ICSE Class 7 Physics Chapter 8 Electricity

Electricity

Syllabus

Sources of electricity - cell/battery/mains.

Effects of electricity - provides heat, light, movement, can make electromagnets, electroplating.

Simple electric circuits - electricity can flow only in a complete circuit starting at one terminal and ending at the other - conductors and insulators - the entire circuit must be made up of conductors - switches.

Circuit diagrams - using symbols for a cell, a bulb, a switch, etc. (Note: arrow in direction of conventional current).

Electricity at home - meters determine consumption and we have to pay for it - fuses/miniature circuit breakers protect the house from electrical fires by breaking a circuit - parallel circuits allow us to use different parts of the household separately (to be explained with a model/simple circuit diagram).

Construction of a simple dry cell - simple cross-section drawing of the same - other types of cells which are commonly available (no construction details required).

Simple rules for the safe use of electricity at home.

Need to conserve electricity.

Caution: Warn students never to conduct any experiments using mains supply. For their experiments, they only need a fresh dry cell, insulation tape, wires and a variety of insulators and conductors.

A simple improvised circuit with a switch and a torch bulb - to be used for checking the conditions under which electricity will flow (see 2 above) and also finding out about conductors and insulators (E).

Locating the meters in a building, watching them at work - learning to read an electric meter (E).

Looking at fuses/MCBs; parallel circuit - set-up on a board (D).

Survey of electrical usage in their houses - for a set period of time daily - listing examples of wastage and identifying possibilities for conservation (E).

Importance Of Electricity

We are well familiar with the word "Electricity". We cannot think of modern life without electricity. It is one of the most useful form of energy available to us. Electricity is brought to our homes, offices, factories and other places with the help of wires from electric power plants. It is used to light our homes and in different ways. We light our homes and other places of work with electricity. It is used to run electric fans, televisions, geysers, electric irons, room heaters, refrigerators, music systems, etc. We use electricity on a large scale to light up streets, to run machines in factories, to run trains, metro rails, etc. Some of the common electrical appliances which we use in our day-to-day life are shown in Fig. 8.1.

Teacher's Note

Every appliance we use daily, from our morning alarm clock to the evening television, depends entirely on electricity generated by power plants and distributed through miles of wires to our homes.

You know that every atom contains one or more electrons. You also know that electrons have a negative charge. Most substances have electrons that can be detached from their atoms and they can move around freely. These loosely bound electrons are called free electrons. When these free electrons are forced to flow in a particular direction under certain conditions, they constitute an electric current.

Electricity - An Introduction

We often observe that when two different types of substances are rubbed together, the free electrons of one substance gets transferred to the other substance. The substance that loses electrons becomes positively charged (anode) and the one which gains electrons becomes negatively charged (cathode). In this way, both the substances become oppositely charged.

Example: Rub a glass rod with a piece of silk. The glass rod loses electrons and becomes positively charged whereas the piece of silk gains electrons and becomes negatively charged.

If both these substances are connected with the help of a metal wire an electric current flows from the glass rod to the silk, in other words, free electrons flow from the silk to the glass rod.

The rate of flow of free electrons through a conductor is known as electric current.

Electricity Generation

There are three types of power plants which are used for generating electricity on a large scale.

Hydroelectric power plants (or hydel power plants) - Hydroelectric power plants use kinetic energy of the moving water.

Thermal power plants - These are based on coal, diesel or natural gas.

Atomic power plants - These are based on the nuclear fission of Uranium-235.

The electric currents supplied to our homes by the electricity supply companies are alternating currents (AC).

Electricity is only a means of transferring energy and not a source of energy. Hence, it is a secondary source, the primary sources being water, air, oil, coal, gas, etc.

Sources Of Electricity

The main sources from where we get electricity are: (i) the dry cell and battery, (ii) the mains provided by the electricity department, (iii) the electric generator, and (iv) a solar cell.

Construction And Working Of A Dry Cell

In a cell, a chemical reaction takes place, due to which chemical energy changes into electrical energy. Thus, the cell becomes a source of electricity. The most commonly used cell is a dry cell. This cell cannot be recharged. It is also called a primary cell.

A dry cell essentially consists of a zinc container, its base acting as the negative electrode. It has a carbon rod placed at the centre with a brass cap. The carbon rod acts as the positive electrode and is surrounded by a mixture of manganese dioxide (MnO2) and charcoal (C) in a muslin bag. The electrolyte used is a moist paste of ammonium chloride (NH4Cl), plaster of Paris, flour, etc. The outer body (except for the base) of a zinc container is insulated with a thick cardboard or plastic material (Fig. 8.2).

Advantages of a dry cell-

Dry cells are light in weight and small in size.

Dry cells can be transported from one place to another very easily.

There is no fear of leakage/spillage in dry cells.

Cells and batteries are mostly used in portable devices.

Dry cells are actually not dry. In fact, a dry cell works only as long as the paste inside it remains moist. The presence of water (or moisture) helps in the movement of the ions within the cell from one electrode to the other. If the cell has not been used for a long time, the chemicals present in it are spent and it stops producing electricity, such a cell is called a dead cell.

These cells are used in a number of household gadgets like a radio, a transistor, a tape-recorder, a calculator, a wrist watch, the remote of a T.V., torch, toys, etc. The working of dry cells in a common gadget like a torch is described in Fig. 8.3.

When the cell is connected to a bulb, the slow ongoing chemical reaction inside the cell becomes fast and a current starts flowing through the bulb. Hence the bulb glows. These dry cells are small source of electricity.

Teacher's Note

A torch is one of the simplest examples of how chemical energy stored in dry cells is converted to light energy, making it a perfect tool to understand the basics of electrical circuits.

Activity 1

To locate the two terminals of a cell.

Material required: A battery cell.

Procedure: Take a dry cell and observe it carefully. Do you find the signs of positive (+) and negative (-) on it. If yes where?

In a battery cell:

There are two terminals labelled as (+) and (-). The positive (+) terminal is at its top, whereas the negative (-) terminal is at its bottom.

All types of cells have two terminals. In some cases, however both the terminals may be on the same side.

The other examples of primary cells are those of simple voltaic cell, Leclanche cell, Daniel cell, etc.

The secondary cells are the cells which can be re-used again and again after recharging. These cells are also called storage cells. They are used in cars, trucks, inverters, etc. They are also known as accumulators.

If two or more cells are used in combination, it is called a battery. A battery is used when we require more electricity.

The Mains

It is another common source of electricity.

The electricity which is produced in a power station is carried through wires to different city sub-stations. From these city sub-stations it is carried to the electric poles fixed in various localities through the transformers. Then, from these poles, electricity reaches the mains board fixed in our houses from where it gets distributed through the wires fixed in all parts of our houses.

All the electrical gadgets in your house like fans, an A.C., a geyser, a TV, an electric iron, bulbs, tubelights, etc., work using this electricity supplied from the mains.

Every residential place or a commercial complex has the mains where meters are fixed to determine the consumption of electricity so that the person may accordingly pay the bill.

Electric Generator

A British Scientist, Michael Faraday, proved that if a copper coil is quickly rotated in a strong magnetic field, electricity is generated in the coil. In this process, mechanical energy is converted into electrical energy. This property is called electromagnetic induction. The property of electromagnetic induction is used to generate electricity and the device is called a generator or dynamo.

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