Lecture 13 Lecture 13 ––DC CircuitsDC CircuitsChapter 31 Chapter 31 --Thursday February 22ndThursday February 22nd•Review: DC circuits and Kirchoff’s laws•Review: Internal resistance•Energy transfer in electric circuits•More examples of applications of Kirchoff’s laws•RC circuitsReading: pages 701 thru 716 (chapters 31) in HRKReading: pages 701 thru 716 (chapters 31) in HRKRead and understand the sample problemsRead and understand the sample problemsWebAssign deadline will be Sunday 25th at 11:59pmWebAssign deadline will be Sunday 25th at 11:59pmHomework set 5 (Ch. 31): E13, P4, P8, E25, E44, E49Homework set 5 (Ch. 31): E13, P4, P8, E25, E44, E49Exam 2: Thursday March 8Exam 2: Thursday March 8ththin class, chapters 29in class, chapters 29--3232I will post a practice exam and there will be a reviewI will post a practice exam and there will be a reviewDC CircuitsDC CircuitsKirchoff’s first law:At any junction in an electrical circuit, the total current entering the junction must equal the total current leaving the junction.Circuit analysisCircuit analysisKirchoff’s second law:The algebraic sum of all differences in potential around a complete circuit loop must be zero.Internal resistanceInternal resistanceiRr=+εabRVRrΔ =+εEnergy transfer in electric circuitsEnergy transfer in electric circuits•A battery does work by providing each coulomb of charge that leaves its positive terminal 1 joule of energy. •If charge flows at a rate of 1 coulomb per second, then the battery does work at a rate of 1 joule per second, i.e.joule coulomb joulePower= × = =wattcoulomb second secondP = Ei = dW/dt•In a resistor, energy is lost in an amount iRper coulomb.⇒ Pcharge= iΔV = i(−iR) = −i2RPheat= i2RRC circuits (charging a capacitor)RC circuits (charging a capacitor)Kirchoff’s 2nd law:0qiRCε −−=dq qRdt Cε =+()//1tRC tRCqC e i eRεε−−= − =RC circuits (charging a capacitor)RC circuits (charging a capacitor)Kirchoff’s 2nd law:0qiRCε −−=dq qRdt Cε =+()//1tRC tRCqC e i eRεε−−= − =RC circuits (discharging a capacitor)RC circuits (discharging a capacitor)Kirchoff’s 2nd law:0ε =0dq qRdt C+=///00tRC tRC tRCqqqe i e eRC Rε−−−==− = −Kirchoff’s 2nd law:0qiRC−− =q0= εCKirchoff’s 2nd law:0ε =0dq qRdt C+=///00tRC tRC tRCqqqe i e eRC Rε−−−==− = −Kirchoff’s 2nd law:0qiRC−− =q0= εCRC circuits (discharging a capacitor)RC circuits (discharging a
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