Physics IGCSE – Circuit Electricity Notes
Specification:
, 2.1
Units:
Ampere (A) is for current, Coulomb (C) is for charge, Joule (J) is for energy,
Ohm (W) is for resistance, Second (s) is for time, Volt (V) is for potential
difference, Watt (W) is for power
2.2
In a mains plug, there are three wires:
The live wire (brown) is at a rapidly changing voltage compared to
the earth (literally, the pl fanet). It is the wire that would push a
dangerous current through you into the ground.
The neutral wire (blue) makes the complete circuit with the live wire
and each component. It is at the same voltage as the ground, so it is
safe to touch.
The earth wire (green + yellow) is literally wired straight into the
ground. Its symbol is three parallel lines, of decreasing length.
Electrical safety
Electrical faults are caused by a stray wire on metal/conductive casing.
This can electrocute you, so there are some precautions:
Earthing - Earth wires are put into anything with a conductive case. If
there is a fault, a large current will flow down into the ground, a low
resistance route.
Earth wires are often used in tandem with fuses…
Fuses & Circuit Breakers -
Fuses detect a high current going through to the earth wire, so
The fuse wire in the live wire will melt
So no more current will flow through and
The metal casing is no longer live, so it is safe to touch.
NOTE: the rating of a fuse e.g. 13A, is the maximum current it can carry
before it melts. So when choosing a fuse size from a list, pick the one that
is a little above the normal operating current.
Circuit breakers
do roughly the same thing. They are currently used more, because:
they are easier to reset, and
they respond faster, reducing the risk of a shock.
Double Insulation - Everything that can be touched is covered in an
insulator like plastic. Examples include device chargers + hairdryers.
2.3
Current is the rate of flow of charge.
Specification:
, 2.1
Units:
Ampere (A) is for current, Coulomb (C) is for charge, Joule (J) is for energy,
Ohm (W) is for resistance, Second (s) is for time, Volt (V) is for potential
difference, Watt (W) is for power
2.2
In a mains plug, there are three wires:
The live wire (brown) is at a rapidly changing voltage compared to
the earth (literally, the pl fanet). It is the wire that would push a
dangerous current through you into the ground.
The neutral wire (blue) makes the complete circuit with the live wire
and each component. It is at the same voltage as the ground, so it is
safe to touch.
The earth wire (green + yellow) is literally wired straight into the
ground. Its symbol is three parallel lines, of decreasing length.
Electrical safety
Electrical faults are caused by a stray wire on metal/conductive casing.
This can electrocute you, so there are some precautions:
Earthing - Earth wires are put into anything with a conductive case. If
there is a fault, a large current will flow down into the ground, a low
resistance route.
Earth wires are often used in tandem with fuses…
Fuses & Circuit Breakers -
Fuses detect a high current going through to the earth wire, so
The fuse wire in the live wire will melt
So no more current will flow through and
The metal casing is no longer live, so it is safe to touch.
NOTE: the rating of a fuse e.g. 13A, is the maximum current it can carry
before it melts. So when choosing a fuse size from a list, pick the one that
is a little above the normal operating current.
Circuit breakers
do roughly the same thing. They are currently used more, because:
they are easier to reset, and
they respond faster, reducing the risk of a shock.
Double Insulation - Everything that can be touched is covered in an
insulator like plastic. Examples include device chargers + hairdryers.
2.3
Current is the rate of flow of charge.
