My Science School

GRAVITY

The universe is made of matter. Matter is held together and moved by forces. One of the basic or fundamental forces is the gravitational force. Any piece of matter from a pinhead to a planet has this gravitational force. It pulls or attracts other matter. The biggest large lump of matter in our daily lives is the Earth. Its gravitational force pulls us and other objects towards it, keeping our feet on the ground. The Earth’s gravitational force is also called gravity.

Gravity means that all matter, from the minute particles that make up atoms to the biggest stars, attracts each other. The nearer an object, the stronger it’s gravitational force on other objects. But the force becomes weaker with increasing distance. Earth is very big and very near, so for us its gravity is very strong. However, a few hundred kilometres above the surface its gravity is weak and objects may drift off into space.

The Earth’s gravity gives matter and objects what we call weight. A big book is weighty because it is being pulled downwards by Earth’s gravity and we have to counteract this force with our muscles when we pick up the book. However, weight varies according to the strength of gravity, and the strength of gravity depends on the amount of matter (and its density) in the two objects that attract each other. We are used to the weight of objects on Earth. The Moon has less matter than Earth, so its gravitational force is less. On the Moon the book would weigh less - about one-sixth of what it weighs on Earth. On a star consisting of vast amounts of matter the book would weigh many tonnes.

Weight varies with gravitational force but mass does not. Weight is a measure of the gravitational force pulling on an object. Mass is a measure of the amount of matter in the object - the numbers and types of atoms. On the Earth, Moon or a star, the book would weigh different amounts, but it would always have the same mass.

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CRYSTALS

In many solid substances, the atoms or molecules are fixed in place but they are not positioned at random. They are arranged in an orderly or regular pattern known as a crystalline framework. The result is that the substance forms crystals. These are not irregular lumps but orderly, geometric shapes with sharp edges and flat sides at certain angles to each other. Many pure metals have a crystalline structure. So do minerals in the rocks, and sugar and salt.

There are seven basic shapes or systems of crystals. Simplest is the cubic shape which is like a box. Diamonds are cubic crystals. The monoclinic system is like a matchbox which has been squashed slightly flat. The calcium-rich mineral gypsum has this shape. Some natural minerals like ruby and emerald form large, shiny crystals with beautiful colours. They are cut and polished as gemstones.

Crystals of quartz in sand grains have a triangular shape. Ice crystals form snowflakes and are six-sided.

            Tiny crystals of minerals in many kinds of rocks fit together like miniature building bricks to form much bigger regular shapes. The hexagonal (six-sided) pillars are made of the rock basalt.

SOLUTIONS

Stir a teaspoon of table salt into a glass of water - and the salt disappears. However, tasting the water shows the salt is still there. It has dissolved. The large grains or crystals of salt have broken down into their individual atoms. These are too small to see and float about freely among the molecules of water. The substance which dissolves, which is usually a crystalline solid, is the solute. The substance it dissolves in, usually a liquid is the solvent. The solute in the solvent is known as a solution.

When substances dissolve, their atoms or molecules usually gain or lose electrons. For example table salt, sodium chloride (NaCl), dissolves and breaks apart into its atoms of sodium (Na) and chlorine (Cl). Sodium loses an electron and becomes positive (Na+), while chlorine gains an electron and becomes negative (Cl-). Atoms which are positive or negative are known as ions. Many solutes form ions.

Shaking salt and sand together produces a mixture, which is different from a solution. In a mixture two or more substances intermingle but they do not dissolve and their molecules do not alter by becoming ions. Add water to the mixture and the salt dissolves. It is soluble. The sand grains do not dissolve. They are insoluble.

            The sea contains dissolved salt. The Dead Sea has so much that no more can dissolve: a saturated solution.

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OXYGEN

We cannot see, smell or taste oxygen. Yet it forms one-fifth of air and is vital for life. We must breathe oxygen to stay alive. So must all animals and plants.

Oxygen is one of the commonest chemical elements on Earth. It forms about half the weight of the planet’s hard outer rock layer or crust. It makes up one-fifth of the air. Joined with hydrogen it forms the water which covers more than two-thirds of Earth’s surface.

Oxygen is a vital part of chemical changes inside each microscopic living cell, which break apart food substances to obtain the energy for life. This is why oxygen is essential for all living things (except for a few specialized types of microbes).

Oxygen is also needed for burning. A substance such as coal or wood burns by splitting apart and joining with oxygen to form new substances. It quickly gives out lots of heat and often light in the process. This is known as combustion (burning).

