | 10
- 11. • Current inan electric circuit flows from the negative side of the voltage source through the load to the positive side of the voltage source (Figure 14-4). Electric Circuits Guibelondo | Ohm’s Law | 11
- 12. • As longas the path is not broken, it is a closed circuit and current flows (Figure 14-5). Electric Circuits Guibelondo | Ohm’s Law | 12
- 13. • However, ifthe path is broken, it is an open circuit and no current can flow (Figure 14-6). Electric Circuits Guibelondo | Ohm’s Law | 13
- 14. • Changing eitherthe voltage applied to the circuit or the resistance in the circuit can vary the current flow in an electric circuit. • The current changes in exact proportion to the change in the voltage or resistance. Electric Circuits Guibelondo | Ohm’s Law | 14
- 15. • If thevoltage is increased, the current also increases. If the voltage is decreased, the current also decreases (Figure 14-7). Electric Circuits Guibelondo | Ohm’s Law | 15
- 16. • On theother hand, if the resistance is increased, the current decreases (Figure 14-8). Electric Circuits Guibelondo | Ohm’s Law | 16
- 17. Georg Simon Ohm Ohm’sLaw Introduction Guibelondo | Ohm’s Law | 17 Born 16 March 1789 Erlangen, Brandenburg-Bayreuthin the Holy Roman Empire (present-day Germany) Died 6 July 1854 (aged 65) Munich, Kingdom of Bavaria in the German Confederation (present-day Germany) Residence Brandenburg-Bayreuth, Bavaria Nationality German Alma mater University of Erlangen Known for Ohm's law Ohm's phase law Ohm's acoustic law Awards Copley Medal (1841) Scientific career Fields Physics (studies of electricity) Institutions University of Munich Doctoral advisor Karl Christian von Langsdorf
- 18. • In 1827,George Ohm first observed Ohm’s law, or the relationship among current, voltage, and resistance. • Ohm’s law states that the current in an electric circuit is directly proportional to the voltage and inversely proportional to the resistance in a circuit. Ohm’s Law Introduction Guibelondo | Ohm’s Law | 18
- 19. • This maybe expressed as follows: • Where: I = current in amperes E = voltage in volts R = resistance in ohms • Whenever two of the three quantities are known, the third quantity can always be determined. Ohm’s Law Introduction Guibelondo | Ohm’s Law | 19 𝐶𝑢𝑟𝑟𝑒𝑛𝑡 = 𝑉𝑜𝑙𝑡𝑎𝑔𝑒 𝑅𝑒𝑠𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝐼 = 𝐸 𝑅 or
- 20. • Example: Howmuch current flows in the circuit shown in Figure 14-9? • Answer: IT = 0.012 amp, or 12 milliamps Ohm’s Law Introduction Guibelondo | Ohm’s Law | 20
- 21. • Example: Inthe circuit shown in Figure 14-10, how much voltage is required to produce 20 milliamps of current flow? • Answer: ET = 24 volts Ohm’s Law Introduction Guibelondo | Ohm’s Law | 21
- 22. • Example: Whatresistance value is needed for the circuit shown in Figure 14-11 to draw 2 amperes of current? • Answer: RT = 60 ohms Ohm’s Law Introduction Guibelondo | Ohm’s Law | 22
- 23. SERIES CIRCUIT • Ina series circuit (Figure 14-12), the same current flows throughout the circuit. Ohm’s Law Applications Guibelondo | Ohm’s Law | 23 𝐼 𝑇 = 𝐼 𝑅1 = 𝐼 𝑅2 = 𝐼 𝑅3 … = 𝐼 𝑅 𝑛
- 24. SERIES CIRCUIT • Thetotal voltage in a series circuit is equal to the voltage drop across the individual loads (resistance) in the circuit. • The total resistance in a series circuit is equal to the sum of the individual resistances in the circuit. Ohm’s Law Applications Guibelondo | Ohm’s Law | 24 𝐸 𝑇 = 𝐸 𝑅1 + 𝐸 𝑅2 + 𝐸 𝑅3 + … + 𝐸 𝑅 𝑛 𝑅 𝑇 = 𝑅1 + 𝑅2 + 𝑅3 + … + 𝑅 𝑛
