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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