Electricity at home: alternating and direct current
 Direct current flows in one direction (from negative terminal to
positive terminal)
 A battery is a source of direction current.
 Alternating current (and voltage) change size and direction in a
regular and repetitive way.
 Mains electricity (a plug socket) or a generator are sources of
alternating current.
 The average voltage from the mains supply is 230V
Electrical safety
There are four electrical safety features which you need to know
about:
1. The three-pin plug
2. Fuses
3. Double insulation
4. The earth wire
Electricity at Home summary notes
1. The three-pin plug
 You need to know the name and colour of the three wires and
the location of their correct pin.
Name
Colour
Location of pin
The live wire is always connected to the fuse. The live and neutral
wire have the same size of current flowing through them.
A switch should be placed on the live wire of side of the appliance (not
the neutral wire)
If the appliance is working correctly, the current in the earth wire
will be zero.
Electricity at Home summary notes
2. Fuses
A fuse will protect the appliance from damage. If a larger current
than is expected flows the wire in the fuse will melt, breaking the
circuit (a user can still be shocked before the fuse melts!)
To select the current fuse for an appliance, we need to know its power
rating. The voltage from the mains socket will 230V. We can then use
the electrical power equation to find the current that the appliance
requires. The fuse should be above this current.
Questions
The following appliances all require either a 3A, 5A or 13A fuse.
Decide which appliance requires which fuse:
i)
A 1550W microwave
ii)
A 2200W kettle
iii)
A 800W torch
iv)
A 650W computer
Electricity at Home summary notes
3. The earth wire
The earth wire prevents damage to
the user. It is connected to the
metal casing of an appliance (if it has
one!).
If a fault occurs and the casing
becomes “live” (the live wire touches
the casing) then the earth wire offers the current a path of very low
resistance, the current to flow away. This will cause a large current
to flow into the appliance, causing the fuse to blow.
4. Double insulation
The circuit in an appliance is encased in plastic (an insulator). All
metallic attachments/casings are fitted to this plastic case.
No external casing touches the circuit/electrical parts
An appliance which has double insulation does not require an earth
wire
Electricity at Home summary notes
True or false?
Are the following statements true or false?
 The earth wire is connected to the fuse
 A source of direct current is a battery
 The earth wire is yellow and green
 The fuse protects the user
 The earth wire protects the user
 An appliance with double insulation does not require an earth
wire
 Alternating current flows from one side of a battery to the
other.
 If the appliance is working correctly, the earth, neutral and
live wire should all have the same current
 The live wire is blue
 The neutral wire is blue
 The earth wire has a very high resistance
 A source of direct current is a mains plug socket in your house
 Double insulation is when the circuit of an appliance in enclosed
in plastic
 The live wire should be connected to the metallic casing of an
appliance
Electricity at Home summary notes
Two way switches
This is a circuit which enables 2 switches to each independently
control the circuit (often lights in a hall or two storey house)
In the diagram below the boxes represent switches. At the moment
the light is off, but if either switch is changed it would turn on
(there would be a complete circuit)
Electricity at Home summary notes
Paying for electricity
Units of electricity are measured in kilowatt-hours(kWh)
1 kilowatt-hour = energy delivered to a 1000W appliance if it is in use
for 1 hour.
Number of Kilowatt-hour units = power (kW) x time (hours)
Total cost = Number of units (kW) x price (pence)
Questions
If a washing machine uses 720W and runs for 3 hours how much
energy is used in kilowatt-hour units?
Calculate the cost of using a mobile phone charger power 10W for 6
hours if each kWh costs 12p.
An electric heater requires 2200W and is used for 4 hours. Calculate
the cost of use if each kWh costs 12p
Electricity at Home summary notes
Questions
1. You use a 1200-watt hair dryer for 10 minutes each day.
a) How many minutes do you use the hair dryer in a month?
(Assume there are 30 days in the month.)
b) How many hours do you use the hair dryer in a month?
c) What is the power of the hair dryer in kilowatts?
d) How many kilowatt-hours of electricity does the hair dryer use
in a month?
e) If your town charges 12p per kWh, what is the cost to use the
hair dryer for a month?
2. Complete the table below:
Present
reading
23,458
345,602
12,489
Previous
reading
22,603
344,852
11,639
Units used
855
Cost per
unit
15p
15p
15p
Cost
£128.25
£
£
Electricity at Home summary notes