            Substances burn by joining with oxygen and giving out energy as heat and light. The welding torch burns acetylene gas.

HYDROGEN

The most abundant element in the Universe is hydrogen. It forms the bulk of most stars. On Earth, most hydrogen (chemical symbol H) is joined to oxygen (O) to form water (H2O). Hydrogen is also the simplest and lightest chemical element because each of its atoms has only two sub-atomic particles, one proton and one electron.

Hydrogen joins with carbon to form the substances known as hydrocarbons. Many of the fuel gases obtained from natural gas or crude oil, such as propane and butane, are hydrocarbons. Hydrogen also joins with carbon and oxygen to form carbohydrates. Starches in foods like potatoes and rice, and sugars in cane or beet, are carbohydrates.

            Pure hydrogen gas is much lighter or less dense than air. It filled the great airships of the early 20th century to keep them aloft. However hydrogen also burns very easily. After several disasters where airships caught fire, hydrogen was no longer used. Today airships use another light gas, helium, which does not burn.

            The Sun and other stars are mostly made of hydrogen. In the star’s centre, tremendous temperatures and pressures squash hydrogen atoms together to form atoms of the gas helium. As this happens, huge amounts of energy are released as heat and light. The energy travels up to the Sun’s glowing surface and then passes through space to Earth. This is known as nuclear fusion because the centres, or nuclei, of the hydrogen atoms join or fuse together. Nuclear energy on Earth is obtained by splitting nuclei apart, known as nuclear fission.

MOLECULES

Atoms make up all the objects and substances in our world. But they are rarely single atoms, alone or unattached. They are usually attached or joined to other atoms. For example, the oxygen gas that makes up one-fifth of the air does not float about as single atoms of oxygen, O. It is in the form of oxygen atoms joined together in pairs, O2. Two or more atoms linked or joined together make a molecule. O2 is a molecule of oxygen.

If atoms of one chemical element join or combine with atoms from other elements, this forms a compound. O2 is a molecule of oxygen but not a compound. Two atoms of hydrogen and one of oxygen form H2O, which is a molecule and a compound. Some compounds, like minerals in rocks, have 50 or 100 atoms in each molecule from 15 or 20 different elements. Other compounds, like certain plastics, have millions of atoms in each molecule but usually from only a few elements, mainly carbon, hydrogen, oxygen and nitrogen. The links between atoms are called bonds. There are different types of bonds depending on the atom’s structure and the conditions such as temperature and pressure.

One of the main features of the chemical element carbon is that it joins or bonds easily with many other types of atoms and also with itself. Carbon atoms can join like links in a chain to form enormously long molecules. Often the chain is made of the same groups of atoms, called sub-units, which are repeated hundreds or thousands of times along its length. This type of molecule is called a polymer and the repeated sub-units are monomers. Many types of plastics and artificial fibres like rayon, acrylic and nylon are polymers. So are molecules in living things like cellulose in plants, chitin in insect body casings and the carrier of genetic information, DNA.

The molecule known as DNA, found in our genes, is based on a group of atoms called ribose sugar, which is repeated millions of times in a long, coiled chain.

PRECIOUS METALS

Certain types of metals are precious or valuable. This may be because they are rare and difficult to obtain from their ores, so owning them has become a symbol of power and wealth. Some metals are prized for their beautiful colours and lustre or sheen. Some are valuable because they are easy to hammer or cast into detailed, intricate shapes such as thin wires or leaves.

Two of the main precious metals are gold and silver. People have fought wars and killed for them. These metals have become even more valuable recently because they are excellent carriers of electricity. They are used in switches, circuits and other devices in electrical equipment. Silver is also mixed with another metal, mercury, to make tooth fillings.

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METALS

Chemical elements can be divided into several groups. The largest group, forming about three-quarters of all elements, is the metals. Metals have several features that the other elements or non-metals lack. They carry heat and electricity very well compared to non-metals. They are solid at normal or room temperature. They are strong, hard and tough, and they can be polished to give a smooth, shiny surface. When they are squeezed under great pressure, they change shape or deform and become squashed, rather than splinter apart or shatter. These features are true of most metals, but not all. The metal sodium is very soft, while the metal mercury is a silvery liquid at normal temperature.

