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  1. 1. ElectricityVolume B Chapter 18
  2. 2. What we will Learn…• What is a current?• How to draw electric circuit diagrams• What is potential difference, voltage and E.M.F?• What is resistance?• S/E: Circuits arranged in series versus parallel• S/E: Fixed resistors versus variable resistors• Effects of electric current
  3. 3. Definition:Electricity is a form of energy that canbe easily changed to other forms.What is Electricity???
  4. 4. Where does Electricity come from?Mainly 2 sources:1) Power Stations- Supply a lot of electricity- Used in many electricalappliances2) Electric Cells (batteries)- Supply a little electricity- Portable- Safe
  5. 5. How does an ElectricalAppliance Work?• To make an electrical appliancework, electricity must flow through it.• The flow of electricity is called an electriccurrent.• The path along which the electric currentmoves is called the electric circuit.
  6. 6. What is an ElectricCurrent???Definition:An electric current is the rate of flow of electriccharges in a circuit.connectingwireelectriccellfilamentflow of electrons
  7. 7. Electric Charges• Electric charges are made up of positivecharges (protons) and negative charges(electrons).• When these charges flow in a circuit, acurrent is produced.
  8. 8. How does electricity flow?• The battery in a circuit gives energy tothe electrons and pushes them arounda circuit, from the negative terminal ofthe cell, round the circuit and back tothe positive terminal of the cell.
  9. 9. • The SI unit for electric current is ampere (A).• Smaller currents are measured inmilliamperes (mA).• Different electrical components andappliances require different sizes of currentto turn them on.How to Measure Current?1 A = 1,000 mA1 mA = 0.001 A
  10. 10. Instrument to Measure Current• An ammeter is an instrument used formeasuring electric current.
  11. 11. Ammeter• It must be connected in series in the circuit.• Positive side of ammeter must be connectednearest to the positive terminal of the battery(electric cell), and vice versa.
  12. 12. Electric Circuits• Electric circuits aremade up of electricalcomponents• These componentsmust be joinedtogether without anygap in between toform a closed circuit.connecting wireselectric cellcircuit boardlight bulbNote:Components refer to thelight bulb, wires, battery
  13. 13. Electric Circuits• Incomplete circuits are called open circuits.connecting wire ismissingno source ofelectrical energyBoth the circuits in the diagram are incomplete, hence they are knownas “open circuits”.
  14. 14. An electric current flowsonly when there is:• a source of electrical energy and• a closed circuitconnecting wireselectric cellcircuit boardlight bulb
  15. 15. How to draw Circuit DiagramsComponent Symbol Component SymbolAn electric cell BatteryLight bulb (lamp) SwitchSwitch (open)Switch (closed)Connecting wires(not joined)Connecting wires(joined)+ +Symbols are used to represent the various electricalcomponents in circuits.
  16. 16. Examples of circuit diagrams:
  17. 17. SwitchesA switch is used to open or close a circuit.Main switch usedin buildingsSwitches used on circuit
  18. 18. Circuit diagrams for openand closed circuitsOpen circuit.Bulb does notlight up whenthe switch isClose circuit.Bulb will lightup when theswitch is
  19. 19. Series and ParallelThere are 2 ways in which anelectric circuit can bearranged:1. Series2. Parallel
  20. 20. Series Circuit• A series circuit connects the components oneafter the other• A single loop is formed• A break in any part of a series circuit stops theflow of current in the whole circuit.
  21. 21. Parallel Circuit• A parallel circuit divides into two or more branches.• The current divides and flows through each parallelbranch.• If a component breaks or is removed, the othercomponents remain on.
  22. 22. Which of the following is aseries circuit?Which is a parallel circuit?Series Circuit Parallel Cir
  23. 23. Draw the circuit diagram forthe following set up and statewhether it is a series orparallel circuit.
  24. 24. Draw the circuit diagram forthe following set up and statewhether it is a series orparallel circuit.
  25. 25. Which is brighter?
  26. 26. Which of the following is aseries circuit?Which is a parallel circuit?Series Circuit Parallel Circ
  27. 27. Voltage• An electric cell gives energy to the electrons andpushes them round a circuit. Voltage is a measureof how much energy the electrons receive.• Different voltages are supplied by different cells andbatteries.12 V Car Battery1.5 V Dry Cell9 V Dry Cell
  28. 28. How to MeasureVoltage?• The SI unit for voltage is volt(V).• A voltmeter is an instrumentused for measuring voltages.
  29. 29. Voltmeter• Voltmeters must be connectedin parallel to the circuit.• The positive side of voltmeteris connected to the positiveterminal of the cell, and viceversa.
  30. 30. Recall: Ammeter!!!• It must be connected in series in thecircuit.• Positive side of ammeter must beconnected nearest to the positiveterminal of the battery (electriccell), and vice versa.
  31. 31. Electromotive Force(e.m.f)• Electromotive force is thesame as voltage.• E.m.f refers to the amount ofenergy supplied by the electricsource (eg. battery) to each unitof electric charge.
  32. 32. Electromotive Force(e.m.f)
  33. 33. Potential Difference(P.d.)
  34. 34. Potential Difference(p.d)• Remember Diffusion?• High  Low• Similarly, electric chargeswill flow from a point ofhigher potential (energy) tolower potentialconnectingwireelectriccellfilamentflow of electrons•This difference in electric potential between 2 points in acircuit is known as the potential difference. It is the sameas VOLTAGE also.
