P2 UNIT 5

P2 UNIT 5

1. 5.1 ALTERNATING CURRENT

   • WHEN YOU SWITCH A LIGHT ON AT HOME, YOU USE THE ALTERNATING CURRENT BECAUSE MAINS ELECTRICITY IS AN AC SUPPLY
     • REPEATEDLY REVERSES DIRECTION
   • MAINS CIRCUITS
     • EVERY MAINS CIRCUIT HAS A LIVE WIRE AND NEUTRAL WIRE. IVE WIRE IS DANGEROUS BECAUSE ITS VOLTAGE IS REPEATEDLY CHANGING FROM POSITIVE TO NEGATIVE.
   • MEASURING AN ALTERNATING POTENTIAL DIFFERENCE
     • AN OSCILLOSCOPE CAN BE USED TO MEASURE THE PEAK POTENTIAL DIFFERENCE AND THE FREQUENCY OF A LOW VOLTAGE AC SUPPLY.
   • DIRECT CURRENT
     • ONE DIRECTION ONLY ALTERNATING CURRENT REPEATEDLY REVERSES ITS DIRECTION

2. 5.2 CABLES AND PLUGS

   • PLUGS, SOCKETS AND CABLES
     • OUTER CASINGS OF ALL OF THE ABOVE ARE MADE OF HARD WEARING ELECTRICAL INSULATORS. THIS IS BECAUSE PLUGS, SOCKETS AND CABLES CONTAIN LIVE WIRES. MOST MAINS APPLIANCES ARE CONNECTED WIA A WALL SOCKET TO THE MAINS USING CABLE AND A THREE PIN PLUG.
       • SOCKETS ARE MADE OF STIFF PLASTIC MATERIAL WITH THE WIRES INSIDE.
         • THE EARTH WIRE OF THIS CIRCUIT IS CONNECTED TO THE GROUND AT YOUR HOME. THE LONGEST PIN OF A THREE PIN PLUG IS DESIGNED TO MAKE CONTACT WITHT HE EARTH WIRE OF A WALL SOCKET CIRCUIT. SO WHEN YOU PLUG AN APPLIANCE WITH A METAL CASE TO A WALL SOCKET THE CASE IS AUTOMATICALLY EARTHED.
       • CABLES USED FOR MAINS APPLIANCES CONSIST OF TWO OR THREE INSULATED COPPER WIRES SURROUNDED BY AN OUTER LAYER OF RUBBER OR FLEXIBLE PLASTIC MATERIAL.
         • COPPER IS USED FOR THE WIRES BECAUSE IT IS A GOOD ELECTRICAL INSULATOR AND IT BENDS EASILY : PLASTIC IS A GOOD ELECTRICAL INSULATOR AND THEREFORE PREVENTS ANYONE TOUCHING THE CABLE FROM RECEIVING AN ELECTRIC SHOCK.

3. 5.3 FUSES

   • A FUSE CONTAINS A THIN WIRE THAT HEATS UP, MELTS AND CUTS OFF THE CURRENT IF THE CURRENT IS TOO LARGE. A FUSE IS ALWAYS FITTED IN SERIES WITHT HE LIVE WIRE. THIS CUTS THE APPLIANCE OFF FROM THE LIVE WIRE IF THE FUSE BLOWS.
   • THE IMPORTANCE OF EARTHING
     • THE EARTH WIRE MEANS THAT APPLIANCES WITHIN A METAL CASING DO NOT INSULATE ELECTRICAL CURRENT AND THEREFORE DO NOT ELECTROCUTE PEOPLE WHEN THEY TOUCH THE CASING.
   • CIRCUIT BREAKERS
     • IS AN ELECTROMAGNETIC SWITCH THAT OPENS AND CUTS OFF THE CURRENT IF TOO MUCH CURRENT PASSES THROUGH THE CIRCUIT BREAKER. A MAINS APPLIANCE WITH A PLASTIC CASE DOES NOT NEED TO BE EARTHED BECAUSE PLASTIC IS AN INSULATOR AND CANNOT BECOME LIVE.

4. 5.4 ELECTRICAL POWER AND POTENTIAL DIFFERENCE

   • CALCULATING POWER
     • THE CURRENT THROUGH IT IS THE CHARGE THAT FLOWS THROUGH IT EACH SECOND : THE POTENTIAL DIFFERENCE ACROSS IT IS THE ENERGY TRANSFERRED TO THE APPLIANCE BY EACH COULOMB OF CHARGE THAT PASSES THROUGH IT : THE POWER SUPPLIED TO IT IS THE ENERGY TRANSFERRED TO IT EACH SECOND. THIS IS THE ELECTRICAL ENERGY IT TRANSFERS EVERY SECOND.
   • CHOOSING A FUSE
     • YOU USE THE POWER RATING OF THE APPLIANCE AND ITS POTENTIAL DIFFERENCE. THE NEXT TIME YOU CHANGE A FUSE, DO A QUICK CALCULATION TO MAKE SURE ITS RATING IS CORRECT FOR THE APPLIANCE.

5. 5.5 ELECTRICAL ENERGY AND CHARGE

   • CALCULATING CHARGE
     • CHARGE = CURRENT X TIME
       • WHEN AN ELECTRICAL CHARGE FLOWS THROUGH A RESISTOR, ENERGY TRANSFERRED TO THE RESISTOR MAKES IT HOT.
   • ENERGY TRANSFER IN A CIRCUIT
     • WHEN CHARGE FLOWS ROUND A CIRCUIT FOR A CERTAIN TIME, THE ELECTRICAL ENERGY SUPPLIED BY THE BATTERY IS EQUAL TO THE ELECTRICAL ENERGY TRANSFERRED TO ALL TGE COMPONENTS IN THE CIRCUIT.

6. 5.6 ELECTRICAL ISSUES

   • FAULT PREVENTION
     • IF A FUSE BLOWS OR A CIRCUIT BREAKER TRIPS WHEN A MAINS APPLIANCE IS IN USE, SWITCH THE APPLIANCE OFF. THEN DONT USE UNTIL AN ELECTRICAL PROFESSIONAL ADVISES. IF AN APPLIANCE OVERHEATS TURN OFF AND DONT TOUCH.
     • AN ELECTRICIAN SLECTING A CABLE FOR AN APPLIANCE NEEDS TO USE : A TWO CORE CABLE IF THE APPLIANCE IS DOUBLE INSULATED AND NOE EARTH WIRE IS NEEDED : A THREE CORE CABLE IF AN EARTH WIRE IS NEEDED BECAUSE THE APPLIANCE HAS A METAL CASE : A CABLE WITH CONDUCTORS OF SUITABLE THICKNESS SO THE HEATING EFFECT OF THE CURRENT IN THE CABLE IS INSIGNIFICANT.
   • NEW BULBS FOR OLD
     • FILAMENT BULBS ARE VERY INEFFICIENT : LOW ENERGY BULBS ARE MORE EFFICIET SO THEY DONT BECOME HOT LIKE FILAMENT BULBS DO : LOW ENERGY COMPACT FLUORESCENT BULBS ARE NOW USED FOR ROOM LIGHTING INSTEAD OF FILAMENT BULBS : LOW ENERGY LIGHT EMITTING DIODES USED FOR SPOTLIGHTS ARE MORE EFFICIENT THAN FILAMENT OR HALOGEN BULBS AND THEY LAST LONGER.