Household Electricity

Uses of electricity in the home

• Lighting
• Heating
• Powering electrical devices e.g. motors

Electrical Power

• Is the rate of using electrical energy e.g. If a bulb is marked 100W, it is converting electrical energy to heat and light at rate of 100J/s
• The SI units of power is the watt (W)
• P = VI

Examples 1: Find the power of an electric iron operating on a 240V supply using a current of 0.5A

Example 2:  Find the voltage of an electric motor if its power is 3kW and the current passing through it is 12,5A.

Example 3:  What is power of an electric light bulb if it is drawing a current of 0.25A from the mains electricity supply of 240V?

Example 4:  What current is drawn by a 1.5kW heater which operates on a 240V mains supply?

Electrical Energy

• Electrical energy is the amount of electricity that is connected to other forms of energy.
• Electrical energy can be found by multiplying the quantities of power and time
• 𝐸 = 𝑉𝐼𝑡
• SI unit for energy is the joule (J)

Cost of electricity

• The cost of electricity consumption is based on the number of kWh of electrical energy used
• The kWh is the energy is the electrical energy used by a 1kW appliance in 1 hour – Metres are installed to measure the amount of energy used by each house.
• One unit of electricity = 1kWh

  Energy(number of units) = power(kW) x time(h)

• The cost of electricity each month is determined by reading the electricity meter and multiplying the units by the cost per unit.

Cost = number of units x cost per unit

A 12V accumulator delivers a current of 3A through a car headlamp for 4 hours. Calculate;

• The power supplied in watts
• Energy supplied in kilowatt-hour
• Cost of 300 units of electricity at 14 cents per unit

Methods of saving electricity

• Use of energy saving bulbs
• Use of alternative sources of electricity e.g. solar panels, biogas
• Switch off appliances not in use
• Use of low power rating appliances
• Using appliances with heating elements sparingly as they consume the most energy.

Electrical hazards and safety precautions Electrical hazards

• Damaged insulation – fire risks and electric shocks
• Overheating cables – fire risks because there is maximum current in a circuit
• Damp conditions – increase the severity of an electric shock because water lowers the resistance of the path to the earth.

Safety precautions

• Earthing all appliances
• Avoid overloading circuits
• Use insulated cables
• Do not handle appliances with wet hands
• Put on rubber foot wear when using appliances

Three pin plug

• Connects appliances to power circuits via power socket

• Earth: carries electric current into the ground when there is a short circuit

• Fuse: melts when too much current flows

• Live and neutral: carry electric current to and from the appliance

• The neutral (N) wire (blue) completes the circuit by forming a path for the current back to the supply. It is usually at zero volt
• The earth (E) wire (yellow with green) is a low resistance wire usually connected to the metal casing of the appliance.
• The live (L) wire (brown) delivers the energy at high alternating voltage to the appliance
• To wire the plug
  • Remove sufficient amount of insulation from each of the three wires.
  • Twist the wire strands together gently.
  • Secure the three wire stands to the correct terminals according to the colour code by means of wrap-round screws. Make sure that the insulation on the wires extends right to the pins.
  • Firmly, tighten the cord grip to grip the cord firmly.

Earthing

• The earth pin on a 3 pin plug is connected to the metal case of the appliance which is thus joined to earth by a path of almost zero resistance.
• Earthing protects the user of the appliance from an electric shock, if the metal casing should accidentally become live. The earth wire carries excess current to the ground thus protecting the user from electric shocks.

Fuse

• The fuse should be connected to the live wire so that the appliance will not become live after the fuse has blown.
• A fuse is a component made from a metal with low melting point. The fuse melts if too large a current flow through it thus breaking the circuit. This prevent overheating and damage of appliance and use from electric shocks. – Fuses are normally rated 1A, 2A, 5A, 10A and 13A

Switches

– All switches are designed to break or complete an electrical circuit. The switch must be fitted onto the live wire so that switching off disconnects the high voltage from an appliance.

Two pin plug

• Double insulation is a safety feature in an electrical appliance that can substitute for an earth wire. Only the live and neutral wires are required for the appliance.
• This safety feature provides two levels of insulation i.e. the electric cable is insulated from the internal components of the appliances and the metal parts which could become live if a fault developed are also insulated from the external casing.
• Appliances with this feature normally have non metallic casing such as plastic or wood e.g. radio, tv

Solar photovoltaic systems

• Photovoltaic means using light to produce voltage.
• Photovoltaic cells covert solar energy to electrical energy. It is made up of semiconducting material i.e. silicon.
• The electrons in the silicon gain energy from the sun and create a voltage which can produce current in a closed circuit.
• A certain amount of light is required to cause a significant current, thus making solar power inefficient during times with low levels of sunlight. In bright sunlight, each cell produces 0.5V and a current of 0.03A.
• Greater voltages and current are obtained by connecting cells in series and in parallel on a panel. Cells are connected in series to give a higher voltage and in parallel to give a higher current. Cells in series supply steady current which last longer.