IGCSE PHYSICS – TOPIC 2
ELECTRICITY
a) MAINS ELECTRICITY
• FUSES – a safety device designed to CUT OF FLOW of electricity to an
appliance if the current becomes too large sue to a fault/surge
o It is usually made from a glass cylinder and a thin metal wire:
o If the current becomes TOO LARGE the wire heats up and
melts which causes the circuit to break and the current to stop
o An RCD (circuit breaker) trips the switch
• EARTH WIRE – many electrical appliances have metal cases – safety
hazard. If a live wire came into contact with the case it would become
electrified – electrocution hazard - an earth wire reduces this risk
o In a plug there are 3 wires: neutral, live and earth:
o BLUE WIRE – kept at 0V – completes circuit – the neutral wire
o BROWN WIRE – carries 230V a.c. – the live wire
o YELLOW/GREEN – stops the appliance becoming live – the earth
wire
o The EARTH WIRE provides a low resistance path to the Earth – this
causes a SURGE OF CURRENT IN WIRE (earth and live) – the HIGH
CURRENT causes the fuse to break which cuts of the electricity to
the appliance – safe!
• DOUBLE INSULATION – some appliances have two layers of insulation –
insulation AROUND THE WIRE and a NON-METALLIC CASE as a second
layer – these appliances do not need an earth wire and are marked
with:
• Electricity in metals is caused by THE FLOW OF ELECTRONS – as the
electrons pass through the metal THEY COLLIDE WITH IONS – the ions
RESIST the flow of electrons:
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, o When electrons collide they lose energy, giving it to the ions –
they START TO VIBRATE
o As a result the metal HEATS UP
o This effect is used in many electric heaters, ovens, electric hobs,
toasters and kettles
• Power is the RATE OF ENERGY TRANSFER:
Power (w) = Current x Voltage
P=IxV
• SELECTING FUSES – fuses come in a variety of sizes – 3A, 5A AND 13A
o In order to select the right one, you need to know the CURRENT
of the appliance (P = IV)
o The fuse should always have a HIGHER CURRENT than needed –
but not too high!
Energy Transferred (J) = Current x Voltage x Time or Power x Time
E = I x V x T or P x T
• ALTERNATING CURRENT – (a.c.) a current that changes direction
continuously (e.g. mains, generators)
• DIRECT CURRENT – (d.c.) a current that is steady and constantly flowing
from + -> - (e.g. batteries)
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ELECTRICITY
a) MAINS ELECTRICITY
• FUSES – a safety device designed to CUT OF FLOW of electricity to an
appliance if the current becomes too large sue to a fault/surge
o It is usually made from a glass cylinder and a thin metal wire:
o If the current becomes TOO LARGE the wire heats up and
melts which causes the circuit to break and the current to stop
o An RCD (circuit breaker) trips the switch
• EARTH WIRE – many electrical appliances have metal cases – safety
hazard. If a live wire came into contact with the case it would become
electrified – electrocution hazard - an earth wire reduces this risk
o In a plug there are 3 wires: neutral, live and earth:
o BLUE WIRE – kept at 0V – completes circuit – the neutral wire
o BROWN WIRE – carries 230V a.c. – the live wire
o YELLOW/GREEN – stops the appliance becoming live – the earth
wire
o The EARTH WIRE provides a low resistance path to the Earth – this
causes a SURGE OF CURRENT IN WIRE (earth and live) – the HIGH
CURRENT causes the fuse to break which cuts of the electricity to
the appliance – safe!
• DOUBLE INSULATION – some appliances have two layers of insulation –
insulation AROUND THE WIRE and a NON-METALLIC CASE as a second
layer – these appliances do not need an earth wire and are marked
with:
• Electricity in metals is caused by THE FLOW OF ELECTRONS – as the
electrons pass through the metal THEY COLLIDE WITH IONS – the ions
RESIST the flow of electrons:
11
, o When electrons collide they lose energy, giving it to the ions –
they START TO VIBRATE
o As a result the metal HEATS UP
o This effect is used in many electric heaters, ovens, electric hobs,
toasters and kettles
• Power is the RATE OF ENERGY TRANSFER:
Power (w) = Current x Voltage
P=IxV
• SELECTING FUSES – fuses come in a variety of sizes – 3A, 5A AND 13A
o In order to select the right one, you need to know the CURRENT
of the appliance (P = IV)
o The fuse should always have a HIGHER CURRENT than needed –
but not too high!
Energy Transferred (J) = Current x Voltage x Time or Power x Time
E = I x V x T or P x T
• ALTERNATING CURRENT – (a.c.) a current that changes direction
continuously (e.g. mains, generators)
• DIRECT CURRENT – (d.c.) a current that is steady and constantly flowing
from + -> - (e.g. batteries)
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