CHAPTER 4: Electric Energy, Distribution and Consumption

Keywords

consumer units

domestic electricity

domestic wiring

insulation

Kilowatt hour

meter

transformer

mains supply

By the end of this chapter, you will be able to:

a) understand the distribution of energy from the source to the consumer units.

b) understand how to use the mains electricity safely and know the insulation colour codes used in domestic wiring.

c) know the dangers of mains electricity and understand how these may be minimised by safety devices and by sensible precautions.

d) understand the energy transformations in common domestic electrical devices and how energy can be saved.

e) know how to read a domestic electricity meter and its significance.

f) appreciate the importance of the use of energy saving appliances.

4.1: Introduction

Electric energy is one of the most commonly used forms of energy. This is because it can be transformed to different other forms by different appliances.

Most domestic and industrial devices operate using electricity. Hence, there is need to produce and supply electricity in large quantities to satisfy the ever growing demand of different communities. Identify the various sources of electricity. Which of these sources are available in your community?

In this chapter, you will be able to understand how the generated electricity is distributed from the source to the consumers. You will also be able to understand how the consumption of electric power can be done sparingly and safely.

4.2: The Mains Electricity Supply

Activity 4.1 Identifying sources of electricity

Key question: What is the major source of electricity in Uganda?

What you need • Internet resource • Library resource

What to do

1. Identify the major sources of electricity in your community.

2. Mention at least three hydro-electric dams that you know in Uganda.

3. How do you compare the electricity produced from the dams with that produced from other sources?

4. Are there any hydro-electric dams in your district? If yes, name them.

5. How does the electricity produced in a hydro-electric dam reach final users in their different locations?

6. Why is there a need to have electricity in our homes?

ASSIGNMENT 4.1

1. Why do you think there are many dams in Uganda?

2. How do people living around these dams benefit from the existence of these dams?

3. Do you think there are problems that may rise due to the establishment of dams? If yes, what are they? If no, explain.

4.3: Transmission/Distribution of Electricity

Figure 4.1: Transmission lines alongside a road in Uganda

You might have always asked yourself some questions like:

a) Why do we have wires running in the space above the ground as those shown in Figure 4.1?

b) Why are we not allowed to touch or hang our clothes on these wires that sag downwards? Using any relevant resources:

1. What explanation can you give for (a) and (b) above?

2. Describe how electricity finally enters into our houses. The wires running in the space above the ground are used to transmit electricity from one place to another. When you touch a wire carrying current, electric charges flow into your body and can ionise the fluids in your body leading to death.

DID YOU KNOW?

Did you know that the symbol for electricity danger is:

Figure 4.2: The grid system

After electricity is generated in a power plant, it has to be transmitted to where it is required. The grid system is the network of devices that are used in the transmission of electricity.

Quiz: Identify the components of the grid system shown in Figure 4.2? High voltage electricity is transmitted through a network of electrically conductive wires of aluminum or copper. These lines are called high-voltage transmission lines.

Quiz: What are the advantages and disadvantages of transmitting power at high-voltage?

4.4: Domestic Electricity

Most homes and schools have got electricity. How do you call that electricity? What do you need to have in your home to access this electricity? How do you know that there is electricity in your home or there isn’t? By discussing this, you will be able to answer all these questions and even more.

Domestic electricity flows through electric circuits, which contain safety devices. These circuits are installed in and around the home.

The safety devices in the home include fuses and circuit breakers, the ring main circuit and the functions of the live, neutral and earth wires.

In Uganda, the standard voltage supply is 240 V at a frequency of 50 Hz. You can use your electric appliances in Uganda, if the standard voltage in your country is in between 220 – 240 V

Fieldwork

1. Go to a nearby or remote area in the village and identify homes with electricity.

2. Seek permission from the chairperson of that village and the person in charge of electricity in that area to read the electricity meters of each household.

3. Are the meters all similar in the way they display information? If not, how are they different? 4. Research on how to read an analog electricity metre.

DID YOU KNOW?

Did you know that an electricity meter is a device that measures the amount of electric energy consumed in a building, tenant space, or electrically powered equipment?

