Household electricity SA - CCEA

• The equation power = current × voltage can be used to calculate and select the appropriate rating of a fuse.

• The unit used in the cost of electricity to the consumer is the kilowatt-hour. One kilowatt-hour (kWh) is the energy supplied when an appliance with a power of 1kW is used for 1 hour.

As charge flows through wires they heat up.

Electrical energy carried by the charge is converted to heat energy.

This lets a kettle heat water or an electric fire heat a room.

Electric power is the amount of electrical energy converted into other forms of energy in one second.

It be calculated using the equation:

electric power = current × voltage

P = VI

where:

Example question

What is the power of a small electric motor if a current of 2 A flows when connected to a 12 V power supply?

Answer

P = VI

I = 2 A

V = 12 V

P = 2 A x 12 V

P = 24 W

The power of the electric motor is 24 W.

In the example above the power of the electric motor is 24 W, but what does a power of 24 W mean?

It means that the electric motor converts 24 J of electric energy into other forms of energy (kinetic energy, heat energy, sound energy), every second.

A fuse breaks the circuit if a fault in an appliance causes too much current to flow.

This protects the wiring and the appliance from overheating, and possibly starting a fire, if something goes wrong.

The fuse contains a piece of wire that melts easily.

If the current going through the fuse is too great, the wire heats up until it melts and breaks the circuit.

Once the fuse has melted, the circuit is broken and no more current flows through the circuit.

Fuses in plugs are made in standard ratings.

The most common are 3 A, 5 A and 13 A.

The fuse should be rated at a slightly higher current than the device needs:

• if the device works below 3 A, use a 3 A fuse;

• if the device works above 3 A but below 5 A, use a 5 A fuse;

• if the device works above 5 A, but below 13 A, use a 13 A fuse.

Key points

• A fuse is a safety feature.

• The wire inside a fuse melts if something goes wrong and the current is too large - this protects the wiring and the device from overheating, and possibly starting a fire.

• Once the fuse has melted, the circuit is broken and no more current flows.

• The most common fuses are 3 A, 5 A and 13 A.

• The fuse selected should be rated slightly higher than the current the device needs to operate normally.

Electricity companies bill customers for the electrical energy they use.

A joule is much too small a unit of energy and so the electricity companies use units called kilowatt-hours (kWh).

One unit of electrical energy is one kilowatt-hour, or 1 kWh.

One kilowatt-hour (kWh) is the energy supplied when an appliance with a power of 1kW is used for 1 hour.

1 unit = 1 kWh

Domestic electricity meters, similar to those above, measure the number of kilowatt-hours of electrical energy used in a home or other building.

The more kilowatt-hours (or units) used, the greater the cost.

The cost of the electricity used is calculated using this equation:

total cost = energy in kWh × cost per unit

The cost per unit is set by the electricity company, for example 30 p per kWh.

This means that each unit, or kilowatt-hour, of electricity costs 30 p.

An electricity bill has two important numbers: present meter reading and previous meter reading.

The number of units used is the difference between these two readings.

Present reading = 40745 kWh

Previous reading = 39990 kWh

Number of units used = present meter reading - previous meter reading

= 40745 – 39990

= 755 kWh

total cost = number of units used × cost per unit

= 755 x 30p

= 22650p

The cost of electricity used is £226.50.

• Use energy efficient light bulbs such as LED bulbs.

• Switch off and unplug devices on stand-by.

• Switch off and unplug chargers when not in use.

• If you are replacing an electrical appliance such as a tumble dryer, kettle or hair dryer choose an energy efficient model.

• Minimize the time spent in an electric shower.

• If possible, hang washing out to dry rather than use a tumble dryer.

The power rating of electrical appliances can be used to calculate the cost of using them.

The following equation enables the number of units used to be calculated:

energy in kWh = power rating in kW x time in hours

When using this equation it is important to remember that:

• the power must be in kW (1 kW = 1000 W);

• the time must be in hours.

Example

Calculate the cost of using a 2 kW heater for 6 hours if the price of a unit is 30 p.

energy in kWh = power rating in kW x time in hours

Time = 6 hours

Power rating = 2 kW

energy in kWh = 2 kWh x 6 h = 12 kWh

total cost = energy in kWh × cost per unit

= 12 kWh x 30 p

= 360 p

= £3.60

The cost of using the heater is £3.60.

Key points

• Electric power is the amount of electrical energy converted into other forms of energy in one second.
• Power is measured in watts, W.
• Electrical energy is measured in joules.
• 1 watt = 1 joule per second.
• A joule is too small a unit of energy for household electricity, so the unit used for household electrical energy is the kilowatt-hour, kWh.
• The cost of the electricity used is calculated using this equation: total cost = energy in kWh × cost per unit.
• Higher tier only: energy in kWh = power rating in kW x time in hours.
• Higher tier only: when using this equation it is important to remember that:
  • the power must be in kW;
  • 1 kW = 1000 W;
  • the time must be in hours.