- 25. PARALLEL CIRCUIT • Ina parallel circuit (Figure 14-13), the same voltage is applied to each branch in the circuit. Ohm’s Law Applications Guibelondo | Ohm’s Law | 25 𝐸 𝑇 = 𝐸 𝑅1 = 𝐸 𝑅2 = 𝐸 𝑅3 … = 𝐸 𝑅 𝑛
- 26. PARALLEL CIRCUIT • Thetotal current in a parallel circuit is equal to the sum of the individual branch currents in the circuit. • The reciprocal of the total resistance is equal to the sum of the reciprocals of the individual branch resistances. • The total resistance in a parallel circuit will always be smaller than the smallest branch resistance. Ohm’s Law Applications Guibelondo | Ohm’s Law | 26 𝐼 𝑇 = 𝐼 𝑅1 + 𝐼 𝑅2 + 𝐼 𝑅3 + … + 𝐼 𝑅 𝑛 1 𝑅 𝑇 = 1 𝑅1 + 1 𝑅2 + 1 𝑅3 + … + 1 𝑅 𝑛
- 27. • Ohm’s lawstates that the current in a circuit (series, parallel, or series-parallel) is directly proportional to the voltage and inversely proportional to the resistance. • In determining unknown quantities in a circuit, follow these steps: • Draw a schematic of the circuit and label all known quantities. • Solve for equivalent circuits and redraw the circuit. • Solve for the unknown quantities. Ohm’s Law Applications Guibelondo | Ohm’s Law | 27 𝐼 ∝ 𝐸 𝑅
- 28. • Ohm’s Lawis true for any point in a circuit and can be applied at any time. • The same current flows throughout a series circuit, and the same voltage is present at any branch of a parallel circuit. Ohm’s Law Applications Guibelondo | Ohm’s Law | 28
- 29. • Example: Whatis the total current flow in the circuit shown in Figure 14-14? • Answer: IT = 0.0054 amp, or 5.4 milliamp Ohm’s Law Applications Guibelondo | Ohm’s Law | 29
- 30. • Example: Howmuch voltage is dropped across resistor R2 in the circuit shown in Figure 14- 16? • Answer: ER2 = 17.50 volts Ohm’s Law Applications Guibelondo | Ohm’s Law | 30
- 31. • Example: Whatis the value of R2 in the circuit shown in Figure 14-18? • Answer: R2 = 2,033.9 ohms Ohm’s Law Applications Guibelondo | Ohm’s Law | 31
- 32. • Example: Whatis the current through R3 in the circuit shown in Figure 14-19? • Answer: IR3 = 13.9 milliamps Ohm’s Law Applications Guibelondo | Ohm’s Law | 32
- 33. • Practice Problem:What is the total circuit current in Figure 14-21? • IT = ? • ET = 12 volts • R1 = 500 ohms • R2 = 1200 ohms • R3 = 2200 ohms • Answer: IT = ? Ohm’s Law Applications Guibelondo | Ohm’s Law | 33
- 34. Gustav Robert Kirchhoff Kirchhoff’s CurrentLaw and Voltage Law Guibelondo | Ohm’s Law | 34 Born 12 March 1824 Königsberg, Province of East Prussia, Kingdom of Prussia (present-day Kaliningrad, Russia) Died 17 October 1887 (aged 63) Berlin, Province of Brandenburg, Kingdom of Prussia (present-day Germany) Residence Prussia / German Empire Nationality Prussian (1824-1871) German (1871-1887) Alma mater University of Königsberg Known for Kirchhoff's circuit laws Kirchhoff's law of thermal radiation Kirchhoff's laws of spectroscopy Kirchhoff's law of thermochemistry Awards Rumford medal (1862) Davy Medal (1877) Matteucci Medal (1877) Janssen Medal (1887)
- 35. Gustav Robert Kirchhoff Kirchhoff’s CurrentLaw and Voltage Law Guibelondo | Ohm’s Law | 35 Scientific career Fields Physics Chemistry Institutions University of Berlin University of Breslau University of Heidelberg Doctoral advisor Franz Ernst Neumann Notable students Loránd Eötvös Edward Nichols Gabriel Lippmann Dmitri Ivanovich Mendeleev Max Planck Jules Piccard Max Noether Heike Kamerlingh Onnes Ernst Schröder
- 36. Kirchhoff’s Current Law andVoltage Law Guibelondo | Ohm’s Law | 36 • In 1847, G. R. Kirchhoff extended Ohm’s law with two important statements that are referred to as Kirchhoff’s laws.