ASSIGNMENT 4.2

1. What are the advantages and disadvantages of an analog electricity meter?

2. What are the advantages and disadvantages of a digital electricity meter?

4.4.2: Calculating the Cost of Electricity

DID YOU KNOW?

Did you know that the energy-efficient appliances are essential to help you save money and energy, protect the environment and enhance your lifestyle? Name some of the energy efficient appliances that you know.

The amount of electrical energy consumed by an appliance depends on its powe rating and on the length of time it is connected to the power supply. The kilowatt hour (kWh) is the unit of electrical energy used for calculating electricity bills. kWh is the electrical energy consumed by a 1 kW appliance in 1 hour.

The equation relating the electrical energy transferred to power and time is: Energy transferred (kWh) = Power (kW) x time (h) The cost of electricity used = Energy transferred to appliance x Rate of electricity

EXAMPLE 4.1

1. Covert 1800 s to hours.

2. An electric kettle labelled 5 kW is connected to an electric power supply for 20 minutes. Find the amount of energy consumed in kWh and the cost of electricity used in 20 minutes if each kWh costs 20shs.

EXERCISE 4.1

1. An electric fire needs 2 kW of electricity to function. It is switched on for 3 hours. Find:

(i) The amount of electrical energy it consumed in the stated time.

(ii) The cost of the electricity used in the stated time if each kWh of electricity costs 15 shs.

2. A 100 W light bulb is left on for 3 days. If each unit of electricity costs 12 shs, how much does it cost to run the light?

DID YOU KNOW?

Did you know that 1 kWh of energy is equal to 3,600,000 joules?

4.5: Energy Transformations in Common Electric Devices

Activity 4.3 Analysing energy changes in electrical appliances

Key question: Describe the energy changes that take place in different electric devices.

What you need

• Abulb • A source of electricity (dry cells in a holder) • Connecting wires • Internet resource

What to do

1. Connect the bulb to a source of electricity using the connecting wires.

2. What happens to the bulb?

3. Explain all the energy changes that took place in your observation in 2 above.

According to the law of conservation of energy, “Energy can be transformed from one form into another”. In Physics, the term energy describes the capacity to produce certain changes within a system, without regard to limitations in transformation imposed. Electrical appliances use the law of conservation of energy to transform electrical energy into many forms mechanical/kinetic, sound, heat, light and other forms of electro-magnetic radiation. The table below summarises changes in energy in different devices. Fill in the energy changes.

Table 4.1: Changes in energy in different electrical devices

NOTE: The greater the amount of the supplied electrical energy that the appliance transfers to useful energy stores, the more efficient the device will be.

4.6: Safety Measures

Mains electricity (electricity from the power station) enters the house via the Live wire. The live wire carries the incoming electricity and is therefore at 240V so it is very dangerous.

Mains voltage on the live wire can kill somebody. As a result of this, fuses and switches are always connected to the live wire. When a switch is turned off (or when a fuse blows), a device connected to the live wire will no longer be connected to it, thus saving the device from burning due to high voltages.

DID YOU KNOW?

Did you know that a fuse is a safety device designed to cut off the flow of electricity to an appliance if the current becomes too large (due to a fault or a surge)?

ASSIGNMENT 4.3

1. Obtain a three-pin plug and dismantle it to see what is inside. Can you see the fuse? Identify the three wires in the plug. Now, reassemble it and test if it works.

2. Identify the colour codes for each of the wires in the plug.

Once the electricity from the live wire has given its energy to the appliances in the household, the current travels back out of the house via the neutral wire. Hence, the neutral wire has a lower voltage than the live wire does. The Earth wire is used for safety purposes and carries the current away when there is a fault.

Figure 4.5: A fuse box for mains electricity

4.7: Dangers of Mains Electricity

Mains electricity is good if used properly. However, if not used properly, it can prove to be very dangerous. Explain some of the dangers that can arise from improper use of mains electricity.

4.7.1: How to Use Mains Electricity Safely

In order to use mains electricity safely, the following precautions must be observed: Replace or repair damaged power cords.

