1. Electric currents

2. Static Electricity Static electricity is when charge “builds up” on an object and then stays “static”. How the charge builds up depends on what materials are used: + - + - + + - - - + + + - - + + + - - -

3. Static Electricity + + + - - - - - - - - -

4. Static shock

5. Measuring Charge  The charge on an electron is very small, so we measure charge using units called “coulombs” (C).  One electron has a charge of 1.6 x 10 -19 C.  Charge can be measured using a coulombmeter, and they usually measure in nanocoloumbs (1nC = 10 -9 C).  For example, a charged polythene rod may carry a charge of a few hundred nanocoulombs

6. Electric Current Electric current is a flow of negatively charged particles (i.e. electrons). We call them “charge carriers” Note that electrons go from negative to positive -+ e-e- e-e-

7. Define electric current. It is sufficient for students to know that current is defined in terms of the force per unit length between parallel current-carrying conductors.

8. Answers

9. Calculating Charge (Q) By definition, current is the rate of flow of charge. In other words, its how much charge flows per second. One amp (1 A) is equal to one coulomb per second (1 Cs -1 ). Charge and current are related by the equation: Current = rate of flow of charge I = Δ q Δ t 1.A battery supplies 10 C over a period of 50 seconds. What is the current? 2.Another battery is connected for 2 minutes and provided a current of 0.4 A. How much charge flowed? 3.A car battery has a capacity of 24 Ah (amp hours). If it provides a current of 48A how long can it be used for? How much charge (in coulombs) does it contain?

10. Conventional Current As we said, technically electrons go from negative to positive. However, we usually talk about “conventional current” and we say that current moves from positive to negative: +-

11. How long will it take for the electrons to complete 1 circuit?

12. Electron Drift What happens inside a conducting material? The following model of a metal wire could help: At normal temperatures, with no current flowing, electrons hurtle around continuously. They collide with ions but because their movement is random there is no net energy transfer. IonsElectrons

13. Electron Drift Now apply a voltage: This time we can see that the electrons are accelerated from negative to positive. This movement is superimposed on top of the random velocities and is responsible for electrical effects. IonsElectrons NegativePositive

15. What does an electric current look like? DownloadRun Now!

16. Conductors….. How many free electrons?

17. Hyperlink

18. Basic ideas… Electric current is when electrons start to flow around a circuit. We use an _________ to measure it and it is measured in ____. Potential difference (also called _______) is how big the push on the electrons is. We use a ________ to measure it and it is measured in ______, a unit named after Volta. Resistance is anything that resists an electric current. It is measured in _____. Words: volts, amps, ohms, voltage, ammeter, voltmeter

19. Voltage Earlier on we said that current is when electrons move: -+ e-e- e-e- “Voltage” is the energy that allows the electrons to move. For electrons to move there must be a “voltage difference”, sometimes called a “potential difference” (p.d.). A higher p.d. means a stronger push, which causes an increase in current.

20. Define electric potential difference The potential difference between 2 points in a circuit is…….. (1 volt = 1 joule per coulomb)

21. Georg Simon Ohm 1789-1854 Resistance Resistance is anything that will RESIST a current. It is measured in Ohms, a unit named after me. The resistance of a component can be calculated using Resistance = Voltage (in V) (in  )Current (in A) V RI Define resistance. Students should be aware that R = V/I is a general definition of resistance. It is not a statement of Ohm’s law. Students should understand what is meant by resistor.

22. Resistance Resistance is anything that opposes an electric current. Resistance (Ohms,  ) = Potential Difference (volts, V) Current (amps, A) What is the resistance of the following: 1)A bulb with a voltage of 3V and a current of 1A. 2)A resistor with a voltage of 12V and a current of 3A 3)A diode with a voltage of 240V and a current of 40A 4)A thermistor with a current of 0.5A and a voltage of 10V

23. Resistivity Apply the equation for resistance in the form where ρ is the resistivity of the material of the resistor. What effect does doubling the diameter have on the resistance?

25. Compare ohmic and non-ohmic behaviour. For example, students should be able to draw the I–V characteristics of an ohmic resistor and a filament lamp. V A Now determine the I-V graphs for a resistor and a bulb.

26. Current-voltage graphs I V Consider a resistor: Current increases in proportion to voltage R V Resistance stays constant

27. Current-voltage graphs I V Now consider a bulb: R V Resistance increases as the bulb gets hotter As voltage increases the bulb gets hotter and resistance increases

28. Current-voltage graphs I V Now consider a diode: I V Resistance decreases as the (“negative-temperature- coefficient”) thermistor gets hotter A diode only lets current go in the “forward” direction Now consider a thermistor:

29. LDR and Thermistor How does their resistance vary with light and heat?

30. Two simple components: 2) Thermistor – resistance DECREASES when temperature INCREASES 1) Light dependant resistor – resistance DECREASES when light intensity INCREASES Resistance Amount of light Resistance Temperature

31. Current in a series circuit If the current here is 2 amps… The current here will be… And the current here will be… In other words, the current in a series circuit is THE SAME at any point.

