Mains electricity

Question 1

What is direct current and what supplies it?

Answer 1

Current that always passes in the same direction

Cells and batteries

Question 2

What is mains electricity?

Answer 2

Alternating current (a.c) supply, constantly changes direction

Question 3

Describe the mains supply in the UK

Answer 3

Frequency of 50 cycles per second (50 hertz, Hz)

230V

Question 4

How are most electrical appliances connected to the mains?

Answer 4

Cable and a three-pin plug

Question 5

What are the components in a three-pin plug? What materials are they made from and what colours (when relevant)?

Answer 5

Live wire (copper with red/brown rubber insulation), neutral wire (copper with blue rubber insulation), earth wire (copper with green and yellow striped rubber insulation), fuse (thin metal wire that melts at relatively low temperatures), cable grip (thermosetting plastic) and outer insulation (thick rubber)

Question 6

Difference between two-core and three-core cables

Answer 6

Two-core has live and neutral wires

Three-core has an additional earth wire

Question 7

What might happen if there is an electrical fault?

Answer 7

The current becomes too great, and the circuit is disconnected by a fuse or circuit breaker in the LIVE wire

Question 8

How does a fuse work?

Answer 8

When the current exceeds a set point in a fuse wire, it will melt, breaking the circuit

Question 9

How does a Residual Current Circuit Breaker (RCCB) operate?

Answer 9

Detects a difference in current between the live and neutral wires and then breaks the circuit

Question 10

Advantage of using a Residual Current Circuit Breaker (RCCB) over a fuse?

Answer 10

Operates much faster than a fuse

Question 11

What types of appliances need an earth wire, and why don’t others?

Answer 11

Appliances with metal (or conductive) cases

Double insulated appliances (i.e. insulated wires and a case made of plastic/wood/etc.) have no earth connection

Question 12

What do the earth wire and fuse do?

Answer 12

Protect the wiring of the circuit

Prevent leakage and therefore electric shocks when there is a fault

Question 13

What is the link between cable thickness and fuse value? Why?

Answer 13

The fuse value must be much less than the maximum current capacity of the cable
The fuse must melt before any other component - it must be the ‘weakest link’

Question 14

Describe the energy transfers through a resistor

How can we reduce energy waste in filament bulbs?

Answer 14

Electrical -> heat

Using power saving lamps such as Compact Fluorescent Lamps (CFLs) - i.e. using more EFFICIENT lamps

Question 15

Relationship between power, energy and time

Answer 15

P = E/t [P is power in Watts, W; E is energy in Joules, J; t is time in seconds, s]

Question 16

Relationship between power, potential difference and voltage

Answer 16

P = I x V [P is power in Watts, W; I is current in Amperes (amps), A; V is potential difference (voltage) in Volts, V]

Question 17

Relationship between energy, potential difference and charge

Answer 17

E = V x Q [E is energy in Joules, E; V is potential difference (Voltage) in Volts, V; Q is charge in coulombs, C]