• Don’t overload your outlets.

Avoid extension cables as much as possible.

Keep electrical equipments or outlets away from water.

Protect small children from hazards.

• Give your appliances proper space for air circulation to avoid overheating.

• Always follow appliance instructions for improved electrical safety.

EXERCISE 4.3

Explain why it is important to take the precautions listed in 4.7.1.

ASSIGNMENT 4.4

1. According to you, why are electricity users sometimes not bothered with enforcing safety precautions when using electricity?

2. Who is responsible for sensitising people on how they should use this mains electricity safely?

3. What advice would you give to everyone who does not follow the set safety guidelines while using electricity and electric appliances?

4.8: Domestic Wiring in Mains Electricity

Domestic wiring involves laying of cables in a house for purposes of distributing electricity to different parts of the house. Wiring is subject to safety standards for design and installation of electricity in a domestic house. Recommended/Approved cable types and sizes are specified according to the operating voltage and electric current capability of the circuit in which they are to be installed. Wiring must take into account environmental conditions, such as working temperature range, moisture levels and exposure to sunlight and chemicals.

Project work

With the following objects provided to you:

• A box

• A pair of scissors

5 dry cells

• Super glue

• Connecting wires

LED lamps or bulbs

Switches. Socket

1. Make a model house of three (3) rooms using the materials provided.

2. Design a plan for wiring the house. Ensure that each room in the house has a socket, an electric lamp and a socket.

3. After wiring, test your circuit by switching on and off the switches. Comment on your wiring techniques.

If a live wire inside an electric appliance, such as a cooker, becomes loose and touches the metal casing, it could cause an electric shock. The earth terminal is thus connected to the metal casing so that in such instance the current would go through the earth wire to the earth instead of causing an electric shock.

A strong current surge flows through the earth wire because it has a very low resistance. The surge in current breaks the fuse or trips the circuit breaker shown in Figure 4.7 and disconnects the appliance. Note that the switches and fuses are placed onto the live wire.

Figure 4.7: A circuit breaker

Quiz: What is the function of a circuit breaker in a circuit? At what point in domestic wiring should a circuit breaker be placed? 4.8.1: Insulation and Colour Codes

a) Insulating wires

Insulating materials are materials that prevent the current flowing through a circuit from spilling over. Manufacturers select insulating materials based on their flame-test ratings, cost, and desired transmission properties.

ASSIGNMENT 4.5

With the help of your teacher, invite any electrician within your community. Ask him/her to tell you the different types of insulating materials used on the cables that they install around in the community. Why did they choose to use the insulations mentioned above?

b) Wire Colours for D.C Power You had earlier been asked to identify the colour code for the wires that enter the electric plug. According to the British system:

1. A red insulation represents the live wire.

2. A black colour represents the neutral wire.

3. A white or grey wire represents the earth wire.

Figure 4.8: Wires used in wiring

Quiz:

1. Identify the live, neutral and earth wires in Figure 4.9.

2. State the advantages of using standard colour codes.

3. Do you think the same colour codes are used in d.c and a.c circuits? Explain your answer.

Chapter Summary

In this chapter, you have learnt that: • an electric current flows as either direct current or alternating current. the electricity that comes into your house through the wall sockets is called the mains electricity. the network that connects power stations in the country to make sure that electricity is accessed everywhere is called National Grid. the production of electrical energy involves chains of transformations to other forms of energy using a range of sources such as brown coal, gas, hydro, wind and solar. we use energy at home mainly to heat, cool, illuminate and move things. The power rating of an appliance or tool (in watts) provides a useful way of comparing how fast these devices are transforming energy. electric power, P, is the rate (in watts) that energy is supplied by a source or dissipated by a device. It is the product of current and voltage. The expressions for electrical power is P = IV. A electrical energy is transformed into many forms – mechanical/kinetic, sound, heat, light, and other forms of electro-magnetic radiation. electricity meter is a device that measures the amount of electric energy consumed by a building, tenant space, or electrically powered equipment. There are two types of electric meters and these are Analog and Digital electric meter.

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