Metals are very important in the modern world. Because of their strength and hardness they are used to make all kinds of buildings, structures, machines and engines. The most widely-used metal in industry is iron - but not usually in its pure form. Iron is mixed with small amounts of other substances, especially carbon, to form steel. A metal mixed with other metals and substances like this is known as an alloy. There are hundreds of different kinds of alloy steels, each with slightly different amounts of carbon and other elements, and each designed to do a different task. Stainless steel is used for sinks and cutlery. Titanium and vanadium steels resist very high temperatures without melting.

            A modern plane like the Boeing 747 Jumbo has more than four million parts, and many of these are made of metals. The main panels of the fuselage, wings and tail are aluminium. This metal is strong but very lightweight.

            Not all metals stay hard and solid. Some can burn very brightly, especially in powdered form. Fireworks contain mixtures of powdered metals such as magnesium as well as other substances, to make them flare up with bright colours.

MAKING METALS

Very rarely a lump of pure metal is found lying on the ground, such as a gold nugget. But most metals occur inside rocks. Rock especially rich in a certain metal is called a metal ore. The metal is separated from its ore by various means. Iron is obtained from ores by heating them until they melt, a process called smelting.

Aluminium ore is known as bauxite. To separate the aluminium, it is treated with chemicals and electricity is passed through it, a process called electrolysis.

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CARBON

One of the most important chemical elements is carbon - partly because it makes up one-fifth of the human body. It is also the main element in all living things and the sixth most common element in the Universe.

Atoms of carbon can join or bond easily with each other and also with numerous other atoms. This allows carbon to be the basis of a vast variety of substances, from wood to plastics. Indeed carbon is such a common and adaptable element that it has its own branch of science, known as organic chemistry.

The structures and substances in all living things are based on carbon. This includes our own skin, hair, blood, muscles, bones and brain, as well as the body parts of birds, fish, insects and worms, and all the parts of plants. Even the chemicals which form our genes, known as DNA, have carbon as their main element. This is why the chemistry of carbon is often called “the chemistry of life itself”.

            Atoms of carbon easily join with a variety of other chemical elements such as oxygen, nitrogen, and hydrogen. In different combinations they form substances such as the sugars and starches found in living things.

            One of the carbon atoms main features is that it joins to itself in long chains. Oil (crude petroleum) is a complicated mixture of many hundreds of substances like this. They include one carbon atom linked to four hydrogens as the gas methane, and eight carbons in a row forming the gas octane.

            Unlike most elements, pure carbon can exist in different forms. If its atoms join as six-sided rings they form the soft, black, slippery powder graphite, used as “lead” in pencils. If they join in a box-like pattern they form the hardest substance of all, diamond.

            Atoms of carbon can even join to each other to form necklace-like rings. One type of ring has six carbon atoms. This is known as the benzene ring. If each carbon in the ring is also joined to a hydrogen atom, the result is the substance known as benzene. It is a very important chemical in many industries.

            The entire living world is based on carbon. It joins with other substances to form snail shells, spider’s legs, ants’ eggs, plant roots and countless other parts.

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MATTER

Everything is made of matter. Every object, substance, chemical and material is matter. This includes not only things you can see easily, like the paper of page and the ink that forms the words and pictures. It also includes specks of dust too small to notice, houses and cars, living things like trees and your own body, the rocks of the Earth, the clouds in the sky and the invisible air around you. And not only objects and substances on Earth are made of matter. All of the planets and stars in deep space contain matter. In fact everything in the entire Universe is made of matter. All matter is made of tiny building blocks called atoms.

There are places where there is no matter. If there is no matter then there is nothing at all. The total or complete absence of matter is called a vacuum. However a total vacuum is very unusual. “Space” is named because it is supposed to be just empty space, with no matter. But even in the depths of space, a few micro-particles of dust or some wispy bits of gas are floating about. These tiny bits of matter may be several metres apart, instead of crammed together like they are on Earth. But they are still present. Here on Earth, powerful vacuum pumps can suck most of the matter out of a container, but never quite all of it.

            Why is a butterfly like a lump of rock whizzing through space? Both are made of atoms. The butterfly is living matter, the rock non-living matter.

STATES OF MATTER

Matter exists in three main forms, called the states of matter. These are solid, liquid and gas. In a solid such as ice, the molecules are very close together and joined in a rigid pattern. They can hardly move. So a solid object stays the same volume and does not change its shape. In a liquid such as water, the molecules are still quite close together but they are not joined to each other. They can move about, which means the whole liquid can change shape and flow, although, like the solid, it still takes up the same volume. In a gas like water vapour, the molecules can move nearer together or farther apart. So a gas can also get bigger or smaller, to fill the container it is in.

