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Circuit

Basic Definitions 1. Circuit. Next Slide. Basic Definitions. Circuit : path through which charges flow. Three parts : source (e.g. cell), load (e.g. lamp), conductors (e.g. copper wires). Revision of symbols for circuit diagrams. Diagram. Simple circuit diagram : continuous flow of charge.

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Circuit

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  1. Basic Definitions 1 Circuit Next Slide Basic Definitions • Circuit : path through which charges flow • Three parts : source (e.g. cell), load (e.g. lamp), conductors (e.g. copper wires) • Revision of symbols for circuit diagrams Diagram • Simple circuit diagram : continuous flow of charge Diagram

  2. Basic Definitions 2 Circuit Next Slide Basic Definitions • Conventional current : flow of +ve charge • Flow of electrons : flow of -ve charge Diagram • Current : rate of flow of charge with respect of time, through the circuit

  3. Basic Definitions 3 Circuit Next Slide Flow of current • Flow of electrons due to the battery • Conservation of energy : Chemical energy in the battery  Electrical potential energy in the electrons  Light and internal energy of the lamp Diagram

  4. Basic Definitions 4 Circuit Next Slide Electromotive force and potential differenece • Electromotive force (e.m.f.) of a battery : the potential energy given to every coulomb of charge passing through the battery • Potential difference (p.d.) of a lamp : the electrical potential energy given out and changed into other form by one coulomb of charge passing through the lamp

  5. Basic Definitions 5 Circuit Next Slide Electromotive force and potential differenece • Example 1 Calculation • Example 2 Calculation • Ammeter : measure current Diagram • Voltmeter : measure e.m.f. or p.d. Diagram

  6. Ohm’s law 1 Circuit Next Slide Relationship between I and V • Experiment to investigate the relationship between I and V Calculation • Ohm’s Law : • the p.d. across the ends of a conductor is directly proportional to the current flowing through it, provided that the temperature and other physical conditions are constant

  7. Ohm’s law 2 Circuit Next Slide Relationship betweenI and V • Definition of Resistance : (slope of V-I graph) • Experiment to demonstrate the restriction of Ohm’s law Calculation

  8. Resistance Circuit Next Slide Resistance and resistors • Resistance is directly proportional to its length and inversely proportional to its cross-sectional area. Diagram • Resistor : device providing resistance Photo • Variable resistor : device providing variable resistance Photo

  9. Resistor Circuit Next Slide Different kinds of connection • Change the circuit with 2 or more resistors to a simple circuit with only 1 resistor (equivalent resistance) • Resistors in series Calculation Calculation • Resistors in parallel • Example 1 Calculation • Example 2 Calculation

  10. Applications Circuit Next Slide Short circuit and connection of cells • Short circuit Diagram • Different connection of cells Diagram • Internal resistance : • the resistance of the battery which also requires a p.d. to drive a current

  11. END of Circuit

  12. cell ammeter battery voltmeter resistor switch lamp push switch Back to Basic Definitions 1 Circuit Click Back to • Revisions of symbols for circuit diagram

  13. Back to Basic Definitions 1 Circuit Click Back to • A simple circuit is shown below • A continuous flow of charge is maintained.

  14. electron flow current flow Back to Basic Definitions 2 Circuit Click Back to • Electrons (-ve charge) flow from -ve terminal through the lamp to the +ve terminal • Current (+ve charge) flows from +ve terminal through the lamp to the -ve terminal

  15. Basic Definitions 3 Circuit Next Slide • When the electrons leave the -ve terminal of the cell, they contain certain amount of electrical potential energy. • When they pass through the lamp, the electrical potential energy changes into light and internal energy. • After passing through the lamp, they contain no electrical potential energy. Refill of energy happens when they pass through the cell again.

  16. Energy of electrons Basic Definitions 3 Circuit Next Slide • The electrical energy stored inside the electrons can be summarised in the following diagram.

  17. potential Back to Basic Definitions 3 Circuit Click Back to • The potential in the circuit can be summarised in the following diagram.

  18. e.m.f. 2 V 0.4 A Basic Definitions 5 Circuit Next Slide • A cell with e.m.f. 2 V is connected to a lamp with current 0.4 A as shown in the following diagram. (a) What is the amount of charge passing the lamp in 4 s? (b) What is the p.d. across the lamp? (c) What is the amount of energy used up by the lamp in 4 s?

  19. 2 J of energy is given to each coulomb of charge passing through 0.4 A 2 J of energy is used up for each coulomb of charge passing through Back to Basic Definitions 5 Circuit Click Back to

  20. e.m.f. 6 V 1 A p.d. 4 V M N Basic Definitions 5 Circuit Next Slide • A cell with e.m.f. 6 V is connected to 2 lamps with current 1 A as shown in the following diagram. The p.d. of lamp N is 4 V. (a) What is the p.d. across another lamp? (b) Which lamp is brighter?

