ICSE Class 8 Physics Chapter 05 More about Energy

More About Energy

You already have a fair idea about energy, its forms and sources. In this chapter, we will study the sources of energy in some detail. However, let us first recall what you know already.

Energy is the ability to do work. The SI unit of energy is the same as the unit of work, i.e., the joule (J). Energy and work are closely related. Energy can be converted into work, and work can be converted into energy. For example, electrical energy can be converted into the work involved in rotating the blades of a fan or a food processor. And the work done in compressing the spring of a wind-up toy is stored in it as potential energy.

Energy can exist in many forms, and one form of energy can change into another form. This is very useful for us. For example, we can use the chemical energy stored in batteries because it can be converted into electrical energy, which can then be converted into light, sound, the kinetic energy of the moving parts of appliances, and so on. In fact, if you look around you, you will notice that the utilisation of energy always involves energy transformations, or the change of one form of energy into another.

For us, the most useful form of energy is electrical energy. Hence, we usually convert other forms of energy into electrical energy. The main advantages of using electrical energy are as follows.

1. It can be converted easily into other forms of energy.
2. It can be transported easily.
3. It does not cause pollution (though the generation of electricity may).

Sources Of Energy

Right from the time when man first learnt how to light a fire, progress has been driven by the utilisation of energy. From meeting all his need for energy by burning firewood, to harnessing the energy of wind and water, to tapping fossil fuels and generating electricity, man has come a long way in making use of energy resources. Let us now discuss the sources of energy available to us in some detail.

Our present sources of energy can be divided into two broad groups - renewable and nonrenewable. Renewable sources are those which can be replaced by natural processes. There is no risk of exhausting our supply of these sources. Nonrenewable sources are those which cannot be replaced easily by natural processes. We are likely to run out of these in less than 200 years if we use them at the rate at which we are right now.

Nonrenewable Sources Of Energy

Coal, petroleum and natural gas, together called fossil fuels, are at present our chief source of energy. They meet about 85% of the world's energy requirement and generate about two thirds of the electricity we use. Any substance that produces usable energy is called a fuel. Most fuels produce energy when they are burnt. Nuclear fuels, which we will discuss later, are an exception. Fossils are the remains of plants and animals preserved between layers of rock. Coal, petroleum and natural gas are called fossil fuels because they were formed from the remains of plants and animals buried millions of years ago. Our present supply of these fuels took millions of years to form. Hence, if we use them up, we will have nothing left.

Nuclear fuels are minerals. Since our supply of minerals is also not endless, most people consider nuclear energy to be a nonrenewable source of energy.

Coal

Plants that died a long time ago and were buried under swampy land formed coal. Over time, these plant remains got buried deeper and deeper. The action of bacteria and the pressure of layers of rocks and soil slowly converted the carbohydrates present in the plants into almost pure carbon. The process of conversion of plant remains into coal is called carbonisation. As you have learnt in your chemistry lessons, the lowest grade of coal is peat, while the highest grade of coal is anthracite.

Coal is used as a fuel and in the production of coke and coal gas, besides other things. A large portion of the coal produced the world over is used for the generation of electricity in thermal power stations. In a thermal power station, fossil fuels are used to produce steam, which is then used to turn turbines and generate electricity.

Petroleum And Natural Gas

These fuels are often found together. They were formed by the decomposition of marine organisms that died and settled on the sea bed and were later buried under layers of mud (sediments). Petroleum or crude oil is usually found trapped above a layer of impervious (nonporous) rock and is extracted by drilling a well through rocks. When oil is struck, or found, it is pumped out and refined, or processed.

Natural gas is mostly found trapped above crude oil. One advantage of natural gas is that it can be piped or supplied directly from an oil field. The other advantage is that it causes far less pollution than the other fossil fuels. It is therefore called a 'clean' fuel and is used extensively to run public transport in some places.

Petroleum is a mixture of hydrocarbons (compounds of hydrogen and carbon) with different boiling points. The refining of petroleum consists of separating the components of this mixture. In a refinery, crude oil is heated to about 400 degrees Celsius, and the vapour is then led into a cooling tower. The tower is fitted with shelves at different levels. As the vapour climbs up the tower, it cools, and the different components condense at different levels, in order of decreasing boiling point. The gas that remains uncondensed is called petroleum gas. Liquid petroleum gas (LPG) is used widely as a cooking fuel. The other fuels derived from petroleum are petrol, kerosene, diesel and fuel oil.

Disadvantages Of Fossil Fuels

The disadvantages of fossil fuels are that they are nonrenewable and cause pollution. The extraction, processing and use of fossil fuels pollute soil, water and air. We will only discuss air pollution caused by the burning of fossil fuels.

1. Fossil fuels contain carbon, so on burning, they produce carbon dioxide. Carbon dioxide traps heat. Scientists believe that the excessive use of fossil fuels has led to an increase in the amount of carbon dioxide in the air, and hence an increase in temperature the world over. This is called global warming. It is feared that global warming may lead to the melting of the polar ice caps, and drastic changes in climate patterns.
2. The burning of fossil fuels produces carbon monoxide when the supply of air is insufficient. Carbon monoxide is a poisonous gas that can cause suffocation, and even death.
3. The carbon and sulphur present in fossil fuels (especially coal) are converted into oxides when these fuels are burnt. The presence of oxides of sulphur and nitrogen in the air causes respiratory diseases. When these oxides dissolve in rain, they form acids, which come down as acid rain. Acid rain harms plants, pollutes the soil and causes corrosion of metals and stones. The Taj Mahal is one of the many monuments that are threatened by acid rain.
4. Unburnt carbon particles called soot and ash released into the air by the burning of fossil fuels cause respiratory diseases.

The concern over the pollution caused by fossil fuels and the shortage of these fuels has led to a worldwide movement to tap alternative sources of energy. Another step being taken to reduce pollution is to switch to cleaner fuels like CNG, LPG and unleaded petrol. Lead used to be added to petrol to prevent what is called knocking, or thumping in engines. However, it is very poisonous, so it is being replaced by other substances in most countries. Vehicles are also designed to reduce emissions that cause pollution. Some cars, for example, have catalytic converters that convert harmful gases into carbon dioxide, nitrogen and water. Vehicles that run partially or entirely on electricity (produced by cells) are also gaining popularity.

Teacher's Note

Every time you see smoke from a vehicle or power station, you are observing the direct results of burning fossil fuels and the pollution they create in our daily environment.

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