Safety / Intro

1. Safety / Intro • The mains supply and batteries are sources of Electrical Energy • Electrical Appliances are Energy Changers

2. Power • Heat producing appliances have a power rating greater than 1000 Watts • Power Rating is the amount of energy used per second • Appliances greater than 700 W are protected by a 13 A fuse

3. Fuses • This protects the flex from overheating, melting and causing a fire. • Fuses are connected to the LIVE pin. • When the fuse blows it isolates the appliance from the ‘high voltage’ supply

4. Plugs • Mains Plugs

5. Human Conductivity • We conduct electricity very well • When wet the body’s conductivity increases • Bathroom light switches, shower switches are either outside the bathroom or on a cord

6. Earth Wire • Connected to casing of metal appliances • If live wire touches the casing ‘ current flows through earth wire to ground ( often via the water pipes ) • A large current flows , blowing the fuse and isolating the appliance from the high voltage mains.

7. Double Insulation • Appliances with casings made of plastic DO NOT need an earth wire • The casing is an insulator

8. Electric Current • This is a Flow of Charged Particles • Q = I x t • Charge Current Time • Coulombs ( C ) Amps ( A ) Seconds ( s )

9. Voltage • Energy Given to each coulomb of charge • Units are Volts ( V ) • Measured by Voltmeter , • Connected in parallel

10. Current • This is a flow of Charged Particles • Units are Amps, ( A ) • Measured using an Ammeter • Connected in Series

11. Series Circuits • Current is the SAME at ALL points • Voltage across Supply equals the voltage across Resistor 1 + voltage across resistor 2 • Vs = V1 + V2

12. Parallel Circuits • Voltage across each branch is the same • Current through resistor 1 plus current through resistor 2 equals the supply current • It = I1 + I2

13. Resistance • Ohm’s Law V = I x R at constant Temp • Resistance : measure of how easy it is for charges to flow. • V = I x R • Volts ( V ) Amps ( A ) Ohms (

14. Series Resistors

15. Parallel Resistors

16. Variable Resistors • A length of wire is wrapped into a cylinder • Different lengths can be tapped into • The longer the length the bigger the resistance • The bigger the resistance the lower the current

17. Variable Resistors • Uses • Light Dimmer Switches • Volume controls • Speed Controls • Temperature Controls for grills/ ovens … • Computer Joysticks • Fuel gauges

18. Continuity Tester Used to detect ‘Open Circuits’ or Breaks in Circuits such as blown bulbs. Component is blown if there is NO reading on the meter.

19. Power This is the amount of energy transferred per second. Units are Watts ( W ). Heat producing appliances have big power ratings

20. Power • Power can also be calculated from

21. Lamps • Discharge Lamps ( Fluorescent Tubes ) • An electric current passes through the gas • Virtually NO heat Energy is produced • MUCH more efficient than • Filament lamps • An electric current is passed through a piece of resistance wire which heats up • Produces more heat energy than light energy

22. Direct Current • Current flows in one direction ONLY • Electrons flow from Negative to Positive • Size of Voltage remains constant • Batteries supply D.C.

23. Alternating Current • Mains electricity is A.C. • The current constantly changes direction • Mains electricity is 50 Hz , 50 cycles in 1 second • The size of the voltage is constantly changing. Quoted value ( 230 V mains )is smaller than the peak value Peak value, 325 V for mains Quoted value, 230 V Current flows when below 0V line 0V Line

24. Behind the Wall • Household appliances are connected in parallel : same voltage ( 230 V ) and independent switching • Sockets are wired in a RING circuit: • Current flows via 2 paths therefore smaller currents flow and thinner cable can be used • Less cable needed than conventional parallel • Easy to add extra sockets

25. Behind the Wall • Lighting circuit draws less current than Ring circuit for sockets • Lighting circuit conventional parallel • Lighting circuit uses a 10A circuit breaker as opposed to a 30A on the ring circuit

26. Fuses / Circuit Breakers • Mains fuses protect the mains wiring • Circuit Breaker is an automatic switch that switches off when current exceeds stated value • Circuit breakers can easily be reset and used again but fuses must be replaced

27. Kilowatt Hours • I unit of electricity is 1kWh • I.e. A 1kW appliance switched on for 1 hour • total units = Power (kW) x time ( hours ) • 1 kWh = Power x time • 1000 W x 60 x 60 seconds • = 3 600 000 Joules

28. Electromagnetism • When a current flows there is a Magnetic Field around a wire • If a wire is placed inside a magnetic field it experiences a force • a N S S N Current flows out of board Current flows into board S N

29. Electromagnetism • Relays • Motors • Bells • Loudspeakers • Tape Recorders • All use Electromagnetic effect

30. 8 The d.c. motor N +VE S -VE d • Each side of the coil experiences forces acting in opposite directions the coil rotates • a Rotation is produced

31. d.c. motor N +VE S -VE 9 a • If the flow of current WAS NOT reversed then the coil would flip back • A SPLIT RING COMMUTATOR reverses the flow of current • a

32. N +VE S -VE a The d.c. motor • b • The coil continues to rotate

33. Commercial motors • More coils : more powerful motor, smoother turning • Electromagnets instead of permanent magnets : increased magnetic field, switched on / off • Brushes as contacts : mould themselves to shape of commutator

34. Commercial motors