Chapter 13 - Electric Circuits

Announcements Electrical Resistance Magnetic Fields Final QuestionsChapter 13Electric CircuitsElectric Circuits

Announcements Electrical Resistance Magnetic Fields Final QuestionsReading AssignmentRead sections 14.4 - 14.5Homework Assignment 9Homework for Chapters 13 and 14 (due at the beginning of class on Thursday, November 11)Chapter 13: Q5, Q10, Q14, Q22, E2, E10, E14Chapter 14: Q10, Q16, Q22, E6Electric Circuits

Announcements Electrical Resistance Magnetic Fields Final QuestionsElectrical resistanceThe electrical resistance R of an object is a measure of how much an object impedes the flow of current (similarto friction)∆V = IRIn this equation, ∆V is the voltage drop across the resistor and I is the current through the resistorThe SI unit of electrical resistance is the volt-per-ampere, also called the ohm (Ω)Electrical resistance is a property of an object: it depends upon both the material and the shape of theobject (and other influences like the temperature)A conductor whose function in a circuit is to provide a specified resistance is called a resistorFor some materials R in this equation is constant (such devices are said to obey Ohm’s law)Electric Circuits

Announcements Electrical Resistance Magnetic Fields Final QuestionsElectrical resistanceThe electrical resistance R of an object is a measure of how much an object impedes the flow of current (similarto friction)∆V = IRIn this equation, ∆V is the voltage drop across the resistor and I is the current through the resistorThe SI unit of electrical resistance is the volt-per-ampere, also called the ohm (Ω)Electrical resistance is a property of an object: it depends upon both the material and the shape of theobject (and other influences like the temperature)A conductor whose function in a circuit is to provide a specified resistance is called a resistorFor some materials R in this equation is constant (such devices are said to obey Ohm’s law)ResistorsThe rate at which energy is dissipated across a resistor is P = I 2 R = ∆V 2 /R, where I is the currentacross the resistor, R is the resistance of the resistor, and ∆V is the voltage drop across the resistorEnergy can not be created or destroyed, so where is this energy going?Electric Circuits

Announcements Electrical Resistance Magnetic Fields Final QuestionsElectrical resistanceThe electrical resistance R of an object is a measure of how much an object impedes the flow of current (similarto friction)∆V = IRIn this equation, ∆V is the voltage drop across the resistor and I is the current through the resistorThe SI unit of electrical resistance is the volt-per-ampere, also called the ohm (Ω)Electrical resistance is a property of an object: it depends upon both the material and the shape of theobject (and other influences like the temperature)A conductor whose function in a circuit is to provide a specified resistance is called a resistorFor some materials R in this equation is constant (such devices are said to obey Ohm’s law)ResistorsThe rate at which energy is dissipated across a resistor is P = I 2 R = ∆V 2 /R, where I is the currentacross the resistor, R is the resistance of the resistor, and ∆V is the voltage drop across the resistorEnergy can not be created or destroyed, so where is this energy going? (it is being converted into thermalenergy)Therefore, when a current is directed across a resistor, the resistor gets hot and radiates energyAn example of a resistor is the filament inside a lightbulb!Electric Circuits

Announcements Electrical Resistance Magnetic Fields Final QuestionsSeries combinationElectric Circuits

Announcements Electrical Resistance Magnetic Fields Final QuestionsSeries combinationThe current through each resistor is the same (I 1 = I 2 = I) (why?)The total voltage difference ∆V tot across resistors connected in series is the sum of the voltage differencesacross the individual resistors (∆V tot = ∆V 1 + ∆V 2 + ...)The equivalent resistance is the algebraic sum of the individual resistancesR eq = R 1 + R 2 + R 3 + ...Therefore, the equivalent resistance of a series combination of resistors is always greater than anyindividual resistanceElectric Circuits

Announcements Electrical Resistance Magnetic Fields Final QuestionsParallel combinationElectric Circuits