  21. e.m.f. 6 V p.d. 4 V p.d. 2 V 2 J of energy is used up for each coulomb of charge passing through 4 J of energy is given to each coulomb of charge passing through Back to Basic Definitions 5 Circuit Click Back to (a) By conservation of energy, p.d. = 6 V - 4 V = 2 V (b) The lamp with p.d. 4 V is brighter.

  22. Back to Basic Definitions 5 Circuit Click Back to • An ammeter is connected in a circuit as shown in the following figure.

  23. Back to Basic Definitions 5 Circuit Click Back to • A voltmeter is connected in a circuit as shown in the following figure.

  24. battery rheostat eureka ammeter voltmeter Ohm’s law 1 Circuit Next Slide • The relation between the current and the p.d. across a conductor (eureka wire) could be investigated by the following circuit (ammeter-voltmeter method)

  25. p.d. (V) p.d. (V) current (A) 0.8 0.2 1.6 0.4 2.4 0.6 3.3 0.8 4.2 1.0 0 current (A) Back to Ohm’s law 1 Circuit Click Back to • Rheostat is used to vary the magnitude of current. • The result is shown in the following table . • p.d.  current • Slope of the graph is defined as resistance

  26. battery rheostat Light bulb ammeter voltmeter Ohm’s law 2 Circuit Next Slide • We use the ammeter-voltmeter method to investigate the current and p.d. across a lamp as shown.

  27. p.d. (V) Ohm’s law is only obeyed initially 0 current (A) Back to Ohm’s law 2 Circuit Click Back to • The graph of p.d. vs. current is shown. • As temperature rises, the resistance of the lamp increases.

  28. A A L 2 L Resistance : R Resistance : 2 R A 2A L L Resistance : R Resistance : R/2 Back to Resistance Circuit Click Back to • Resistance R  Length L : • Resistance R  1/Cross-sectional A :

  29. Back to Resistance Circuit Click Back to • Some common resistors are shown in the following photo :

  30. Resistance Circuit Next Slide • Two different variable resistors are shown in the following photos

  31. current Back to Resistance Circuit Click Back to • Structure of rheostat is shown in the following diagram. • The longer the resistance wire the larger the resistance.

  32. original circuit equivalent circuit e.m.f. V e.m.f. V I I I I I I R (equivalent resistance) V V Resistor Circuit Next Slide • Equivalent resistance should have the same current (I) and p.d. (V)

  33. Back to Resistor Circuit Click Back to • The current I is the same at all points throughout the circuit. • The equivalent resistance of two or more resistors connected in series is the sum of the individual resistance.

  34. original circuit equivalent circuit e.m.f. V e.m.f. V I I I I I V R (equivalent resistance) V V Resistor Circuit Next Slide • The circuit with R (equivalent resistance) should have the same current (I) and p.d. (V) as the original circuit.

  35. Back to Resistor Circuit Click Back to • The p.d. across each resistors are the same as the e.m.f. of the cell V.

  36. e.m.f. 9 V 2  4  Resistor Circuit Next Slide • A cell with e.m.f. 9 V is connected to two resistors 2  and 4  in series. Find the current , the p.d. across the 2  resistor and the p.d. across the 4  resistor.

  37. original circuit equivalent circuit e.m.f. 9 V e.m.f. 9 V I I I 2  4  R = 2 + 4 = 6  9 V Resistor Circuit Next Slide • The circuit is changed into a circuit with 1 equivalent resistor.

  38. Back to Resistor Circuit Click Back to

  39. e.m.f. 12 V 3  6  Resistor Circuit Next Slide • A cell with e.m.f. 12 V is connected to two resistors 3  and 6  in parallel. Find the main current , the current through the 3  resistor and the current through the 6  resistor.

  40. original circuit equivalent circuit e.m.f. 12 V e.m.f. 12 V I I 3  I R = 2  V 6  Resistor Circuit Next Slide • The circuit is changed into a circuit with 1 equivalent resistor.

  41. Back to Resistor Circuit Click Back to

  42. copper wire (short circuit) Back to Applications Circuit Click Back to • A short length of copper wire is connected across the battery (or the cell). • The wire has very small resistance and draws a large amount of current (parallel connection). • It is called short circuit and the wire becomes very hot as well as the lamp dies out. It is very dangerous and may cause fire.

  43. e.m.f. = 4.5 V e.m.f. = 1.5 V Back to Applications Circuit Click Back to • Cells (each 1.5 V) in series (larger e.m.f.) • Cells (each 1.5 V) in parallel (the same e.m.f. as one cell)

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