            A hot-air balloon contains matter in the form of gas – air. Heat from the burner causes the air’s molecules to rush farther apart, so taking up more room. Soon there are fewer molecules in the hot air inside the balloon than in the normal air outside. The balloon is lighter or less dense and rises.

CHANGING STATES

Matter or substance can change state from solid to liquid, or liquid to gas. This usually happens by adding heat. Matter can also change state the other way from gas to liquid or liquid to solid. This usually happens by cooling (taking away heat). A common example which is all around us is water. The world’s water is always on the move and changing state in a never-ending process, the water cycle.

In the water cycle, the Sun warms the sea. The heat makes liquid water turn into a gas, invisible water vapour, which rises into the sky. It is cooler high up so water vapour changes state back into a liquid, forming tiny droplets. These are so light that they float as clouds. Wind blows the droplets over the land. Some clumps together, become too heavy to float and fall as rain. Some droplets blow even higher, up over a mountain, and become even colder. They change state again, freezing solid into snowflakes. The snow falls to the ground and melts into liquid water. With the rain, it flows into streams and rivers, and finally into the sea and so the cycle continues.

PROPERTIES OF MATTER

Matter has many features, or properties. One of the main properties is its state -solid, liquid or gas. Another property is the type of atoms it is made of. Each kind of pure substance, like iron, carbon, oxygen or sulphur, has a different kind of atom. It is known as a chemical element.

A third property of matter is density. This is the amount of matter in a certain place or volume. The more matter within a certain volume, the denser or heavier the substance or object. Dense substances like iron have lots of large atoms packed close together. Density is important because it determines whether things float or sink. If an object is less dense than water, such as a lump of wood, it floats. A lump of iron is more dense than water and so it sinks. But if the iron is made into a boat’s hull its shape contains lots of air, which has a very low density and is extremely light. The overall density of the iron-plus-air is less than the density of water, and makes the boat float.

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ATOMS

All matter is made of atoms. An individual atom is far too small to see, even with the most powerful microscope. But atoms joined together make up every solid object, substance, chemical and material in the Universe. A pinhead, for example, contains about one billion billion atoms.

Atoms able to move about make up liquids and gases. Atoms which are more fixed and unable to move much make up solids. Not all atoms are the same. There are about 92 different kinds of atoms that occur naturally. Scientists have made another 17 or so artificial kinds in laboratories. Each kind of atom has individual properties that distinguish it in some way from another kind. A substance made from just one kind of atom is known as a chemical element. Examples of six elements are:

Atoms of different chemical elements have different numbers of particles. Simplest is Hydrogen (1), a very light gas that makes up most of the Sun. Neon (2) is a gas used in coloured strip-lights. Egg yolks are rich in sulphur (3). Calcium (4) is needed for healthy bones. Silver (5) is a shiny valuable metal. Lead (6) has many particles and so is very heavy. It is used to make small weights or shot.

Atoms are not solid, like marbles. In fact, they are mostly empty space. But this space contains even smaller pieces of matter known as subatomic particles. There are three main kinds of subatomic particles - protons, neutrons and electrons. The protons and neutrons are gathered together in the middle of the atom, forming its central part or nucleus. The electrons are much smaller and move at speed around the nucleus. They do not move at random, but stay in certain layers known as shells.

Elements differ in their numbers of subatomic particles. Hydrogen is the simplest because its atom has just two particles, one proton and one electron. In most atoms there are the same numbers of protons as electrons. This is because a proton has a tiny positive electrical charge and an electron has the same amount of negative charge. The two sets of opposite charges balance each other out so the whole atom has no charge. This makes it stable or unlikely to break up.

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RADIOACTIVITY

Most atoms are stable. They remain the same through time. Others are unstable - they are likely to break up. As they do so they give off some of their particles or energy in the form of rays. These particles or rays are known as radioactivity. Examples of chemical elements with radioactive atoms include uranium, plutonium and radium. As atoms give out particles or rays they change into the atoms of simpler elements. For example, uranium changes into lead. This change is called radioactive decay. It happens at different speeds or rates for different radioactive elements. The time taken for half of a number of atoms to decay is known as the half-life of that element. Radioactivity can be dangerous since it harms living things. But under controlled conditions it is very useful in medicine and scientific research.

