ICSE Class 8 Physics Chapter 08 Electricity

8 Electricity

Theme

In this theme the aim is that children will develop the ability to estimate consumption of electricity by knowing the power rating of appliances used. They will also appreciate and understand the need and importance of taking certain precautions and using of safety devices to protect themselves and others against electrical hazards. Previous learning stressed on electricity due to charges in motion, i.e. current electricity. However, objects can be charged, where charges are static, not in motion. This is known as static electricity. This leads to many phenomena in nature, like lightning and thunder during rainy season. How an object that is charged may be detected using a simple device known as an electroscope.

In This Chapter You Will Learn To

- describe household consumption of electricity.

- identify live wire, neutral wire and earth wire in terms of their energy and path they travel.

- describe safety components (fuses, circuits breakers).

- describe phenomenon of static electricity.

- explain conservation of charges.

- describe conduction and working of an electroscope.

- describe a lightning conductor.

- identify dangers of electricity.

- conduct scientific experiments keeping in mind all the parameters.

- study the impact of energy consumption and drawn conclusions from the same and suggest alternate approaches.

- learn the use of safety precautions while dealing with electrical appliances.

Learning Objectives

- Revising previous concepts learnt by children.

- Building on children's previous learning.

- Calculating energy consumption using household electricity bills by children.

- Helping children identify live, neutral and earth wires.

- Demonstrating safety components and their uses.

- Demonstrating static electricity.

- Demonstrating induction and conduction.

- Engaging children in activities related to static electricity.

- Demonstrating the construction and working of an electroscope.

Knowing Concepts

- Household consumption of electric energy (kilowatt hour).

- Identify live wire, neutral wire and earth wire in terms of their energy and path they travel.

- Safety components (fuses/circuit breakers (qualitative approach only))/grounding).

- Static electricity.

-- Conservation of charges.

-- Induction.

- Lightning conductor.

-- Conduction.

-- Electroscope (Gold leaf electroscope).

(A) Household Electricity

Introduction

In our daily life, we all use electricity in different ways such as to light our home, school, office etc. to run fan, television, heater, radio and all other electrical appliances. The different sources of electricity are - cells (or battery), mains, electric generator (or dynamo) and solar cells. In earlier classes, we have read the use of cells (or battery) in torches, watches, calculators, remote controls etc. In this chapter, we shall study the use of mains as a source of electricity for household purposes.

The cell or battery provides us current which remains constant with time. This current is called direct current (or D.C.). The mains and electric generator provide us the alternating current (A.C.) which changes its magnitude and polarity with time. Generally we use A.C. of frequency 50 Hz in which the polarity is 50 times positive and 50 times negative in each one second.

Electrical Energy and Power Consumed in a Circuit

In class VII, we have read that current from a cell flows in a circuit due to the motion of electrons in the metallic wires used in that circuit. A cell has a potential difference (or voltage) between its electrodes namely the anode and cathode. Potential difference is defined as the work done in moving a unit charge from one electrode to another electrode. It is expressed in the unit volt (symbol V) after the name of the scientist Alessandro Volta.

We have defined current as the rate of flow of charges in a unit time. It is measured in ampere (symbol A) after the name of the scientist Andre Marie Ampere.

In Fig. 8.1, suppose a current I flows through a conductor of resistance R for time t, when a source of potential difference V is connected across its ends. We are to find the amount of electrical energy supplied by the source.

By definition, potential difference is the work done in moving a unit charge, so work

needed to move a charge Q through a potential difference V is

W = QV

But current I = charge Q / time t

- Q = It

Hence,

W = VIt

This gives the electrical energy W supplied by the source (Battery or mains) in providing the current I for time t in the conductor under a potential difference V.

Now power is the rate of doing work. So power supplied by the source

P = W / t = VIt / t = VI

Thus, power (in watt)

= (potential difference in volt) x (current in ampere)

Thus 1 W = 1 V x 1 A

or 1 watt = 1 volt x 1 ampere.

Transmission of Power from the Power Generating Station to the Consumer

Electric power is generated at the power generating stations which are usually located very far from the areas where it is consumed. At the generating station, the electric power is generated at 11 kV. The voltage generated has alternating frequency of 50 Hz (i.e. its polarity at the terminals changes 100 times a second, 50 times + and 50 times -). The voltage of the power generated at the generating station is first raised from 11 kV to 132 kV to reduce loss of energy in transmission due to heating of line wires and then is transmitted to the grid sub station. At the grid sub station, its voltage is reduced from 132 kV to 33 kV before it is sent to main substation where its voltage is further reduced to 11 kV. Once reduced, the power is supplied to the city substation where its voltage is further reduced to 220 V before it is supplied to the consumer.

