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DC Circuits and Power Lecture

This Physics lecture covers DC circuits, circuit diagrams, Kirchhoff's laws, and the concepts of power and energy in circuits. Examples and problem-solving techniques are included.

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DC Circuits and Power Lecture

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  1. Physics 122C Electricity and Magnetism Lecture 14 DC Circuits and Power April 27, 2007 Martin Savage

  2. Lecture 14 Announcements • Lecture HW Assignment #5 has been posted on the Tycho system and is due Wednesday at 10 PM. • Remember that Midterm 2 is next friday Physics 122B - Lecture 14

  3. A Circuit Elements & Diagrams These are some of the symbols we will use to represent objects in circuit diagrams. Variable elements are indicated by adding an arrow. Other symbols: inductance, transformer, diode, transistor, ammeter, etc. Physics 122B - Lecture 14

  4. A Circuit Diagram Actual Circuit Circuit Diagram Physics 122B - Lecture 14

  5. Kirchhoff’s Laws Physics 122B - Lecture 14

  6. Basic Circuit Physics 122B - Lecture 14

  7. High pressure V1 I P1 I I V2 Constriction Pump I P2 Low pressure Plumber’s Analogy* The “plumber’s analogy” of this circuit is a pump (=battery) pumping water in a closed loop of pipe that includes a constriction (=resistor). The pressure (=V1) is the upper part of the loop is higher than in the lower part (=V2). There is a pressure drop (=V1-V2) across the constriction and a pressure rise across the pump. The water flow I is the same at all points around the loop. Pump = Battery Constriction = Resistor Pressure = PotentialWater Flow =Current Physics 122B - Lecture 14

  8. Example:A Single-Resistor Circuit Physics 122B - Lecture 14

  9. Example:A More Complex Circuit (1) • Analyze the circuit shown, to: • Find the current in and potential difference across each resistor. • Graph the potentials around the circuit Physics 122B - Lecture 14

  10. Example:A More Complex Circuit (2) Physics 122B - Lecture 14

  11. Question What is the DV across the indicated circuit element in the direction of the current, I? • -4 V; (b) -2 V; (c) 0 V; (d) +2 V; (e) +4 V; Physics 122B - Lecture 14

  12. Applying Kirchhoff’sLoop Law to Many Loops* i1 – i3 • Define a minimum set of current loops. Label all elements. • Write a loop equation for each loop. (Battery or 0 = SDV). • Solve equations for currents • Calculate other variables of interest. R1= i3 R2= i1 Vbat= R4= R3= R5= Loop equations: i2 i1 – i2 Physics 122B - Lecture 14

  13. Solving for the loop currents • -8 -6 I1 V • -8 32 -24 I2 = 0 • -6 -24 54 I3 0 • -8 -6 I1 V • -1 4 -3 I2 = 0 • -1 -4 9 I3 0 Gaussian Elimination 0 0 36 I1 V -1 0 3 I2 = 0 0 2 -3 I3 0 I1 = 2 A , I2 = 1 A , I3 = 2/3 A Physics 122B - Lecture 14

  14. Applying Kirchhoff’s JunctionLaw to Many Junctions* • Define a minimum set of junction potentials. You can select one ground point. Label all elements. • Write a junction equation for each unknown junction . • Solve these equations for the unknown junction potentials. • Calculate the other variables of interest. J1 R1= R5= Vbat V1 R3= R2= R4= Junction equations: J2 V2 V=0 Physics 122B - Lecture 14

  15. Energy and Power (1) Example: A 90 W load resistance is connected across a 120 V battery. How much power is delivered by the battery? Physics 122B - Lecture 14

  16. Energy and Power (2) The energy delivered by the battery is dissipated by the resistor. Physics 122B - Lecture 14

  17. Example: The Power of Light How much current is “drawn” by a 100 W light bulb connected to a constant 120 V outlet? Physics 122B - Lecture 14

  18. 8 W Example: The Power of Sound Most loudspeakers are designed to have a resistance of 8 W. If such a loudspeaker is connected to a stereo amplifier with a rating of 100 W, what is the maximum possible current in the loudspeaker? Physics 122B - Lecture 14

  19. Example: A Dim Bulb How much power is used by a 60 W light bulb rated at 120 V if it is operated using a dimmer switch at 100 V? (Assume the resistance is constant.) (Actually, the resistance at this lower operating potential will be less than the R-value calculated above because the temperature will be lower, so the actual power used will be somewhat higher.) Physics 122B - Lecture 14

  20. End of Lecture 14 • Before the next lecture, read Knight, sections 31.5 through 31.8. • Lecture HW Assignment #5 has been posted on the Tycho system and is due next Wednesday at 10 PM. • Remember that next Friday we will have Exam 2. Physics 122B - Lecture 14

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