            The name “radioactivity” was invented by Polish-born scientist Marie Curie (1867-1934). She studied various rocks and minerals from the Earth and gave the name to the invisible rays or particles that some of them gave off, which affected photographic paper and various electrical equipment. In particular Marie worked with the substance pitchblende, a raw material used to obtain the metal uranium. Pitchblende, gave off more radioactivity than expected from uranium alone. Marie purified the substances which gave off this extra radiation and so discovered two new elements, polonium and radium.

            How do we know the age of mummies from Ancient Egypt? By measuring the tiny amounts of radioactive substances they contain. This is known as radiocarbon dating.

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               There is perhaps no human activity older, more varied or stranger, than fishing. He tricks and catches fish in different ways, such as using his bare hand, or fishing even with harpoon guns in whaling! But the method most used today is the one by which it produces the biggest share of commercial fishing known as trawling. Do you know how do trawlers fish?

               Trawlers fish with a bag-size net. It is let out on long warps or ropes. The fish are swept in at the wide, open end and then get trapped at the narrower, closed end. The trawler may be between 100 to 1500 metres long or more. In this system, the motorized fishing boats trawl by towing a large net in three different ways to keep the mouth of the net open. Firstly, a beam can be placed across the head of the net; secondly a pair of boats can be used - one at each side of the net to tow it and thirdly, some floating weights, called otter boards can be attached to the sides of the mouth of the net.

               However, the beam trawl is only used on a few small fishing crafts, and on the other hand, pair trawling is used to catch fishes from the bottom of the sea to enormous depths, sometimes at the range of 1500 metres or more. When the net is full, powered winches haul it on the board through a ramp. The otter trawl is widely used and is employed on almost every fishing technique except the smaller trawlers.

               The net gathers in everything including eggs, newly hatched fishes and algae. But this system is considered to be very destructive and alarming in the context of overfishing along the seas. Sometimes an entire fleet of fishing vessels is headed by a large factory ship fitted out just for processing of the catch. A single “sweep” of the net often taken in terms of tonnes of fish provides an idea of the quantity of fish caught in rich seas. Deep sea fishes like sardines and herrings together account for eighteen percent of the world’s catch.

               Today, the large motor fishing vessels are fitted with sonar or echo-sound equipments to locate a shoal of fish.

 

               You might say that the ball would fall behind the person who throws it because he would have moved forward with the moving train. But in fact this is not correct.

               You can perform a simple experiment to answer this question. You would be surprised to find that the ball lands right in your hand when thrown upward inside the moving train. Do you know why it happens so?

               In a moving train everything inside the train also moves with the speed of the train, for example, the fans, passengers, you and the ball in your hand. When you throw up the ball, a part of the speed of the train is imparted to it. It acquires a vertical motion in addition to its horizontal motion. The passengers in the train cannot see its horizontal motion but only its upward and downward movements.

               Imagine a man outside the train, who is watching your experiment. As we have said the ball possesses both vertical and horizontal motions, both these motions combined together make the ball travel along a parabolic path. The observer outside the train will see the ball moving in a parabolic path but a passenger in the train will see only the up and down motions of the ball.

               Now the question arises whether the ball follows the parabolic path or just moves up and down? Out of these two which one is right? In fact, all motion is relative to the observer. There is nothing like absolute motion and hence the motion of the ball is different for the two observers. 

               When you push the button of an electric door bell or calling bell it keeps on ringing as long as the button remains pressed.

               Do you know how does it function? An electric bell is a simple device based on the magnetic effects of electric current. It is used in offices, houses, industries and for fire alarms.

               It consists of a U-shaped electromagnet and a soft-iron armature. The armature has a small hammer for striking the gong. This hammer hits the gong repeatedly and produces sound. The gong is made of a metal. For operating the bell, a push button is pressed. In an electric bell, the button is a switch that connects the supply of electricity to the bell.

               When the button of the bell is pressed, the current flows through electromagnet winding, armature, contact spring and the contact screw. The flow of the current magnetizes the soft-iron core of the electromagnet. This attracts the armature, causing the attached hammer to strike the metal gong and thereby produce sound.

               As the armature moves forward due to magnetic attraction the contact spring moves away from the contact screw. This breaks the circuit and the current stops flowing. As a result, the soft-iron core loses its magnetism. It, therefore, no longer attracts the armature which, then, is pulled back by the contact spring to its original position. As soon as the armature comes to its original position the electric circuit is again completed and the soft iron becomes magnetized. It again attracts the armature and thereby the hammer strikes against the gong and produces sound. As long as the push button remains pressed, the circuit is alternately broken and completed causing the hammer to strike the gong. Thus an electric bell keeps ringing.