Announcements Electrical Resistance Magnetic Fields Final QuestionsSeries combinationThe current through each resistor is the same (I 1 = I 2 = I) (why?)The total voltage difference ∆V tot across resistors connected in series is the sum of the voltage differencesacross the individual resistors (∆V tot = ∆V 1 + ∆V 2 + ...)The equivalent resistance is the algebraic sum of the individual resistancesR eq = R 1 + R 2 + R 3 + ...Therefore, the equivalent resistance of a series combination of resistors is always greater than anyindividual resistanceParallel combinationThe voltage difference across each resistor is the same (∆V 1 = ∆V 2 = V) (why?)The total current I is the sum of the currents across the individual resistors (I = I 1 + I 2 + ...)The inverse of the equivalent resistance is the algebraic sum of the inverses of the individual resistances1R eq= 1 R 1+ 1 R 2+ 1 R 3+ ...Therefore, the equivalent resistance of a parallel combination of resistors is always less than the smallestindividual resistance in the groupElectric Circuits

Announcements Electrical Resistance Magnetic Fields Final QuestionsQuestion #1A circuit is constructed using a 9-V battery and two resistors with resistances of 3-Ω and 6-Ω are placed, in series.What is the current in the circuit?Electric Circuits

Announcements Electrical Resistance Magnetic Fields Final QuestionsQuestion #1A circuit is constructed using a 9-V battery and two resistors with resistances of 3-Ω and 6-Ω are placed, in series.What is the current in the circuit?AnswerFor two resistors placed in series, the equivalent resistance is R = R 1 + R 2 = 9 ΩTherefore, the current in the circuit isI = ∆VR = 9 V9 Ω = 1 AElectric Circuits

Announcements Electrical Resistance Magnetic Fields Final QuestionsQuestion #1A circuit is constructed using a 9-V battery and two resistors with resistances of 3-Ω and 6-Ω are placed, in series.What is the current in the circuit?AnswerFor two resistors placed in series, the equivalent resistance is R = R 1 + R 2 = 9 ΩTherefore, the current in the circuit isI = ∆VR = 9 V9 Ω = 1 AQuestion #2A voltage drop ∆V is connected across a resistor with a resistance R, causing a current I through the resistor.Rank the following variations according to the change in the rate at which electrical energy is converted to thermalenergy in the resistor, greatest first: (a) ∆V is doubled with R unchanged, (b) I is doubled with R unchanged, (c)R is doubled with ∆V unchanged, (d) R is doubled with I unchanged.Electric Circuits

Announcements Electrical Resistance Magnetic Fields Final QuestionsQuestion #1A circuit is constructed using a 9-V battery and two resistors with resistances of 3-Ω and 6-Ω are placed, in series.What is the current in the circuit?AnswerFor two resistors placed in series, the equivalent resistance is R = R 1 + R 2 = 9 ΩTherefore, the current in the circuit isI = ∆VR = 9 V9 Ω = 1 AQuestion #2A voltage drop ∆V is connected across a resistor with a resistance R, causing a current I through the resistor.Rank the following variations according to the change in the rate at which electrical energy is converted to thermalenergy in the resistor, greatest first: (a) ∆V is doubled with R unchanged, (b) I is doubled with R unchanged, (c)R is doubled with ∆V unchanged, (d) R is doubled with I unchanged.Answer(a) = (b), (d), (c)Electric Circuits

Announcements Electrical Resistance Magnetic Fields Final QuestionsA brief history of magnetismabout 4,000 years ago: Legend has it that while herding sheep in an area of Northern Greece calledMagnesia, an elderly shepherd named Magnes found that both the nails in his shoes and the metal tip ofhis staff became firmly stuck to a large, black rock upon which he was standing (magnetite)1269: Pierre de Maricourt describes the poles of a magnet; subsequent experiments reveal that everymagnet has two poles, called north (N) and south (S) which exert forces on other poles1600: William Gilbert suggests that the Earth is a permanent magnet1819: Hans Christian Oersted discovers that an electric current in a wire can deflect a nearby compassneedle (relationship between electricity and magnetism)1831: Michael Faraday and Joseph Henry independently show that a changing magnetic field creates anelectric field (electromagnetic induction)1861: James Clerk Maxwell finds that a changing electric field creates a magnetic field (Maxwell’scorrection)Electric Circuits

Announcements Electrical Resistance Magnetic Fields Final QuestionsReading AssignmentRead sections 14.4 - 14.5Homework Assignment 9Homework for Chapters 13 and 14 (due at the beginning of class on Thursday, November 11)Chapter 13: Q5, Q10, Q14, Q22, E2, E10, E14Chapter 14: Q10, Q16, Q22, E6Electric Circuits