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Electricity in the home (Cables & Plugs (Earthing Metal case - auto…
Electricity in the home
Alternating Current
AC-DC
Torch battery -
d.c. = one direct current
(/p.d.)
Mains -
a.c.
supply =
repeat reverse direction
Successive cycles each second
works w/ bulbs
Frequency - No. of cycles per second (Hz)
1 cycle = 0.02; 0.02/1 = 50 Hz;
Mains Circuit
always
Live, Neutral
; Possible
Earth
Polarity repeats reverse -
a.c.
Neutral earthed at local electric substation
Live, Neutral - dangerous
; p.d. change +/-
P.d. = diff. of Live and Earth; 325 V max (UK)
National Grid(NG)
Buildings supplied from
power stations
Network
of cables,
transformers
Typical generates (+/-)25 kV
Transformers
Increase p.d.
- less current for same power
Power loss from
Resistance heating reduced
NG is efficient in power transfer
Step-up Transformers
Stations to NG -
25 kV to 132 kV
increase
Step-down Transformers
NG to buildings -
132 kV to 230 V
(d.c.)
(Factories - 100 kV to 33 kV)
Cables
& Plugs
Earthing
Metal case - auto connect to earth wire
Prevents live case if wire touches it
Double Insulation
All UK plastic case appliances need it
No need for earth wire (plastic earth pin)
Plugs, Sockets, and cables
Hard-wearing insulators
due to live wires
Most connect to socket with
three-pin plug
Sockets - stiff plastic
, protect wires
Earth - 0 V
; ground connect; current in fault
Longest pin
, case then
auto earthed
plugged
Inside three-pin plug
Brass pins
(good conduct, hard, no rust)
Case - wires don't touch
when sealed
Fuse - In live
; melts, cuts w/ hi current
Blue - Neutral
; Brown - Live;
Green, Yellow - Earth
(only 3-core)
Cable
2/3 wires in rubber, flexible material
Cu
in wires (good conduct, malleable)
Plastic insulator
, prevents shock
2-core; plastic cases
(radios, hairdryer)
Wall socket cables, thicker
(more current)
Wires thick, less resist, not as hot
Short circuits
Live touches neutral, big current passes
Fuse blow (contact)
Electric shock(lethal)
People are 0 V; big p.d. w/ live wire
Current flows through body to ground
Electric power
& P.d.
Power
Current in appliance transfers energy
Energy from power source connected
Power = Energy Transferred / Time
E = Pt; P = IV; P IIR
Current
Rate of flow of charge :star:
Potential Difference
Energy transferred per unit charge :star:
Resistance
Difficulty for charge to flow :star:
Power
Energy transferred in joules per second :star:
Power Calculation
4 A, 12 V Motor =
4 x 12 = 48 W
0.1 A,(x) 3 V Lamp =
0.1 x 3 = 0.3 W
Fuse Choose
Domestic - 3/5/13 A fuse
Power / Voltage = Current
E.g. 2.2 A needs 3 A fuse
E.g. 10.4 A needs 13 A fuse
Resistance heating
Resistor dissipates current through heat
P = IIR; Power proportionate to II
(Double current - Quadruple power)
Electrical currents
& Energy transfer
Charge Calculation
Appliance on; e- forced travel by p.d. of power unit
Q = It
Energy & Potential Difference
Resistor connect to battery
Work done for
e-
to pass resistor
e- collides w/
metal resistor ions
Ions gain
KE
, vibrate more;
heating
Chem battery energy to Resistor thermal
When charge flows through resistor, energy
is transferred to resistor, thus it heats up
E = QV; E = Pt; E = VIt
Energy transfer in circuit
12 V = 12 J/C(oulomb)
Energy increase component thermal store
Bulbs - heat up, light;
Resistors heat
surroundings
Energy transfer to surrounding
by components
Battery energy transfer =
Components energy transfer
V = WQ
;
V = IR
Appliances
& Efficiency
Energy Calculations
1 kW for 2 hr = 2 kW for 1 hr
Work out Energy transfer over time
By appliance power rating and time used
E = Pt
;
P = IV
;
P = IIR
Efficiency
Need power input, useful energy transfer
Efficiency = Percentage or ratio/decimal
Never =>100%
, always below
Waste = Wire heating, motor friction etc
Efficiency = Input / Useful Output
Waste = Efficiency - Useful Output