P2 - Mains Electricity Flashcards
What makes a material an electrical conductor?
Freely moving charged particles
Why are electrical insulators needed?
The human body conducts electricity and this needs to be prevented
What do we do to prevent electric shocks when a live wire is touching a metal casing?
Attach an earth wire to the metal casing to provide a safe route for the current
Why does the earth wire from the casing have a low resistance?
If the live wire touches the casing, there would be an increase in current in the live wire which would cause the fuse in the live wire to melt
What are the colours of the live, neutral and earth wires?
Live is brown
Neutral is blue
Earth is green and yellow
If facing an electric plug, what is each socket? (Live, neutral and earth)
The bottom-right is the live
The bottom-left is the neutral
The top is the earth
What are the live, neutral and earth wires for?
The live carries the electricity to the socket of the plug
The neutral completes the circuit
The earth is connected to the casing of the appliance providing a safe route for the current
Why do some plugs have a plastic earth pin?
The casing is not conductive meaning the earth wire is not needed
What is difference between AC and DC?
AC can change direction but DC travels in one direction only
How often does mains current alternate?
50 Hz
50 times a second
What type of electricity do batteries and cells supply?
Direct current
What is the voltage of mains electricity?
230 volts
Describe how cathode ray oscilloscopes work
If you plug in an AC supply into an oscilloscope, you get a trace on the screen that shows how voltage changes with time
You get a sine wave that goes positive then negative then positive
With a DC supply, you get a straight line
How do you calculate the voltage from an oscilloscope?
For an AC supply, the peak of the sine wave is the voltage
For a DC supply, the height of the straight line is the voltage
How do you calculate the frequency of an AC supply using an oscilloscope?
Calculate the time period by looking at the time between 2 consecutive peaks
Calculate the frequency by doing 1 / time period
On an oscilloscope, what do the gain and timebase dials do?
The gain dial controls how many volts each centimetre division represents on the vertical axis
The timebase dial controls how many milliseconds each division represents on the horizontal axis
What are the 9 electrical hazards in the home?
Long cables
Frayed cables
Cables in contact with something hot or wet
Water near sockets
Shoving things into sockets
Damaged plugs
Too many plugs in one socket
Lighting sockets without bulbs in
Appliances without their covers on
What is the voltage of the neutral wire?
0 volts even though it carries the electricity back
How should fuses be rated?
So that their maximum current is slightly higher than the current of the circuit
How are earth wires different to the love and neutral wires?
They are thicker and haveess resistance so that a higher current can flow in order to melt the fuse
What is double insulation?
When an appliance has a plastic casing and no metal parts showing, it is double insulated
This means that no earth wire is needed because there is no metal casing
What are circuit breakers?
An electrical safety device that detect surges in current and break the circuit by opening a switch
What is the most commonly used circuit breaker and how does it work?
Residual current circuit breakers
Normally, the same current flows through the live and neutral wires but if someone touches the live wire, the neutral wire will carry back less current than the live wire
The device detects this difference in current and quickly opens a switch
What are the 3 advantages of circuit breakers?
Can be reset by flicking a switch where fuses have to be replaced
Break the circuit much faster than fuses
Work for tiny current changes that might not be large enough the melt a fuse
What is the disadvantage of circuit breakers?
Much more expensive than fuses