- 37. Kirchhoff’s Current Law Guibelondo |Ohm’s Law | 37 • The first law, known as Kirchhoff’s current law, states the following: ➢The algebraic sum of all the currents entering and leaving a junction is equal to 0. • Here is another way of stating Kirchhoff ’s current law: ➢The total current flowing into a junction is equal to the sum of the current flowing out of that junction. • A junction is defined as any point of a circuit at which two or more current paths meet. In a parallel circuit, the junction is where the parallel branches of the circuit connect.
- 38. Kirchhoff’s Current Law Guibelondo |Ohm’s Law | 38 • In Figure 14-22, point A is one junction and point B is the second junction. Following the current in the circuit, IT flows from the voltage source into the junction at point A. There the current splits among the three branches as shown. Each of the three branch currents (I1, I2, and I3) flows out of junction A.
- 39. Kirchhoff’s Current Law Guibelondo |Ohm’s Law | 39 • According to Kirchhoff ’s current law, which states that the total current into a junction is equal to the total current out of the junction, the current can be stated as • Following the current through each of the three branches finds them coming back together at point B. Currents I1, I2, and I3 flows out into junction B, and IT flows out. Kirchhoff ’s current law formula at this junction is the same as at junction A: 𝐼 𝑇 = 𝐼1 + 𝐼2 + 𝐼3 𝐼1 + 𝐼2 + 𝐼3 = 𝐼 𝑇
- 40. • Practice Problem:Refer to Figure 14-23. What are the values of I2 and I3? • Answer: I2 = ? I3 = ? Kirchhoff’s Current Law Guibelondo | Ohm’s Law | 40
- 41. Guibelondo | Ohm’sLaw | 41 • Kirchhoff ’s second law is referred to as Kirchhoff’s voltage law, and it states the following: ➢The algebraic sum of all the voltages around a closed circuit equals 0. • Here is another way of stating Kirchhoff’s voltage law: ➢The sum of all the voltage drops in a closed circuit equals the voltage source. Kirchhoff’s Voltage Law
- 42. Guibelondo | Ohm’sLaw | 42 • In Figure 14-24 there are three voltage drops and one voltage source (voltage rise) in the circuit. If the voltages are summed around the circuit as shown, they equal 0. Notice that the voltage source (ET) has a sign opposite that of the voltage drops. Therefore the algebraic sum equals 0. Kirchhoff’s Voltage Law 𝐸 𝑇 − 𝐸1 − 𝐸2 − 𝐸3 = 0
- 43. Guibelondo | Ohm’sLaw | 43 • Looking at this another way, the sum of all the voltage drops equals the voltage source. • Both of the formulas shown are stating the same thing and are equivalent ways of expressing Kirchhoff’s voltage law. • The key to remember is that the polarity of the voltage source in the circuit is opposite to that of the voltage drops. Kirchhoff’s Voltage Law 𝐸 𝑇 = 𝐸1 + 𝐸2 + 𝐸3
- 44. • Practice Problem:Refer to Figure 14-25. What is the total voltage applied to the circuit? • Answer: ET = ? Kirchhoff’s Voltage Law Guibelondo | Ohm’s Law | 44
- 45. Summary Guibelondo | Ohm’sLaw | 45 • An electric circuit consists of a voltage source, a load, and a conductor. • The current path in an electric circuit can be series, parallel, or series-parallel. • A series circuit offers only one path for current to flow. • A parallel circuit offers several paths for the flow of current. • A series-parallel circuit provides a combination of series and parallel paths for the flow of current. • Current flows from the negative side of the voltage source through the load to the positive side of the voltage source. • Changing either the voltage or the resistance can vary current flow in an electric circuit.
- 46. Summary Guibelondo | Ohm’sLaw | 46 • Ohm’s law gives the relationship of current, voltage, and resistance. • Ohm’s law states that the current in an electric circuit is directly proportional to the voltage applied and inversely proportional to the resistance in the circuit. • Ohm’s law applies to all series, parallel, and series-parallel circuits. • To determine unknown quantities in a circuit: • Solve for equivalent circuits. • Draw a schematic of the circuit and label all quantities and redraw the circuit. • Solve for all unknown quantities. 𝐼 = 𝐸 𝑅
- 47. Summary Guibelondo | Ohm’sLaw | 47 • Kirchhoff ’s current law: The algebraic sum of all the currents entering and leaving a junction is equal to 0; it may be restated as the total current flowing into a junction is equal to the sum of the current flowing out of that junction. • Kirchhoff ’s voltage law: The algebraic sum of all the voltages around a closed circuit equals 0; it may be restated as follows: The sum of all the voltage drops in a closed circuit equals the voltage source.