  35. 35. Resistance• When an electric current flowsthrough a circuit, there will besome resistance that opposes it.(similar to friction)• It can be measured by dividingvoltage by the current.R =VIR = ResistanceV = VoltageI = Current
  36. 36. Resistance• Good conductors of electricityhave LOW RESISTANCE. (Eg. Metalobjects)– Electricity is able to flowthrough them very easily• Poor conductors of electricityhave HIGH RESISTANCE. (Eg.Wood, cloth)– Electricity is not able to
  37. 37. • The SI unit for resistance is ohm ()• Different electrical componentshave different resistance• For example, nichrome wires have ahigher resistance than copper wires.ResistanceSo should we usenichrome or copperto make wires?
  38. 38. Example 1• An electric rice cookeroperates at 240 V and uses acurrent of 8 A. What is theresistance of the rice cooker?Voltage (V)= 240 VCurrent (I)= 8 AR =VIR =2408= 30 
  39. 39. Resistors• An electrical component that is specially made to have acertain resistance is called a resistor.• They can be connected in a circuit to resist the currentflow.
  40. 40. Fixed Resistors andVariable Resistors• Fixed resistors have only one resistance value• Variable resistors can be adjusted to changethe resistance.fixed resistor symbol variable resistor symbol•Variableresistors areuseful in lightdimmers and
  41. 41. S/E: Resistors• Resistors can be connected inseries or parallelsingle resistorextra resistor inseries results indimmer bulbextra resistor inparallel resultsin brighter bulbResistors connectedin SERIESResistors connectedin PARALLEL
  42. 42. S/E: Resistors inSeries• When resistorsare connected inseries, theresistance willadd up andincrease.• R = R1 + R2 + R3…2  3 The total resistance for this circuit is:2  + 3  = 5 
  43. 43. • When resistors areconnected in parallel, theresistance will decrease• This is because they willprovide alternate routes forthe current to flow.S/E: Resistors inParallel
  44. 44. S/E: Resistors inParallel6 6 1R=1R1R1R+ +1 2 3What is theresistance of thecircuit?1R=1616+1R=2613= R = 3 The finalresistance is
  45. 45. What we will Learn…• What is a current?• How to draw electric circuitdiagrams• What is potential difference, voltageand E.M.F?• What is resistance?• S/E: Circuits arranged in series versusparallel
  46. 46. Heating Effect ofElectric Current• When an electric current flows througha wire, the wire heats up. Electricalenergy has been converted into heatenergy.• The greater the resistance of the wire,the greater the amount of heatproduced. This heating effect is used incommon electrical appliances.Iron Kettle Hair Dryer
  47. 47. A kettle uses both copper and nichrome wires.Copper has low resistance while nichrome has high resistance.Which material, copper or nichrome, should be used for the heatingelement, and for the external wire?Use copper wirefor the externalwire as it has lowresistance &produces less heatUse nichrome wire for the heatingelement as it has high resistance&produces a lot of heat
  48. 48. filament wire producesheat and lightIn a light bulb, theheated filament which isalso a resistancewire, becomes so hotthat light is alsoemitted.Heating Effect ofElectric CurrentARGON (inert/unreactive gas)
  49. 49. Chemical Effect ofElectric CurrentWhat is ELECTROLYSIS?Definition:• Electrolysis is the chemical change that occurs when an electriccurrent passes through solutions or molten compounds.
  50. 50. ElectrolysisThere are 2 uses ofelectrolysis:1) Electroplating2) Extraction of Metals
  51. 51. 1. Electroplating• In electroplating, a key iscovered with a thin layer ofcopper when electricity ispassed through the solution.key copperwirecoppersulphatesolutionstart after a few minutescopper onthe key- +
  52. 52. Uses of ElectroplatingMetal objects can beplated with athin layer of another
  53. 53. 2. Extraction of metals• Some metals (eg. sodium, aluminium) are obtainedby electrolysis.• To extract the metal– heat the solid compound of the metal until it melts– pass an electric current through the moltencompound
  54. 54. Magnetic Effect of aCurrent• A straight wire is placed near acompass. When an electric currentflows through the wire, thecompass needle is deflected. Thisshows that an electric current hasa magnetic effect.compass needle isdeflectedcurrentin wire
  55. 55. Electromagnet• An electromagnet is a magnet thatis made by using electricity• It consist of a wire coiled around ametal rod (eg. Iron)coil of wireto batteryiron corecompass needle is stronglyattracted to iron core
  56. 56. ElectromagnetThe magnetic effect of the electromagnet can beincreased by:1. increasing the current (by using more batteries)2. increasing the number of turns of wire in the coil3. Using an iron rod
  57. 57. Differences between anelectromagnet and a magnet:Electromagnet MagnetA temporary magnetwhich can be turnedon and off usingelectric current.A permanent magnetwhich retainsmagnetism until it ispurposelydemagnetised(spoilt).Magnet can be madestronger or weakerMagnet remains thesame strength
  58. 58. Uses ofElectromagnets• Cranes that liftiron/steel• Iron/steelseparators• Electric bells• Magneticallylevitated trains• Electric motorscraneElectric motor in fansElectric bell