32. Kirchoff’s First Law 6A For example: If the current through here is 4A... …and the current through here is 2A… … then the current here will be 6A Gustav Kirchoff (1824-1887) “The sum of the currents leaving a point is the same as the sum of the currents entering that point.”

33. What happens if you have a choice?

34. Current in a parallel circuit A PARALLEL circuit is one where the current has a “choice of routes” Here comes the current…And the rest will go down here… Half of the current will go down here (assuming the bulbs are the same)…

35. Current in a parallel circuit If the current here is 6 amps The current here will be… And the current here will be…

36. What is the current in each bulb? 3A 6A

37. More basic ideas… If a battery is added the current will ________ because there is a greater _____ on the electrons so they move ______ If a bulb is added the current will _______ because there is greater ________ in the circuit, so the electrons move _____ Words – faster, decrease, slower, increase, push, resistance

38. Voltage in a series circuit V VV If the voltage across the battery is 6V… …and these bulbs are all identical… …what will the voltage across each bulb be? 2V

39. Voltage in a series circuit V V If the voltage across the battery is 6V… …what will the voltage across two bulbs be? 4V

40. Voltage in a parallel circuit If the voltage across the batteries is 4V… What is the voltage here? And here? VV 4V

41. Summary In a SERIES circuit: Current is THE SAME at any point Voltage SPLITS UP over each component In a PARALLEL circuit: Current SPLITS UP down each “strand” Voltage is THE SAME across each”strand”

42. An example question: V1V1 V2V2 6V 3A A1A1 A2A2 V3V3 A3A3

43. An example question: V A 1)What is the resistance across this bulb? 2)Assuming all the bulbs are the same what is the total resistance in this circuit? Voltmeter reads 10V Ammeter reads 2A

44. Solve problems involving potential difference, current and resistance. V1V1 V2V2 10V 3A A1A1 A2A2 V3V3 A3A3

45. An example A 15 Ω A A 6V Calculate the missing values (from A-level June 2006) 0.24A R 4Ω4Ω V ? ? ? ?

46. More examples… 12V 3A 6V 4V 2A 1A 2V What is the resistance of these bulbs?

47. Resistors in Series V1V1 V2V2 R1R1 R2R2 VTVT I V T = V 1 + V 2 V T = IR T But V 1 = IR 1 and V 2 = IR 2 IR T = IR 1 + IR 2 “In a series circuit current stays the same but voltage splits up” R T = R 1 + R 2

48. Resistors in Parallel ITIT R1R1 R2R2 I1I1 I2I2 ITIT V “In a parallel circuit voltage stays the same but current splits up” I T = I 1 + I 2 I T = V R T V = V + V R T R 1 R 2 1 = 1 + 1 R T R 1 R 2

49. Example questions Calculate the equivalent resistance: 1) 3) 2) 4) 10 Ω 40 Ω 20 Ω 100 Ω 20 Ω 100 Ω 50 Ω

50. Question 1 Calculate the total resistance of the circuit. Question 2 Calculate the current I. Question 3 Calculate the voltage across the points A and B. Question 4 Calculate the current I1. Question 5 How much charge will pass through the cell in 20s?

51. Each of the resistors in the circuit below has a resistance R Ohms. What is their total resistance (in terms of R). Calculate the total resistance of the four resistors in the circuit shown below Calculate the total resistance of the same four resistors when a wire of very low resistance is connected across points X and Y.

54. Describe a real life situation for……

55. Ideal ammeters have ……. resistance and are connected in …………. Ideal voltmeters have ……. resistance and are connected in …………. Describe the use of ideal ammeters and ideal voltmeters.

56. Emf and internal resistance The Emf of a cell is the total energy required to move each coulomb of charge around the complete circuit.

57. Apparatus required: Cell Two multimeters (or an ammeter and voltmeter) Rheostat (approx. 20 Ω) Leads Circuit: Procedure Start with the rheostat on its maximum resistance. Record V and I. Gradually reduce the rheostat to its lowest resistance (zero) measuring V and I a minimum of 7 times over the range. Don’t leave the circuit connected for long when the resistance is low (current high) because this will run the cell down quickly. Plot a graph of V against I. Determine the Emf and r Measuring Emf and internal resistance A V

58. Analysis E V I =r

61. The electronvolt What is an electronvolt? What are the units of the the electronvolt? Why do we need to use the electronvolt? How are electron beams produced? How fast do electrons move in an electron beam?

62. Define the electronvolt Click to play

63. Summary of units and symbols QuantitySymbolSI unitCommentsEquations potential difference Vvolt (V)also use mV and kV V = W / Q chargeQcoulomb (C) Q = It currentIamp (A)also use mA energyE or Wjoule (J)also use kJ, MJ powerPwatt (W)also use mW, kW, MW P = W / t, P = I V = V 2 / R = I 2 R resistanceR ohm (  )also use k  R = V / I timetsecond (s)