P2.4 Using mains electricity safely Flashcards
What is a direct current?
a current that always flows in the same direction
it is always at the same voltage
What type of current does mains electricity supply?
an alternating current
What is an alternating current (a.c) ?
A current that is constantly changing direction
What voltage is the mains electricity in the UK?
about 230 v
What is the frequency of the mains electricity in the UK?
50 cycles per second (50 hertz)
Describe the oscilloscope trace of an alternating current
a regular repeating wave.
some of the time it is positive and some of the time it is negative
Describe the oscilloscope trace of a direct current
a straight line
What is the time period of a wave?
The time taken to complete 1 cycle
On an oscilloscope trace, how do you find the time period of the wave?
measure the horizontal difference between 2 peaks then multiply by the time base which will be given.
On an oscilloscope trace, how do you find out the frequency of the wave?
frequency = 1 (divided by) time period
On an oscilloscope, what does the timebase dial control?
how many milliseconds each devision represents on the horizontal axis
On an oscilloscope, what does the gain dial control?
how many volts each cm division represents on the horizontal axis
On an oscilloscope screen, what does the vertical hight of the a.c show?
the input voltage at that point
How can you measure the potential difference of an a.c supply on an oscilloscope?
By measuring the hight of the trace
How can you measure the potential difference of a d.c on an oscilloscope screen?
measure the distance from the straight line trace to the centre line
What are most electrical appliances connected to the mains using?
a cable and three-pin-plug
What type of current do cells and batteries supply?
a direct current (d.c)
Describe the structure of an electric cable
3 wires inside, each with a core of copper and a coloured plastic coating
Live wire- brown, alternates between a positive and negative voltage
Neutral wire- blue, always at 0V
Earth wire- green and yellow- protects the wiring and for safety. works together with the fuse to prevent fire and shocks. It is attached to the metal casing of the appliance and carries electricity to the earth if something went wrong or if the live or neutral wires touched the metal case
Describe the function of the earth wire in an electrical cable and three-pin-plug
Earth wire- green and yellow
protects the wiring and for safety.
works together with the fuse to prevent fire and shocks.
It is attached to the metal casing of the appliance and carries electricity to the earth if something went wrong or if the live or neutral wires touched the metal case
What materials are used in a three-pin plug?
main parts are made of copper or brass- as good conductors
case, cable grip, cable insulation - rubber or plastic as they are good insulators and flexible
What are some safety features of a three-pin-plug?
right coloured wire is connected to each pin, and firmly screwed in
no bare wires showing in the plug
cable grip is tightly fastened over the cable outer layer
What happens if an electrical fault causes too great a current?
the circuit is disconnected by a fuse or circuit breaker in the LIVE wire
What happens when the current through a fuse exceeds the rating of the fuse?
The fuse will melt, breaking the circuit
Why does the fuse rating needed for cables increase with cable thickness?
Thicker cables have less resistance, so carry more current
What does it mean if an appliance is double insulated?
if the appliance has a plastic casing and no metal parts are showing. It does not need an earth wire.
What are two-core-cables?
cables that only carry the live and neutral wire
They do not need an earth wire as the appliance is double insulated
What is ‘earthing’ an appliance?
appliances with metal cases are usually earthed to reduce the danger of electric shock
means the case must be attached to an earth wire.
What does RCCBs stand for?
Residual Current Circuit Breakers
What do RCCBs do and how do they work?
detect a difference is the current between the live and neutral wires and break the circuit by opening a switch
What are some advantages of using RCCBs over fuses?
Safer- operate much faster as no time is wasted waiting for the current to melt the fuse
More effective- work for small currents that may not be enough to melt a fuse
more convenient- can easily be reset by flicking a switch whereas fuses need to be replaced
What happens when an electrical charge flows through a resistor?
the resistor gets hot
How do filament lamps work and why is a lot of energy wasted?
work by passing a current through a very thin wire. - means there is lots of resistance- heated up so much that it glows.
therefore, lots of energy is wasted as heat
What is an example of a power saving lamp?
Compact Fluorescent Lamps (CFLs)
In the equation
P= E (divided by) t
what do the letters stand for and what are the units?
P- power (W)
E- energy (J)
t- time (s)
In the equation
P= I x V
What do the letters stand for and what are the units?
P- power (W)
I-current (A)
V- potential difference (V)
In the equation
E= V x Q
what do the letters stand for and what are the units?
E- energy (J)
V- potential difference (V)
Q- charge in coulombs (C)
Explain why the kinetic output of a motor will always be less than predicted?
As the motor will not be 100% efficient.
some of the energy will be transformed into heat and sound
Why may an appliance not be safe if there is no earth wire?
The case may become live if there is an electrical fault
this causes a danger of electric shock
Describe the structure of a cable that connects a cooker to the mains electricity supply
contains a live, neutral and earth wire
theses wires should be insulated with plastic
What is the difference between an alternating current and a direct current?
a. c is constantly changing direction
d. c flows in one direction only
