Direct current: cells/battery supply - current passes in one direction
Alternating current (ac): mains supply - passes in one direction, reverses and passes in other direction
Frequency of UK mains supply = 50Hz - changes direction 50 times/second, voltage of mains = 230V
Live wire of mains supply alternates between positive and negative potential with respect to neutral wire (which stays at 0V)
Live wire alternates between peak voltages of +325V and -325V - equvilent to direct pd of 230V
Frequency of ac supply determined from oscilloscope trace using:
f = 1 / T
f - frequency of ac - Hz
T - time for one cycle - s
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P2.5.2 - Cables and Plugs
Most electrical appliances connected to sockets of mains supply using cable and three-pin plug
Outer cover made of plastic or rubber: insulators, plug pins made of brass: conductor, hard, won't rust or oxidise
Important cable grip fastened tightly over cable - should be no bare wires showing and correct cable must be connected firmly to pin
Brown wire: live pin, blue wire: neutral pin, green and yellow wire: earth pin
Earth wire - only in three-core cable, must have one for appliances with metal cases: case attatched to earth wire in cable, connected to longest pin, appliances with plastic cases don't need to be earthed: double-insulated and connected with two-core cable
Different thickness cable = different purpose - more current = thicker
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P2.5.3 - Fuses
Plastic cased appliances don't need to be earthed: insulated - metal cased appliances are earthed: fault occurs and live wire touches case, case becomes live and may give someone a shock
Fuse - fitted in series with live wire: cuts appliances off if it blows - fault occurs in earthed appliance: large current flows to earth and melts fuse
Fuse rating should be slightly higher than normal working current: too high = fuse won't melt soon enough, too low = fuse will melt when turned on
Circuit breaker - can replace fuse - electromagnetic switch that opens and cuts of supply if current exceeds certain value
Residual current circuit breaker (RCCB) - cuts off current in live wire if different to neutral wire current - faster than fuse or normal circuit breaker
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P2.5.4 - Electrical Power and Potential Difference
Appliances transfer electrical energy other energy forms - rate: power
P = E / t
P - power - W
E - energy transferred - J
t - time - s
More usual to measure current through appliance and pd across than energy transferred and time in electrical circuits
P = I X V (used to calculate current in appliance: work out fuse needed)
P - power - W
I - current - A
V - potential difference - V
Power rating shown on electrical appliances - pd of mains: 230V - fuse size used is slightly higher than fuse value calcualted
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P2.5.5 - Electrical Energy and Charge
Electrical current: rate of flow of charge
Q = I X t
Q - charge - C
I - current - A
t - time - s
Electrical energy changed to other forms when charge flows through appliance
In resistor, electrical energy transferred to resistor: resistor becomes hotter
E = V X Q
E - energy - J
V - potential difference - V
Q - charge - C
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P2.5.6 - Electrical Issues
Electrical faults may occur due to damaged sockets, plugs, cables or appliances: equipment checked regularly for wear - worn/damaged items replaced/repaired by qualified electrician
Aviod overloading sockets - cause overheating: risk of fire
Safely handle appliances and keep away from water
Cable shoud be appropriate for use
Electrical appliance should be chosen according to power, efficiency rating and cost
Filament and halogen bulbs less efficient and don't last as long as low-energy bulbs - number of different low-energy bulbs avaliable
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