1. DC Circuits

2. Topics 1.Circuit Symbols 2.5 principles of DC Circuits 3.2 tips for solving DC Circuits 4.Putting it all together – Electricity Problem Solving Toolkit

3. CIRCUIT SYMBOLS

4. Circuit Symbols (pg 324) Sources of E.M.F.: – Cell & Battery – D.C. Power Supply – A.C. Power Supply Resistor – Fixed Resistor – Variable Resistor (Rheostat) – Light Dependent Resistor (LDR) – Thermistor – Light Emitting Diode (LED) Meters – Ammeter – Voltmeter Switch Light Bulb (Lamp) Fuse Bells Variable Potential Divider (Potentiometer)

5. Cell (Battery)

6. D.C. Supply

7. A/C Supply

8. Fixed Resistor

9. Variable Resistor (Rheostat)

10. Light Dependent Resistor (LDR) LDRs decrease their resistance when exposed to light Often used as components for light- sensitive circuits (e.g. turn on lamps when it is dark)

11. Thermistor Thermistors decrease resistance when exposed to heat

12. Semiconductor Diode Allows current to flow in one direction Blocks current in opposite direction

13. Light Emitting Diode (LED) Same as a diode, except now it emits light when current is flowing in the allowed direction

14. Potentiometer Also called “variable potential divider” Is actually just a wire attached to a metre rule Comes with an apparatus called a “jockey” Will use this in the lab to prepare for SPA 2

15. Ammeter

16. Voltmeter

17. Galvanometer

18. Switch

19. Light Bulb

20. Fuse

21. Bell

22. Earth

23. Transformer

24. ELECTRICITY PROBLEM SOLVING TOOLKIT

25. Toolkit A handyman has a toolbox with many tools (e.g. hammer, spanner, screwdriver, etc.) In each situation he won’t need to use ALL his tools, but different situations call for different tools. Similarly, when solving electricity problems there is a total of 11 tools you can use. You won’t need to use ALL your tools for any one question, but different questions call for different tools.

26. Electricity Problem Solving Toolkit 4 equations Definition of Current Definition of Resistance Electrical Power Electrical Energy 2 arrangements of Resistors In series In parallel 5 Principles Current in Series Current in Parallel P.d. in series P.d. in parallel Potential Divider 3 tips check for short circuit redraw diagram replace cluster of resistors

27. Equations Definition of Current I = Q/t Definition of Resistance R = V/I

28. Arrangement of Resistors In series R total = R 1 + R 2 + R 3 +…. In parallel 1/R total = 1/R 1 + 1/R 2 + 1/R 3 + …. 2 parallel resistors only (optional) R total = (R 1 R 2 )/(R 1 + R 2 )

29. Electricity Problem Solving Toolkit 4 equations Definition of Current Definition of Resistance Electrical Power Electrical Energy 2 arrangements of Resistors In series In parallel 5 Principles Current in Series Current in Parallel P.d. in series P.d. in parallel Potential Divider 3 tips check for short circuits redraw diagram replace cluster of resistors

30. 5 PRINCIPLES OF DC CIRCUITS

31. Current in Series In a series circuit (i.e. no parallel circuits), the current is the same at all points of the circuit

32. Worked Example 1 What is the reading of Ammeter X? A A Ammeter Reading = 0.2 A Ammeter X

33. Practice Task GLM Pg 307 2(b), Pg 308 Qn 3(a)

34. Current in Parallel In a parallel circuit, there must be branches Current follows the “what goes in must come out” rule

35. Worked Example 2 What is the value of I? I 0.3 A 0.2 A

36. Worked Example 3 What is the value and direction of current in wire X? 0.2 A 0.3 A 0.2 A X

37. Practice Task GLM Pg 311 Qn 1(b) Pg 320 Qn 1

38. Potential Difference in Series Total p.d. is equals to the sum of the individual p.d. components across the series This is similar to calculating resistance of resistors in series Note that p.d. across wire (without resistors) is zero

