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PHY 184

PHY 184. Spring 2007 Lecture 18. Title: Resistor Circuits. Announcements. Midterm 1 will take place in class tomorrow Chapters 16 - 19 Homework Sets 1 - 4 You may bring one 8.5 x 11 inch sheet of equations, front and back, prepared any way you prefer. Bring a calculator

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PHY 184

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  1. PHY 184 Spring 2007 Lecture 18 Title: Resistor Circuits 184 Lecture 18

  2. Announcements • Midterm 1 will take place in class tomorrow • Chapters 16 - 19 • Homework Sets 1 - 4 • You may bring one 8.5 x 11 inch sheet of equations, front and back, prepared any way you prefer. • Bring a calculator • Bring a No. 2 pencil • Bring your MSU student ID card • We will post Midterm 1 as Corrections Set 1 after the exam • You can re-do all the problems in the Exam • You will receive 30% credit for the problems you missed • To get credit, you must do all the problems in Corrections Set 1, not just the ones you missed 184 Lecture 18

  3. Section 2 Seating Instructions Thursday • Please seat yourselves alphabetically. • Sit in the row (C, D,…) corresponding to your last name alphabetically. • For example, Bauer would sit in row C, Westfall in row O. • We will pass out the exam by rows. Fall Semester 2006 Midterm 1 Section 1 Alphabetical Seating Order 184 Lecture 18

  4. Review - Temperature Dependence • The temperature dependence of the resistance of metallic conductors is given by • Ris the resistance at temperature T • R0is the resistance at temperature T0 • is the temperature coefficient of electric resistivity for the material under consideration 184 Lecture 18

  5. Review – Par and Ser Resistors • We can replace n parallel resistors with one equivalent resistor given by • We can replace n series resistors with one equivalent resistor given by 184 Lecture 18

  6. Example: Network of Resistors • Consider the network of resistors shown below • Calculate the current flowing in this circuit. 184 Lecture 18

  7. Example: Network of Resistors (2) • Ok, let’s look at it. R3 and R4 are in series • Now note that R34 and R1 are in parallel 184 Lecture 18

  8. Example: Network of Resistors (3) • And now R2, R5, R6, and R134 are in series 184 Lecture 18

  9. Clicker Question • Consider the circuit on the right. • Which statement is correct? A) R2 and R3 are in parallel B) R1 and R3 are in series C) R1 and R2 are in parallel D) Several statements above are correct 184 Lecture 18

  10. Clicker Question • Consider the circuit on the right. • Which statement is correct? C) R1 and R2 are in parallel R1 and R2 have the same voltage across them. R2 and R3 do not have the same voltage drop, so they cannot be in parallel. R1 and R3 do not have the same current flowing through them, so they cannot be in series. 184 Lecture 18

  11. More resistors … • The figure shows a circuit containing one ideal 12 V battery (no internal resistance) and 4 resistors with R1=20, R2=20 , R3=30, and R4=8 . • What is the current through the battery? Idea: Find the equivalent resistance and use Ohm’s Law. R2 and R3 are in parallel. 184 Lecture 18

  12. More resistors … • R23=12 W • What is the current through the battery? R1, R23 andR4 are in series. 184 Lecture 18

  13. More resistors … • The circuit contains one ideal 12 V battery (no internal resistance) and 4 resistors with R1=20, R2=20 , R3=30, and R4=8 . • What is the current i2 through R2? Key Idea 1: R2 and R3 are in parallel, so they have the same voltage drop V2=V3=V23 Key Idea 2: R1, R23 and R4 are in series so they have the same current V23=iR23 =(0.3 A)(12)=3.6 V 184 Lecture 18

  14. More resistors … • The figure on the right shows a circuit containing one ideal 12 V battery (no internal resistance) and 4 resistors with R1=20, R2=20 , R3=30, and R4=8 . • What is the current i3 through R3? Key Idea: Conservation of charge tells us that the current i going through R23 must be equal to the sum of the currents through R2 and R3. 184 Lecture 18

  15. Light Bulbs in Parallel and in Series +12 V - 12V In parallel: Observation: Take out one bulb, nothing happens to the others Assume: the bulbs are almost identical and have the same resistance 184 Lecture 18

  16. Clicker Question +12 V - 12V In parallel: What voltage drop will be measured across one light bulb? A) 12 V B) 24 V C) 36 V 184 Lecture 18

  17. Clicker Question +12 V - 12V In parallel: What voltage drop will be measured across one light bulb? B) 24 V Since the bulbs are wired in parallel: the voltage drop is the same for all and equal to the voltage supplied by the emf device 184 Lecture 18

  18. Light Bulbs in Parallel and Series +12 V - 12V In series: Observation: Taking one bulb out breaks the circuit. The more bulbs we put in series, the dimmer they get! Assume: the bulbs are almost identical and have the same resistance 184 Lecture 18

  19. Clicker Question +12 V - 12V In series: What voltage drop will be measured across one light bulb? A) 8 V B) 12 V C) 24 V 184 Lecture 18

  20. Clicker Question +12 V - 12V In series: What voltage drop will be measured across one light bulb? A) 8 V In series: Vemf=V1+V2+V3, all resistances are the same. We measure Vemf/3=24/3=8 V across each bulb 184 Lecture 18

  21. Energy and Power in Electric Circuits • Consider a simple circuit in which a source of emf with voltage V causes a current i to flow in a circuit. • The work required to move a differential amount of charge dq around the circuit is equal to the differential electric potential energydU given by • The definition of current is • So we can rewrite the differential electric potential energy as • The definition of powerP is • Pitting it together 184 Lecture 18

  22. Energy and Power • The power dissipated in a circuit or circuit element is given by the product of the current times the voltage. • Using Ohm’s Law we can write equivalent formulations of the power • The unit of power is the watt (W). • Electrical devices are rated by the amount of power they consume in watts. • Electricity bill is based on how many kilowatt-hours of electrical energy you consume. • The energy is converted to heat, motion, light, … with kW h = power times time 1 kW h = 1000 W X 3600 s = 3.6 x 106 joules 184 Lecture 18

  23. Temperature Dependence of the Resistance of a Light Bulb • A 100 W light bulb is connected to a source of emf with Vemf = 100 V. • When the light bulb is operating, the temperature of its tungsten filament is 2520 °C. • Question: • What is the resistance of the light bulb at room temperature (20 °C)? • Answer: • Power when lighted 184 Lecture 18

  24. Temperature Dependence of the Resistance of a Light Bulb (2) • … so • The temperature dependence of the resistance • … solve for the resistance at room temperature, R0 • Look up the temperature coefficient for tungsten … 184 Lecture 18

  25. Total Energy in a Flashlight Battery • A standard flashlight battery can deliver about 2.0 Wh of energy before it runs down. • If a battery costs US$ 0.80, what is the cost of operating a 100 W lamp for 8.0 hours using standard batteries? Answer: $320 184 Lecture 18

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