ICSE Class 7 Physics Chapter 07 Electricity

Electricity

The world has seen enormous changes over the last hundred years. The harnessing of electricity has been the single most important factor behind these changes. You can easily think of the large number of ways in which electricity plays a role in our lives today. In this chapter, we will discuss the basics of electricity.

What Is Electricity?

Streaks of lightning across the sky are caused by electricity produced in the clouds. Cells produce electricity when they are connected to watches, cameras, torches, and so on. The electricity in our homes is carried by wires from giant power stations. To understand how these seemingly different things are related, we need to know about a quantity called charge.

Charge

Charge is a fundamental part of matter. You could say that mass and charge are two things that characterise matter. The tiniest particles that matter is made up of have mass and carry charge. The SI unit of charge is the coulomb, which has the symbol C.

Charge exists in two forms - positive and negative. 'Positive' and 'negative' are just names we have given to the two types of charge. The charges are not positive and negative in the mathematical sense. Many things around us happen because of charges. All these things are together called electricity.

Normally matter contains an equal amount of positive and negative charges. Sometimes, however, there is an imbalance of charges, and a body gets a net negative or a net positive charge. We say that the body is electrically charged. You already know that electrically charged bodies, such as a comb rubbed against hair or nylon, exert a force that is called electrostatic force. This and other effects produced by charges that accumulate or build up in bodies are studied under electrostatics. The term static electricity is used to describe this kind of electricity that is caused by charges that are at rest. Lightning, as you will learn later, is a result of static electricity.

Teacher's Note

Static electricity is observed when you rub a balloon on your hair and it sticks to the wall, demonstrating how charge accumulation creates observable forces in everyday life.

Electric Current

Under certain conditions, charges can flow. When charges flow, they give rise to what is called an electric current. An electric current is defined as the charge flowing past a point per unit time. Thus, when Q coulombs of charge flows past a point in t seconds, the current I at that point is

\[I = \frac{Q}{t} \text{ coulomb/second}\]

The unit coulomb/second is called the ampere, with the symbol A. This is the SI unit of current.

Most of the electrical gadgets we use in our homes work only when an electric current flows through them. In other words, they work because of the effects produced when an electric current flows through them. The effects produced by an electric current (or a flow of charges) are studied under a branch of physics called current electricity, or simply electricity.

Electric Potential

Just as water flows from a higher level to a lower level, charge also flows from a higher to a lower 'level'. The level of charge is described by a quantity called electric potential. Thus, an electric current flows only when there is a difference in potential. The SI unit of potential and of potential difference (p.d.) is the volt, whose symbol is V.

There is a very close relation between the potential difference across two points and the current that flows between them. The higher the potential difference (commonly called voltage), the greater is the current.

Teacher's Note

The concept of voltage is similar to water pressure in pipes - higher pressure (voltage) causes more water (current) to flow through the pipe.

Sources Of Electricity

We can obtain electricity or electric current from different sources. Any source of electricity creates a difference in potential within itself. The common sources of electricity are cells, batteries and the mains.

Electric Cells

Cells come in different shapes and sizes. Common pencil and torch cells usually have a potential difference or voltage of 1.5 V. Button cells (Figure 7.4), used in watches, cameras, and so on, create potential differences of 1.5 V to 3 V. All cells producing electrical energy from reactions between the chemicals stored in them. The cells we are discussing are called dry cells because the chemicals in them are not in the form of a liquid.

Teacher's Note

A simple battery in a flashlight demonstrates how chemical reactions inside cells convert stored energy into electrical current that powers the device.

Figure 7.3(b) shows the inside of a common cell. Such cells have an outer casing or shell made of zinc and a rod made of carbon at the centre. The space inside is separated into two chambers by porous cloth or paper. A mixture of powdered carbon and manganese dioxide is packed around the carbon rod in the inner chamber. And a paste of ammonium chloride and other chemicals is filled in the outer chamber.

You may have noticed that there is a '+' sign and a '-' sign at the two ends of a cell. These denote the positive and negative terminals of the cell. When we use a cell we connect these terminals to the gadget we want it to run. Electric current enters and leaves the cell through these terminals. In the type of cell we are discussing, the carbon rod acts as the positive terminal, while the zinc casing acts as the negative terminal. The carbon rod has a metal cap for better contact, and the top of the cell is sealed to prevent leakage.

When the cell is connected, a chemical reaction between the zinc casing and the chemicals inside produces electricity. When the chemicals are used up, the cell becomes 'dead', or it cannot be used any more. Such cells, which get used up, are called primary cells.

Batteries

The word 'battery' is used in more than one sense. The correct meaning is an arrangement of a number of cells, for example, the 9-V battery shown in Figure 7.5(a). Quite often, however, the common cell is also referred to as a battery.

In this section, we will discuss what is called a storage battery. Unlike the cells we have discussed in the previous section, storage batteries can be recharged. That means when the chemical energy of the battery decreases, it can absorb electrical energy and store it again as chemical energy. In other words, when the chemicals in the battery get used up, they can be restored by a kind of reverse chemical process. This way, storage batteries can be used many times over.

Teacher's Note

Rechargeable batteries in mobile phones and cameras are storage batteries that can be charged repeatedly, unlike disposable primary cells.

Storage batteries can be of two types. The first type is the lead-acid battery or accumulator used in motor vehicles and in inverters (which provide electricity during power cuts). Such batteries consist of several storage cells connected together. Each cell has a plate of lead and another of lead oxide dipped in acid (hence the name lead-acid cell). Lead-acid batteries create or provide a potential difference of 6 V to 12 V.

The dry batteries used in mobile phones, electronic cameras and other portable devices are also storage batteries. As you must know already, they can be recharged. So can some pencil cells available in the market these days. Storage cells are also called secondary cells, as opposed to primary cells, which cannot be recharged.

Mains

The most important source of electricity is what is commonly called the 'mains', or the electric supply to our homes, offices, factories, and so on. This electricity is generated on a large scale in power stations, either by burning fuels (thermal power) or from water stored in reservoirs (hydel power). Electricity is also generated from nuclear energy, wind energy, solar energy and other sources of energy. However, these sources meet only a tiny fraction of our total requirement of electrical energy.

The mains provide or have a potential difference of 220 V. This is a high voltage, so you must never use the mains for any of the activities given here or in any other book.

Teacher's Note

The electricity supplied to homes at 220V comes from power stations far away, demonstrating the large-scale infrastructure needed to provide electricity to entire cities.

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