               If a steel core is used instead of a soft-iron, then the steel core will become a permanent magnet due to passage of electric current through the winding. Consequently, the armature will stay attracted even when the contact spring moves away from the contact screw, so the hammer will strike the gong only once.

 

               It is a well known fact that when resins are put in water they get swollen. This swelling takes place due to the entry of water through the membrane of the resins. Similarly, if grapes are put in sugar solution they shrink. Swelling of resins and shrinking of grapes take place due to a process known as osmosis. Do you know what this osmosis is?

               Osmosis is a process in which a solution of lower concentration passes into a solution of higher concentration through a semipermeable membrane. A semipermeable membrane is one that allows some, but not all, substances to pass through it. This contains very small pores. When resins are put into water, the covering acts as a semipermeable membrane. Water is less concentrated than the substance present inside the resins and so the water moves into the resins through its semipermeable membrane. Similarly, fluid from grapes moves out through the semipermeable membrane, as the concentration of sugar solution is more than that of the grapes. There is a tendency for solutions separated by a membrane to become equal in molecular concentration.

               In osmosis, the movement is always from a dilute solution into a solution of higher concentration. This reduces the concentration of the stronger solution. The rate of osmosis depends upon the comparative strengths of the two solutions. The greater the difference, the faster the rate of osmosis. This process continues until both solutions are of equal strength. When this equilibrium is reached, osmosis stops.

               Osmosis is an ongoing process among the living beings. The membranes of cells are semipermeable. Plants absorb water and dissolved minerals from the soil by osmosis; they use osmosis to move the water and dissolved minerals through the plant, cell by cell. Osmosis also maintains turgor pressure. Turgor pressure is the pressure of water on the cell. It gives the cell form and strength. When there is a decrease in turgor pressure, the plant will soon wilt and lose its regular stiffness.

               Osmosis allows the transfer of water and dissolved nutrients in the human body from the blood into the cells.

 

Electric heaters, immersion heaters, electric irons, electric kettles, etc. are appliances which produce heat through electricity. All these appliances are based on the heating effects of electric current. When electric current is passed through a wire, it gets heated up. Heating of a wire depends upon two facts: first, on the resistance of the wire and then on the amount of electric current passed. The heat produced in the wire is directly proportional to the resistance of the wire and that of the square of the current. The amount of heat produced also depends upon the time for which the current passes through the wire.

Based upon this property of current, many domestic electric appliances have been developed. The working principle of all these appliances is almost the same, the difference lies only in their construction. An electric heater consists of a coil of nichrome wire which is in the form of a spring. This coil is mounted on an insulating base plate made of clay. When electric current is passed through the coil, it gets heated up. Room heaters are also made in a similar way, the only difference being that nichrome wire is wound around an insulating rod and a reflector is mounted at the back of the coil which reflects the heat radiation.

Immersion heaters also consist of a nichrome wire which is enclosed in a metal tube. To isolate the wire from the metal tube, an insulating powder is filled in the tube. This powder acts as an insulator for electricity but conducts heat. When the two terminals of the wire are connected to an electric source, the current starts flowing through the wire and it gets heated up. The immersion heater is put inside a bucket full of water to heat the water.

An electric iron is used to remove the wrinkles from washed clothes. This appliance also consists of a ribbon of nichrome wire which is enclosed between two sheets of mica. This spreads the heat uniformally along the base plate of an electric iron. Mica sheets are mounted on a heavy metal plate. This metal plate, when pressed against the surface of the cloth, removes the wrinkles from the cloth.

Electric irons are of two types: automatic and manual. Automatic one is fitted with a thermostat control which regulates the temperature. Manual irons do not have such a device. When the iron is cold, thermostat provides and maintains a constant temperature by the use of a device that cuts off the supply of heat when the required temperature is exceeded.

An electric kettle is used to prepare tea or coffee. It also consists of a heating element fitted at the bottom of the vessel and is isolated from it. Water is put into the vessel which gets heated when current is passed through the heating element.

For all electrically heated appliances, it is very essential to have an earth connection. Immersion heaters should not be switched on, until there is water in the bucket. The electric bulb is also a similar device whose filament gets heated up when the electric current is passed through it and it produces light.