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DC Circuits

DC Circuits. Topics. Circuit Symbols 5 principles of DC Circuits 2 tips for solving DC Circuits Putting it all together – Electricity Problem Solving Toolkit. Circuit Symbols. Circuit Symbols ( pg 324). Sources of E.M.F.: Cell & Battery D.C. Power Supply A.C. Power Supply Resistor

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DC Circuits

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  1. DC Circuits

  2. Topics • Circuit Symbols • 5 principles of DC Circuits • 2 tips for solving DC Circuits • 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 • Thermistor’s 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. 5 principles of dc circuits

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

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

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

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

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

  30. 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

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

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

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

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

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

  36. Potential Divider Principle • The ratio of the resistances is the ratio of the p.d. • Equation form (not recommended to memorize): • V1 = [R1/(R1 + R2)]Vɛ

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

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

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

  40. 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. 10 V R R

  41. 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)

  42. Potentiometer 50 cm 50 cm R V R/2 R/2 V

  43. Worked Example 11 • What is the reading of the voltmeter? 5 V 30 cm 70 cm V

  44. 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. 5 V 0 cm 100 cm

  45. Worked Example 13 • Potential divider circuits may also involve the use of LDRs and Thermistors • 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)

  46. 2 tips for dc circuits

  47. Redraw Circuit Diagrams

  48. Replace Resistors in a Cluster

  49. 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 2 tips • redraw diagram • replace parallel resistors

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