Chapter 5 - Mains Electricity Flashcards
What supplies direct current
Cells and batteries
What is alternating current
The current from the mains supply passes in one direction, then reverses and passes in the other direction
What is the frequency of the U.K. mains supply and what does it mean
50 Hz. This means it chances direction 50 times each second
What is the voltage of the mains
230 v
Why is the outer cover of the three pin plug made of plastic.
It’s a good electrical insulator
Why are the pins of the plug made of brass
Brass is a good electrical conductor. It is also hard and will not rust or oxidise
What is the brown wire connected to
The live pin.
What is the blue wire connected to
The neutral pin
What is the green and yellow wire connected to
The earth pin
Why do appliances with metal cases need to be earthed but appliances with plastic cases do not
If a fault develops, and the live wire touched the metal case, the becomes live and could give a shock to anyone who touches it. Plastic is an insulator and cannot become live
What is the purpose of a fuse
A fuse contains a thin wire that heats up and melts if too much current passes through it which cuts off the current.
Why should the rating of the fuse be slightly higher than the normal working current of the appliance
So that it melts at the correct time. If it is much higher, it will not melt soon enough. If it is not higher than the normal current, it will melt as soon as the appliance is switched on
What is a circuit breaker
Something that is used in place of a fuse. It’s an electromagnetic switch that opens and cuts off the supply if the current is bigger than a certain value
What does a residual current circuit breaker do and what are its advantages
It cuts off the current in the live wire if it is different to the current in the neutral wire. It works faster than a fuse or ordinary circuit breaker
Why are filament bulbs very inefficient
Doesn’t last as long as a low energy bulb
