Mains Electricity Flashcards
All 18 cards in this deck
What is direct current (d.c.)?
A movement of charge in one direction only.
What is alternating current (a.c.)?
A current in which the movement of charge repeatedly changes direction.
What type of current do cells and batteries supply?
Direct current (d.c.).
What is the difference between a direct voltage and an alternating voltage?
A direct voltage keeps the same polarity (positive stays positive); an alternating voltage repeatedly swaps polarity, so it pushes charge first one way then the other.
On an oscilloscope or voltage–time graph, how can you tell an a.c. supply from a d.c. supply?
a.c. gives a repeating wave that goes above and below zero; d.c. gives a flat horizontal line at a constant value.
True or false? The current from a cell changes direction 50 times every second.
False. A cell supplies d.c., which flows in one direction only; it is UK mains a.c. that alternates, at 50 Hz.
What are the type, frequency and voltage of the UK domestic mains supply?
a.c., at a frequency of 50 Hz and a voltage of about 230 V.
What is the function of the live wire in a mains cable?
It carries the alternating potential difference from the supply to the appliance.
What is the function of the neutral wire in a mains cable?
It completes the circuit, carrying current back from the appliance to the supply.
What are the potential differences of the live, neutral and earth mains wires (relative to earth)?
Live: about 230 V. Neutral: about 0 V. Earth: 0 V.
True or false? The potential difference between the neutral wire and the earth wire is about 230 V.
False. It is about 0 V; it is between the live and earth (or live and neutral) that there is about 230 V.
What is the function of the earth wire in an appliance with a metal case?
It is a low-resistance safety wire connecting the metal case to earth, so a faulty live case cannot stay at a high potential.
What is the function of a fuse in a mains circuit?
A thin wire that melts and breaks the circuit if the current gets too large.
What are the steps by which the earth wire and fuse make an appliance safe?
e.g. a loose live wire touches the metal case
- The case becomes live and a large current flows through the earth wire to earth.
- This large current melts the fuse in the live wire.
- The circuit breaks, disconnecting the appliance from the live supply, so the case is safe to touch.
In which wire must switches and fuses be fitted, and what does this achieve?
In the live wire — so that when the switch is off or the fuse blows, the appliance is cut off from the 230 V potential and is safe.
What happens if a connection is made between the live wire and earth?
e.g. a person touches a bare live wire
A large current flows through the connection to earth — causing an electric shock, burns, fire, or death.
What are the steps to choose a suitable fuse rating for an appliance?
e.g. an appliance that normally draws 3 A
- Find the normal working current: 3 A.
- Choose the next standard fuse rating above it (3 A, 5 A, 13 A): 5 A.
- Fit the fuse in the live wire.
True or false? A circuit breaker must be replaced after it has switched off the circuit.
False. A circuit breaker can be reset and used again, unlike a fuse, whose wire melts and must be replaced.