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Background information

In your form three physics syllabus, you learnt about current electricity while in form four, you tackled Electromagnetic Induction where alternating and direct current generators were discussed. In this topic, we shall learn more about sources, transmission, installation and consumption of electric energy.

Objectives

By the end of the lesson you should be able to:

a) State the sources of mains electricity

b) Describe the transmission of electric power from the generating station

c) Explain the domestic wiring system

d) Define the Kilowatt Hour

e) Determine the electrical consumption and cost

f) Solve numerical problems involving mains electricity












Introduction

There are several sources of mains electricity in Kenya, with the two main ones being hydroelectricity and geothermal. After generation at the power station, electrical energy is delivered to consumers through cables. The process of distributing power to consumers is called electrical power transmission. Electrical power is distributed to consumers all over the country through a National Grid system which consists of a network of transmission cables carried over structures known as pylons.

Sources of mains electricity


1.This is the Ngong power station. Wind is used to turn the vanes which produce kinetic energy that is converted into electrical energy by the generators. Click on the play button to make your observations.


2. At Ol karia power station, steam from the ground turns the turbines which then rotate the coils of the generators. Click on the play button to make your observations.




3. In a hydro-electric power station, potential energy of the water stored in the high dam is used to rotate turbines in the generators. Click on the play button to make your observations.



4. In nuclear power stations, radioactive matter breaks under nuclear fission releasing a lot of energy which heats up the water into steam. This steam under high pressure is used to rotate the turbines. Click on the play button to make your observations.



5. Tidal generator makes use of the tidal waves (up and down motion) as the source of kinetic energy. Click on the play button to make your observations.


Solar panels have photocells which convert light energy into electrical energy. Batteries are used to store the energy for use. In diesel powered generators, a diesel engine rotates the coil or the magnet of the generator. For a coal powered generator, the water is heated into steam under high pressure using coal.

Sources of mains electricity


1.This is the Ngong power station. Wind is used to turn the vanes which produce kinetic energy that is converted into electrical energy by the generators. Click on the play button to make your observations.


2. At Ol karia power station, steam from the ground turns the turbines which then rotate the coils of the generators. Click on the play button to make your observations.


3. In a hydro-electric power station, potential energy of the water stored in the high dam is used to rotate turbines in the generators. Click on the play button to make your observations.


4. In nuclear power stations, radioactive matter breaks under nuclear fission releasing a lot of energy which heats up the water into steam. This steam under high pressure is used to rotate the turbines. Click on the play button to make your observations.


5. Tidal generator makes use of the tidal waves (up and down motion) as the source of kinetic energy. Click on the play button to make your observations.

Solar panels have photocells which convert light energy into electrical energy. Batteries are used to store the energy for use. In diesel powered generators, a diesel engine rotates the coil or the magnet of the generator. For a coal powered generator, the water is heated into steam under high pressure using coal.

Power transmission

The animation below shows cables carrying electrical power at high voltage. Click on the play button to make your observations.



Explanation

At the generating power station, the voltage produced is stepped up to between 220 and 500 kilovolts for transmission along the grid system and stepped down to a voltage of between 11 and 33 kilo volts at the consumer substation. This voltage is further stepped down to between 415 and 240 volts for domestic use.


Dangers of high voltage transmission

In high voltage transmission, a strong electric field is set up between the electric cables and the earth. This may ionize the air around the cables especially during rainy seasons or when the air is humid. This may cause a danger of electrocution to people or other animals nearby. This danger is usually minimized by supporting the cables high above the ground by pylons. If the cables are to pass through towns and cities, the cables are usually buried underground. The animation below shows cables carrying high voltage current click on its play button to observe.

The domestic wiring system

This is the connection of electric circuits in a building for low voltage consumption. In such a system, there are two main circuits namely: the lighting circuit for lights and the ring main circuit for the sockets. Typically, the voltage is supplied at 240 Volts at a frequency of 50 Hertz to the consumer unit. From there, it is split into the ring main circuit and the lighting circuit. This illustration shows a complete domestic wiring system.


Observation 1

When the 5A fuse is fixed and a switch to a bulb switched on, the bulb lights

Observation 2

The 30A fuse for the cooker when switched on makes the light emitting diode to go on showing that the cooker circuit is complete.

Observation 3

The 30A fuse for the sockets completes the ring main circuit causing the iron box to emit radiations showing the presence of current.

EXPLANATIONS

The first observation indicates what happens in the lighting circuit while the second and third observations indicate the action of the ring main circuit.

Electrical energy consumption and cost

Electricity meter

An electricity meter records the total number of units consumed on the premises where it is located in a given period of time. The cost of electricity consumed during such a time period is calculated as per the units consumed. This cost is equal to the number of kilowatt hours multiplied by the cost per unit.

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