39. Worked Example 4 What are the readings of voltmeters X and Y? V V V V 3.0 V Voltmeter Y 1.0 V Voltmeter X

40. Worked Example 5 What is the reading of voltmeter X? V V V Voltmeter X 4.5 V 1.0 V

41. Practice Task GLM Pg 307 Qn 2(a), Pg 308 3(b)

42. Potential Difference in Parallel p.d. is the same across parallel branches

43. Worked Example 6 What is the reading of voltmeter X? V V Voltmeter X 4.0 V

44. Worked Example 7 What is the reading of voltmeter X? V V Voltmeter X 4.0 V V 1.0 V

45. Practice Task GLM Pg 312 Qn 4(a)

46. Multi-tool Practice Tasks GLM Pg 311 Qn 1(a), 2(b), 3(a), 3(b) Pg 322 Qn 1(a), 1(b)

47. Quiz 18a

48. Assignment 18a TYS Topic 18 Paper 1 Qn 2, 4, 11, 15, 18, 19, 22, 24, 31, 32 Paper 2 Qn 1

49. Potential Divider Principle The ratio of the resistances is the ratio of the p.d. Equation form (not recommended to memorize): V 1 = [R 1 /(R 1 + R 2 )]V ɛ

50. Worked Example 8 What is the reading of the voltmeter? 1 Ω V 3 V

51. Worked Example 9 What is the reading of the voltmeter? 1 Ω 2 Ω V 3 V

52. Worked Example 10 What is the reading of the voltmeter? 3 Ω 4 Ω V 5 V

53. Practice Task GLM Pg 307 Qn 1(b) Pg 308 Qn 4(b), 4(c)

54. How is a potential divider useful? Let’s say I only have a 10 V battery, but I only need 5 V of emf for a circuit. I can use a potential divider to “divide up” my 10 V battery into just 5 V. RR 10 V

55. Potentiometer However, using resistors to divide up emf is inflexible. We cannot change the ratio easily (need to change the resistors manually). An easier method is to use a potentiometer (or variable potential divider)

56. Potentiometer R V R/2 50 cm V

57. Worked Example 11 What is the reading of the voltmeter? V 30 cm70 cm 5 V

58. Worked Example 12 State and explain what will happen to the lamp as the jockey slides from the 0 cm mark to the 100 cm mark. 100 cm 5 V 0 cm

59. Practice Task GLM Pg 319 Qn 1(a), 1(b) Potential divider circuits may also involve the use of LDRs and Thermistors GLM Pg 319 Qn 2(b), Pg 320 Qn 3(b)

60. Worked Example 13 Design a circuit which switches on a lamp automatically when it turns dark (hint: when bright, p.d. across lamp is low. when dark, p.d. across lamp is high)

61. Quiz 18b

62. Electricity Problem Solving Toolkit 4 equations Definition of Current Definition of Resistance Electrical Power Electrical Energy 2 arrangements of Resistors In series In parallel 5 Principles Current in Series Current in Parallel P.d. in series P.d. in parallel Potential Divider 3 tips Check for short circuits redraw diagram replace cluster of resistors

63. 3 TIPS FOR DC CIRCUITS

64. 3 Tips The following are not found in most textbooks Strictly speaking, these are tips not tools, but they can be really helpful nevertheless

65. Tip 1: Checking for Short Circuits In theory, a wire has zero resistance When a wire bypasses a circuit component, all the current flows through the wire instead of the component. This is called “shorting the component”. If a wire shorts ALL the resistors, it is said to form a “short circuit”, which is very dangerous in real life since (current becomes very high)

66. Shorting Components RR R Is the same as

67. Shorting Components RR R R Is the same as

68. Short Circuit (very dangerous) RR Is the same as

69. Tip 2: Redraw Circuit Diagrams A helpful habit is to redraw circuit diagrams such that the arrangement is easy to see: Arrangement of Resistors

70. Practice Task Redraw the following circuit diagrams – GLM Pg 312 Qn 4 – Pg 313 Worked Example 2 – Pg 315 Qn 2

71. Challenge Yourself! Redraw the following resistor arrangements:

72. Tip 3: Replace Resistors in a Cluster A short cut for some calculation questions involve replacing a cluster of resistors with one resistor of same effective resistance 3 Ω 2 Ω convert to 4 Ω

73. Practice Task GLM Pg 307 Qn 1(a) Pg 312 Qn 4(c) [hint: you have already determined I 1 and I 2 from earlier parts of question]

74. Electricity Problem Solving Toolkit 4 equations Definition of Current Definition of Resistance Electrical Power Electrical Energy 2 arrangements of Resistors In series In parallel 5 Principles Current in Series Current in Parallel P.d. in series P.d. in parallel Potential Divider 3 tips check for short circuit redraw diagram replace cluster of resistors

75. Assignment 18b TYS Topic 18 Paper 1 Qn 3, 7, 13, 16, 23, 26, 29, 33, 34 Paper 2 Section A Qn 4 except (c)(iii) Paper 2 Section B Qn 2(a)