Transcript Safety / Intro

Safety / Intro
• The mains supply and batteries are sources
of Electrical Energy
• Electrical Appliances are Energy Changers
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
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
Plugs
• Mains Plugs
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
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.
Double Insulation
• Appliances with casings made of plastic DO
NOT need an earth wire
• The casing is an insulator
Electric Current
• This is a Flow of Charged Particles
»Q = I x t
• Charge Current
Time
• Coulombs ( C ) Amps ( A ) Seconds ( s )
Voltage
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Energy Given to each coulomb of charge
Units are Volts ( V )
Measured by Voltmeter ,
Connected in parallel
Current
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This is a flow of Charged Particles
Units are Amps, ( A )
Measured using an Ammeter
Connected in Series
Series Circuits
• Current is the SAME
at ALL points
• Voltage across Supply
equals the voltage
across Resistor 1 + voltage across
resistor 2
• Vs = V 1 + V 2
Parallel Circuits
• Voltage across each
is the same
• Current through resistor 1
plus current through
resistor 2 equals the
supply current
• It = I1 + I2
branch
Resistance
• Ohm’s Law V = I x R at constant Temp
• Resistance : measure of how easy it is for
charges to flow.
»
V=IxR
» Volts ( V ) Amps ( A ) Ohms ( Ω
Series Resistors
Rs
= R1 + R2 + R3
Parallel Resistors
1
Rp
=
1
1
1
+
+
R1 R2 R3
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
Variable Resistors
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Uses
Light Dimmer Switches
Volume controls
Speed Controls
Temperature Controls for grills/ ovens …
Computer Joysticks
Fuel gauges
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.
Power
This is the amount of energy transferred per
second. Units are Watts ( W ).
Heat producing appliances have big power
ratings
Energy Tranferred
E
Power =
P =
Time
t
One Watt is one joule per sec ond (1W = 1J / s )
Power
• Power can also be calculated from
Power = Voltage x Current
P =V xI
Power = Current squared x resis tan ce P = I 2 x R
V2
Power = Voltage squared ÷ resis tan ce P =
R
Lamps
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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
Direct Current
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Current flows in one direction ONLY
Electrons flow from Negative to Positive
Size of Voltage remains constant
Batteries supply D.C.
Alternating Current
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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
Quoted
mains
value, 230 V
Current flows
0V Line
when below 0V line
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
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
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
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
Electromagnetism
•• aWhen a current flows there is a Magnetic
Field around a wire
• If a wire is placed inside a magnetic field it
experiences a force
N
Current
flows out
of board
S
S
S
N
N
Current flows
into board
Electromagnetism
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Relays
Motors
Bells
Loudspeakers
Tape Recorders
All use Electromagnetic effect
The d.c. motor
d
8
• Each side of the coil experiences forces acting
• ain opposite directions the coil rotates
+VE
N
Rotation is
produced
S
-VE
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d.c. motor
a
If the flow of current WAS NOT reversed
then the coil would flip back
• a
• A SPLIT RING COMMUTATOR reverses
the flow of current
+VE
N
S
-VE
The d.c.
a motor
• The
b coil continues to rotate
+VE
N
S
-VE
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
Commercial motors
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