Unit 2: 5 - Mains Electricity Flashcards
What is alternating current (ac)?
When the mains supply passes the current in one direction, then reverses it in the other direction and repeats.
What is direct current (dc)?
When cells/batteries pass the current round the circuit in one direction.
What is the frequency of UK mains electricity?
50Hz
What does the frequency mean in ac?
How many times the flow changes direction. E.g. 20Hz means it changes direction 20 times a second.
What does the live wire in a plug do?
Gives the mains supply alternating between a positive and a negative potential, with respect to the neutral wire.
What ‘voltage’ is the mains supply in the UK?
230V
What ‘voltage’ is the neutral wire at?
0V
What peak voltages does the live wire alternate between?
+325 and -325 volts
How can the frequency of ac be determined from an oscilloscope?
f=1/T
Frequency = 1 / Time
What colour is the live wire?
Brown
What colour is the neutral wire?
Blue
What colour is the earth wire?
Green and yellow
The more current carried, the _____ the cable needs to be.
Thicker
Why are the pins in a plug made of brass?
Brass is a good electrical conductor, yet also hard and it will not rust or oxidise.
What is the difference between a two-core and a three-core cable?
A 2-core does not have an earth wire, whereas a 3-core does.
What sorts of appliances must be earthed? And why?
Appliances with metal casings. If there is a fault, the live wire could touch the metal case and make it live, causing a shock to anyone who touches it.
A fuse is fitted in ____ with the live wire.
Series
How does a fuse work?
If a fault develops in an earthed appliance, a large current flows to earth and melts the fuse, cutting off the supply.
How must the rating of a fuse compare to the normal working current of the appliance?
It should be slightly higher (else it would straight when the appliance is switched on).
What is a circuit breaker?
A device that can be used instead of a fuse. It is an electromagnetic switch that cuts off if the current is bigger than a certain value.
What is a residual current circuit breaker (RCCB)?
A device that cuts off the current in the live wire if it is different to the current in the neutral wire.
What are the benefits of an RCCB?
It works faster than a fuse or an ordinary circuit breaker.
Why don’t appliances with a plastic casing need to be earthed?
Because plastic is an insulator so will not deliver a shock.
How do we work out the charge in Coulombs (using current)?
Q=I*t
Charge in coulombs = Current in amperes * time in seconds
