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Phys 133 -- Chapter 311 Chapter 31 Fundamentals of Circuits.

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Presentation on theme: "Phys 133 -- Chapter 311 Chapter 31 Fundamentals of Circuits."— Presentation transcript:

1 Phys Chapter 311 Chapter 31 Fundamentals of Circuits

2 Phys Chapter 312 Circuits potential  field  current

3 Phys Chapter 313 KLR: Kirchoff’s loop rule (conservation of energy) Kirchoff’s rules KJR: Kirchoff’s junction rule (conservation of charge) Always true!

4 Phys Chapter Recall lab: rheostat (resistor), tungsten, carbon Ohm’s Law

5 Phys Chapter 315 Question A typical resistance value is 1Ω. A typical copper wire has a radius of 0.5 mm. How long must the typical copper wire be so that it has a 1Ω resistance?  copper = 1.7 x Ωm

6 Phys Chapter no potential difference (“no voltage drop”) --all potential difference across resistors Ideal wire:

7 Phys Chapter 317 Circuit elements: real and schematic

8 Phys Chapter 318 ≈0 Basic circuit +-+- I (+)

9 Phys Chapter Find the magnitude and direction of the current through the 18  resistor. On your way to the solution: Draw a schematic of this circuit. Choose a direction of positive current Indicate the high and low side of the resistor. Apply Kirchoff’s law --Draw a graph of the potential as a function of the clockwise distance around the circuit. Start with V=0 at lower left corner. Problem A

10 Phys Chapter 3110 Problem A (ans) I (+) + - Apply Kirchoff’s law Draw a schematic of this circuit.Choose a direction of positive currentIndicate the high and low side of the resistor. 6 V 3 V V r

11 Phys Chapter 3111 Do Workbook 31.6 a)I out ? I in A.< B.> C.= b) R 1 ? R 2 ? R 3 A.R 1 > R 2 > R 3 B.R 3 > R 1 > R 2 C.R 1 = R 2 = R 3 D.R 2 > R 1 > R 3 c) E ? A.Right B.Left C.E = 0

12 Phys Chapter 3112 Do Workbook 31.9  V 12 = ? A.0V B.3V C.<3V

13 Phys Chapter 3113 Resistors-series Currents are the same In general

14 Phys Chapter 3114 a)Determine the current through R 1 and R 2 ? b)Determine the potential difference ∆V 1 and ∆V 2 ? Problem B

15 Phys Chapter 3115 Problem B (ans) R 1 & R 2 in series I +-+-

16 Phys Chapter 3116 Voltage in a circuit The light bulb in this circuit has a resistance of 1 . What are the values of:  V 12  V 23  V 34 Suppose the bulb is removed from its socket. Then what are values of  V 12  V 23  V 34 A n s w e r s : A.0 V B.1 V C.2 V D.3 V

17 Phys Chapter 3117 Resistors-parallel Potential differences are the same In general

18 Phys Chapter 3118 Problem C a)Determine the current through R 1 and R 2 ? b)Determine the potential difference ∆V 1 and ∆V 2 ? c) What fraction of the current goes through R 1 ? What happens to this fraction if the value of R 2 is increased?

19 Phys Chapter 3119 Problem C (ans) R 1 & R 2 in parallel I +-+-

20 Phys Chapter 3120 Power and energy For battery For resistor

21 Phys Chapter 3121 a)How much power is dissipated by each resistor? b)How much power is provided by each battery? Problem D Series Parallel

22 Phys Chapter 3122 Problem D (ans) Series Parallel

23 Phys Chapter 3123 Real batteries: ideal emf and internal resistance

24 Phys Chapter 3124 Compared to an ideal battery, by what percentage does the battery’s internal resistance reduce the potential difference across the 30  resistor? Problem E

25 Phys Chapter 3125 Compared to an ideal battery, by what percentage does the battery’s internal resistance reduce the potential difference across the 30  resistor? Problem E (ans)

26 Phys Chapter 3126 Conceptual 1 In the circuit below, the battery maintains a constant potential difference between its terminals at points 1 and 2 (i.e., the internal resistance of the battery is considered negligible). The three light bulbs, A, B, and C are identical. How do the brightnesses of the three bulbs compare to each other? A.A = B = C B.B = C > A C.A > B = C D.A > B > C *RTP Lab 3 Homework

27 Phys Chapter 3127 Conceptual 2 A wire is connected from the battery terminal at point 1 to point 4 a)What happens to the potential difference across bulb B? Explain. b)What happens to the potential difference across bulb C? Explain. c)What happens to the potential difference between points 1 and 5? Explain your reasoning. d)What happens to the brightness of each of the three bulbs? Explain. e)What happens to the current through point 3? Explain. *RTP Lab 3 Homework

28 Phys Chapter 3128 seriesparallel VV  V 1 +  V 2 =  V eq V1=V2V1=V2 II 1 =I 2 I 1 +I 2 = I eq Resistors

29 Phys Chapter 3129 SeriesParallel VV addsame Q, Isameadd General relationships

30 Phys Chapter 3130 Find the current through and potential difference across each resistor. (Hint: Collapse and expand.) Problem 31.67

31 Phys Chapter 3131 Problem (ans)

32 Phys Chapter 3132 Find the current (I) and potential difference (  V) across each resistor. (Hint: use KLR and KJR.) Problem: Multi-loop circuit

33 Phys Chapter draw currents (any direction), split at each junction --use KJR at junction(s) --using KLR, go around a loop (any direction) Battery--  V increases going (-  +) of battery --  V decreases going (+  -) of battery Resistor--  V increases upstream (against current) --  V decreases downstream (with current) --solve equations for unknown currents Circuit problem strategy

34 Phys Chapter 3134 Find the current (I) and potential difference (  V) across each resistor. (Hint: use KLR and KJR.) Problem F --draw currents (any direction), split at each junction --use KJR at junction(s) --using KLR, go around a loop (any direction)

35 Phys Chapter 3135 Problem F (ans) + - I1I I3I I2I2 Apply KLR Apply KJR abcea: abcdea: ecde: Junction c: Junction e: 5 equations, 3 unknowns Choose 3, and solve

36 Phys Chapter 3136 Grounding: convention/safety

37 Phys Chapter 3137 Do Workbook 31.32

38 Phys Chapter 3138 Determine the potential at points a through d. Problem 31.34

39 Phys Chapter 3139 Determine the potential at points a through d. Problem (ans) V 4 V 7 V 9 V

40 Phys Chapter 3140 RC circuits: discharge Before the switch closes After the switch is closed Apply KLR

41 Phys Chapter 3141 RC circuits: discharge

42 Phys Chapter 3142 RC circuits: charging

43 Phys Chapter 3143 A 20  F capacitor initially charged to 40  C is discharged through a 5.0 k  resistor. Problem G How long does it take to reduce the capacitor’s charge to 10  C? What is the initial current through the resistor?

44 Phys Chapter 3144 Problem G (ans) Close switch at t=0, then Find t when Q(t)=20µC


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