Key Concepts Learnt: - conventional/electron flow - electric circuit - current - voltage - potential difference, electromotive force - resistance

4. Ampere (A) Coulomb (C) second (s) Measurement of Current Definition: Current is a rate of flow of charge. The amount of charge passing thru a given pt in 1 sec. Chapter 14 Pg 241 Time (t) Charge (Q) Current (I) SI Unit Quantity Q _______ I t Formula: I = or Q = I t Q t

5. Example 1: A current of 10 A flows through an electric heater for 10 minutes. What is the total charge circulated through the heater? [Solution] t = 10 min x 60 = 600 s I = 10 A Q = I t = 10 A x 600 s = 6000 C The total charge is 6000 C Measurement of Current Chapter 14 Q _______ I t

6. Example 2: In an electrical circuit, a charge of 60C flows past a point in 10s. What is the current in the circuit? Measurement of Current Chapter 14 [Solution] t = 10 s ; Q = 60 C Q = I t I = = 60 / 10 = 6 A The current is 6 A Q _______ I t Q t

7. Example 3: A lightning flash carries 25 C of charge and lasts for 0.01 s. What is the current? [Solution] Q = 25 C ; t = 0.01 s Q = I t 25 C = I x 0.01s 25 / 0.01 = I I = 2500A Current is 2500A Measurement of Current Chapter 14

8. Example 4: A current of 2 A is flowing through a conductor. How long does it take for 10 C of charge to pass any point? [Solution] I = 2 A ; Q = 10 C Q = I t 10C = 2A x t 10 / 2 = t t = 5 s Time taken is 5 s Measurement of Current Chapter 14

10. More common symbols can be found on pg 243 Chapter 14 Pg 243 Electric Symbols

11. Circuit Diagram Variable resistor Bulb Ammeter Voltmeter Battery Fixed resistor Switch Chapter 14 Pg 243

14. Electromotive Force (e.m.f) Chapter 14 Pg 245 Definition: Electromotive force is defined as the total work done by a source in driving a unit charge around a complete circuit 1 Unit charge = 1 coulomb of charge

16. 2V Electromotive Force (e.m.f) Hi I’m Mr Coulomb (1 C) 2J of energy 2J of energy 2 J of work is done when 1 C of charge moves round the circuit Mr Coulomb goes back to the source for energy Note: 2J of electrical energy 2J of light and heat energy 2 J of energy is supplied by the cell in moving 1 C of charge round

18. Analogy The pump pushes the water to flow flow of water Work done/ energy is used to move the mill

19. Potential Difference (p.d.) Chapter 14 Pg 246 Definition: The p.d. between two points is the energy required to move 1 C of charge between them. Potential Difference (p.d.) OR Voltage (V) SI Unit : V (volts)

20. The p.d. between 2 points is the energy required to move 1 C of charge between the two points. energy E p.d. = --------------- , V = ------ or E = VQ charge Q e.g. 2V = 2 J/C Potential Difference (p.d.) Formula: E _______ V Q

22. The diagram shows a battery with an electromotive force of 6 V in a circuit. How much energy is needed to drive 30C of charge round the circuit? E = VQ = 6V x 30C = 180 J or [Solution] Example 1 6 V

26. Chapter 14 Pg 247 The resistance is a measure of how difficult it is for an electric current to pass through a substance. Resistance

27. Chapter 14 Pg 247 Definition: The resistance of a conductor is defined as the ratio of the potential difference across the conductor to the current flowing in it. Resistance Formula: R = SI Unit : Ohms (  ) V I where R = resistance V = p.d / voltage I = current or V = IR

28. The size of the current depends on the resistance in the circuit. A A 2  5  10  With the same cell used (i.e. voltage is the same), as resistance, R increases, current, I ____________ Resistance 20 V 20 V 20 V decreases I = 10 A I = 4 A I = 2 A A

30. V = I R 6 = I x 4 6 / 4 = I I = 1.5 A Reading on the ammeter is 1.5 A A 4  resistor is connected in series with an ammeter and a 6 V battery, as shown. What is the reading shown on the ammeter. Example 1 R I _______ V

31. Chapter 14 Pg 249 The resistance R (= V / I) of a metallic conductor is CONSTANT under steady physical conditions Ohm's Law

32. For Ohmic conductors (Conductors that obeys Ohm’s law) e.g. pure metal Chapter 14 Pg 248 For non-Ohmic conductor e.g. filament lamp bulb I /A V/V I /A V/V Metal A Metal B I /A V/V

37. Example 2 C

38. Example 3 C

39. Example 4 B

40. Example 5 B

42. Resistivity Formula: where R = resistance ρ = resistivity l = length A = cross-sectional area

43. Example 6

44. Example 7

45. Resistors in Series Resistors in Parallel Simulation from Crocodile Physics

46. Example 8

47. Example 9