EX unit two: electricity Flashcards
what colour is the earth wire
green and yellow
what colour is the live wire
red/brown
what colour is the neutral wire
blue
when wiring a three pin plug, what must the cable insulation be firmly gripped by
the cable anchor
all mains wiring is insulated with how many layers of insulation
2
why is all mains wiring insulated with 2 layers
it prevents the separate conductors (live, neutral and earth) from touvhing and prevents anyone touching a live (mains voltage) wire.
what does it mean when some appliances are double insulated (as well as the wiring)
that the outer casing is made out of an insulating material like plastic
why is it useful to double insulate appliances
so that no one gets an electric shock.
appliances with a metal outer casing that the user might touch must be…
earthed. the earth wire ensures that the outer casing is held at 0V and provides a very low resistance path for the current in the event of a fault in which the live wire touches the casing. this means the fault current will be very large and cause the fuse to blow immediatly, disconnecting the live supply.
what material are cartridge fuses
ceramic
what happens to a fuse once it has blown
it has to be replaced
what are circuit breakers
they perform the same job as fuses but are usually operated magnetically and may be reset by pressing a button.
what are RCCB’s
residual current circuit breakers
why do cables that carry electricity around our homes have very low electrical resistance
to make the unwanted conversion of electricity as small as possible.
power =
current x voltage
what is current measured in
amps (A)
what is voltage measured in
volts (V)
what is power
the rate at which electrical energy is converted in an appliance.
energy =
power x time
what is energy measured in
joules (J)
what does AC stand for
alternating current
what does DC stand for
direct current
what is alternating current
an alternating supply, like the mains, causes the current to change continuously, with electricity flowing in one direction then the other.
what is direct current
a direct supply, like batteries, makes electricity flow in one direction only.
why are metals good conductors of electricity
they have large numbers of de-localised electrons - electrons that aren’t bound to any particular atom in the structure of the metal. they are free to move, so when an electric force acts on the electrons it forces them to ‘drift’ in the direction of this electric force.
electric current is
the rate of flow of electric charge through ta conductor
what is charge measured in
coulombs (C)
current =
charge / time
what is the symbol for charge
Q
1 volt =
1 joule per couloumb
energy =
charge x voltage
what measures current
an ammeter
what measures voltage
a voltmeter
what does an ammeter tell you
how many coulombs of charge are passing through the lamp per second.
what does voltage tell you
how many joules of energy are being converted to heat and light per coulomb of charge are passing through the lamp.
is an ammeter placed in series or parallel
series
is a voltmeter placed in series or parallel
parallel
why is the current always the same as it flows through a series circuit
there are no alternative routes for the current to follow instead.
why isn’t the current always the same as it flows through a parallel circuit
the current can split to take two or more routes.
what appliances are wired in parallel
mains sockets outlets, mains lights and decorative lights.
in a series circuit, the energy supplied by the cell is _________ between the components so the voltage across each component will _________
shared
vary
in a parallel circuit, each component has its own connections to the cell so does not have to ________. the voltage across each branch in the circuit will be the ______.
share
same
voltage =
current x resistance
what is the symbol for resistance
R
what is resistance measured in
ohms
give a use for a resistor
simple lamp dimmer circuits or volume control in audio equipment
resistors can be varied by
rotary or sliding action
what are thermistors made from
semi-conductors
thermistor’s resistance decreases as…
temperature increases
what are LDR’s made from
semi-conductors
more light shining on an LDR =
less resistance
what are LDR’s used in
light sensing circuits
diodes and LED’s only allow an electric current to pass in the direction of…
the arrow. one direction has huge resistance and the other has very little.
insulators examples
rubber, glass, and many types of plastic
protons are _____ charged
positively
neutrons carry no ________ ________
electric charge
electrons are ________ charged
negatively
the charges on the proton and electron are?
equal in size but of opposite sign
all atoms have equal numbers of electrons and protons, so the charges balance out and the atoms are overall,
neutral.
unlike charges _______ and like charges ________
attract
repel
what happens when you rub two materials together?
some electrons are torn from the surface of one of the materials and transferred to the other. the material which loses electrons now has a net positive charge because there are now more positively charged protons than electrons. the material that gained electrons now has more negatively charged electrons than positively charged protons and is therefore negatively charged
uses of static electricity
inkjet printers, photocopiers, paint spraying, electrostatic precipitators.
problems with static electricity
electric shocks, fuelling tanks and aircraft, handling microprocessors and computer ‘chips’.
