2. CircuitBasic Definitions 1 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 Simple circuit diagram : continuous flow of charge Next Slide Diagram

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

4. CircuitBasic Definitions 3 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 Next Slide Diagram

5. CircuitBasic Definitions 4 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 : Next Slide the electrical potential energy given out and changed into other form by one coulomb of charge passing through the lamp

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

7. CircuitOhm’s law 1 Relationship between I and V Experiment to investigate the relationship between I and V Next Slide 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

8. CircuitOhm’s law 2 Relationship between I and V Definition of Resistance : (slope of V-I graph) Next Slide Experiment to demonstrate the restriction of Ohm’s law Calculation

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

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

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

12. END of Circuit

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

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

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

16. Circuit Next Slide Basic Definitions 3 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.

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

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

19. Circuit Next Slide Basic Definitions 5 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? 0.4 A e.m.f. 2 V

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

21. Circuit Next Slide Basic Definitions 5 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? 1 A e.m.f. 6 V p.d. 4 V MN

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

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

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

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

26. Circuit Click Back to Ohm’s law 1 Rheostat is used to vary the magnitude of current. Back to The result is shown in the following table. 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 p.d. (V) current (A) 0 p.d.  current Slope of the graph is defined as resistance

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

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

29. Circuit Click Back to Resistance Resistance R  Length L : Back to Resistance R  1/Cross-sectional A : A A L2 L Resistance : R Resistance : 2 R A 2A2A L L Resistance : RResistance : R/2

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

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

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

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

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

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

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

37. Circuit Next Slide Resistor 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. e.m.f. 9 V 2  4 

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

39. Circuit Click Back to Resistor Back to

40. Circuit Next Slide Resistor 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. e.m.f. 12 V 3  6 

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

42. Circuit Click Back to Resistor Back to

43. Circuit Click Back to Applications 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. copper wire (short circuit)

44. Circuit Click Back to Applications 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) e.m.f. = 4.5 V e.m.f. = 1.5 V