Supply of Power to a House

To supply electric power to a house from the city substation, either the overhead wires or cable on poles or an underground cable is used. The cable has three wires: (i) live (or phase) wire (L), (ii) neutral wire (N), and (iii) earth wire (E). The neutral and the earth wires are connected together at the local substation so that the neutral and earth wires are at the same potential (i.e. at 0 V). The live wire, also called the phase wire, is at 220 V and it carries current from the source to the distribution board, while the neutral wire is for the return path of current. The earth wire passes the current to the earth.

Do You Know

(1) A.C. voltage is raised up by using a step up transformer while it is lowered down with a step down transformer.

(2) D.C. voltage can neither be increased nor decreased by any device. This is why D.C. power is not used in household circuits.

(3) In household circuits, we use A.C. power at 220 volts.

Colour Coding of Live, Neutral and Earth Wires

In the cable, the live, neutral and earth wires have insulations of different colours so that they can be easily distinguished.

Live wire is red or brown.

Neutral wire is black or light blue.

Earth wire is green or yellow.

To summarize, the voltage, colours and the purpose of the three wires are given below.

Connection from Pole to the Distribution Board

To connect the cable from pole to the meter in a house, first a fuse of high rating (= 50 A) is connected in the live wire at the pole or just before the meter. This fuse is called the company fuse or pole fuse. Only the electric supply company staff are authorized to handle it. The rating of the fuse depends-on the load for which connection is taken from the company. After the company fuse, the cable is connected to an electric meter. The electric meter is usually mounted on the front or outside wall of the house. From the meter, connections are made to the distribution board through a main fuse and a main switch. The main fuse is connected to the live wire, while the main switch is connected to the live and neutral wires.

Commercial Unit of Electrical Energy

Electrical energy is generally measured in a unit called B.O.T (Board of Trade) unit or kWh, (i.e., kilowatt hour).

1 kilowatt hour is defined as the amount of energy consumed when an electrical appliance of power 1 kilowatt is used for 1 hour.

The energy consumed in our houses, shops, factories etc. is measured in kWh.

Energy

Since, Power = ----------

Time

Energy = Power x Time

i.e. Energy (in kWh)

= Power (in kW) x time (in h)

Hence, 1 kWh = 1 kW x 1 h

= (1000 W) x (60 x 60 s)

= 36,00,000 J

= 3-6 x 10^6 J

Electric Meter

The electric meter is also called the kWh meter because it measures the amount of electric energy consumed by the consumer in the unit kWh (called kilo watt hour) for which the electricity bill is paid by him to the electricity board.

Fig. 8.2 shows a kWh meter. The main part of the meter is the armature A which is connected to the main line (Fig. 8.3). Wher any electrical appliance is put on, the electric current flows which rotates the armature. The counter fixed on the armature reads the number of rotations.

Then five dials on the counter read electricity consumption directly in kWh units as shown in Fig. 8.3. These five dials from right to left measure the energy in (i) units, (ii) tens, (iii) hundreds, (iv) thousands and (v) ten thousands respectively. In Fig. 8.3, the reading on the meter is 49180 kWh.

Nowadays, the old meters are being replaced by electronic digital meters which are totally different from the meters shown above. The following activity will make clear how the electrical energy consumed in a given time is measured by the electric meter.

Activity 1

To find the consumption of electricity in a month and pay the electricity bill.

Fig. 8.4 (a) shows the reading on dials of the meter on the first day of a month and Fig. 8.4(b) shows the reading on the last day of the month. You are to find the initial reading, final reading, the electrical energy consumed in the month and the electricity bill at a rate of - 6.25 per kWh.

The initial reading is 49180 kWh and the final reading is 50625 kWh. The total consumption of electricity in the month is 50625 kWh - 49180 kWh = 1445 kWh i.e. the electricity consumed is 1445 units. The rate of electricity per unit is - 6.25 i.e. the bill will be 1445 x - 6.25 = - 9031.25 only.

Teacher's Note

Encourage students to bring their own electricity bills and calculate the monthly consumption and cost to understand the real-world application of energy calculations in their homes.

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