My Science School

Do you know that some people fly on a bag of hot air? Of course, the "bag" is a huge hot-air balloon. A balloon filled with hot air can lift people.

When air is heated, its molecules speed up and begin to push on one another. This makes the air expand, or take up more space. The molecules of hot air push further and further apart until only a few molecules take up a great deal of space.

The molecules of hot air that fill a balloon are further apart than the molecules of colder air outside the balloon. So the hot air weighs much less than the colder air. Because the hot air is lighter, it rises. It pushes up inside the balloon. When the push is strong enough, it lifts the balloon high into the sky.

Ask a grown-up to heat a toaster. Standing I metre away from the toaster, blow soap bubbles so that they float above it. Watch the bubbles rise as they reach the hot air above the toaster! Be careful not to get the toaster wet! Electricity and water are dangerous together!

A burning candle has a soft, glowing flame that gives off light. But when you turn on a light bulb, there is no flame. So how does a light bulb give off light?

When the light is on, electricity runs through a tiny wire inside the bulb. This makes the wire hot. Like everything else, the wire is made of tiny bits of matter called atoms. As the wire in the light bulb gets hot, the atoms soak up energy. Pretty soon, the atoms have soaked up all the energy they can hold. Then they begin to throw off the extra energy as light.

Compact fluorescent lights or lamps (CFL's) are twisted glass tubes filled with gas. CFL light bulbs give off light when electricity passes through the gas, causing the tubes' coating to glow.

Some bulbs produce light when electricity flows through tiny devices called light-emitting diodes (LED's). LED's are used in remote controls, billboards, traffic lights, street lights, and even car headlights.

You don't have to watch a covered saucepan to know when the food or water inside it is boiling. The lid will begin to bounce up and down when the water begins to boil. The push that moves the lid comes from steam.

As the water inside the saucepan is heated, the molecules take up energy. That energy makes the molecules speed up and push hard against one another. When the water is hot enough, some of it changes from a liquid to a gas called steam.

The molecules of gases move around more and are further apart than the molecules of liquids. So the steam expands. It takes up much more space than the hot water did. But there is only one place where the steam can escape - through the spaces around the saucepan lid. It squeezes out with a hard push, and the push bounces the lid up and down.

Steam also can be used to push the moving parts of machines—to run ships, trains, and factories that make electricity. Such big machines need very strong pushes from expanding steam. So huge amounts of water must be boiled.

Sometimes, in cool weather, the insides of our windows look cloudy. They are covered with a thin film of water. Where does this water come from?

It comes from water vapour, water molecules mixed with the air inside the house. The water collects on the windows when the glass is cool.

Water vapour is a gas. The molecules of water vapour are as warm as the air around them in the house, so they move very fast. But when the molecules hit the cool glass in the window, they lose heat. As the molecules grow cooler, they move closer together and slow down. When they are moving slowly enough, they condense, or turn into tiny drops of liquid water.

Sometimes when the weather is very cold, the glass in the windows gets much colder than the air inside the house. Then the molecules of water vapour lose even more heat when they touch the glass. They slow down much more and move much closer together. When they get close enough to pull hard on one another, they freeze. Then the window is covered with frost - thin, feathery bits of solid ice.

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Cold butter on hot toast doesn't stay cold for very long. Some of the heat from the toast passes into the butter, and so the butter becomes warm, too. The heat is a form of energy. Heat energy can spread. Heat energy always flows from something warmer to something cooler. The movement of molecules passes the heat along.

A slice of toast is a solid piece of bread. But the molecules in the bread move. They wriggle and jiggle, even though they are held together. As the bread is toasted, the heat from the toaster makes the molecules speed up.

Cold butter is solid, too. But its molecules are moving very slowly. When you spread the cold butter onto the hot toast, some of the fast-moving toast molecules bump into the slow-moving butter molecules. That makes the butter molecules move faster. The jiggling motion moves from molecule to molecule until the butter is soft and warm.

Put five coins with different dates in a small box. Show them to some friends and explain that you will read your friends' minds. While your back is turned, get your friends to choose one of the coins and remember its date. Ask each friend to hold the coin tightly in one hand for a moment and to concentrate on the date. When everyone has had a turn, ask the last person to drop the coin in the box. Turn around right away and touch each coin lightly. Four of the coins will be cool - but the fifth will be warm, because it has taken heat energy from your friends' hands. Pick that coin, read the date, and amaze your friends!

Can you fry ice cubes? You can try, but if you heat ice cubes in a pan, they won’t be ice cubes anymore. The ice will melt into water. And after a while, the water will boil and turn into steam. What makes the ice cubes change?

Ice melts because something happens to its molecules. Heat energy makes the molecules move faster. As the molecules speed up, they begin to move away from another. Then the ice changes from a solid to a liquid - water.

Heat speeds up the molecules in liquids, too. So as the molecules speed up, they move even further apart. Finally, they lose almost all their pull on one another. Then the liquid evaporates. It becomes a gas.

And that’s what happens when water boils. Heat makes the molecules roll and tumble faster and faster in the pan. When the water molecules are moving fast enough, they become steam. The molecules of steam mix with other molecules in the air.

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On a chilly day, you can rub your hands together to warm them. Why does rubbing your hands make them warm?

You are made up of molecules, just like all the other kinds of matter around you.

When you rub your hands together, you create friction. You make the molecules in your skin bump and push one another.

That makes the molecules speed up. It gives them more energy. The faster the molecules move, the warmer you get. So if you rub hard enough, your hands feel warm.

Biff! The racket hits the tennis ball, and the ball sails across the net. A point is scored - thanks to a hamburger!

Of course, the hamburger did not actually hit the ball. The player did. But the player ate the hamburger for lunch, and the player’s body got energy from the hamburger. Some of this energy was used to swing the racket. And the energy in the swinging racket knocked the ball over the net. So stand up and cheer for the hamburger. It helped score the point!

There are only a few kinds of energy. But each kind of energy can change into other kinds. Chemical energy in the food you eat can be changed into the energy of movement when you run or hit a ball.

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In some parts of the world, energy comes roaring up out of the ground or rolling across the ocean - for free!

One form of this energy is gathered from boiling water and steam. The rocks deep below the surface of the earth are very hot, sometimes as hot as 600 °C. This is about five times hotter than boiling water. If underground water trickles into these hot rocks, it quickly turns into steam. The steam takes up about 4,000 times more room than the water. The steam forces its way into cracks in the rocks and sometimes finds its way to the surface. The steam can be piped into an electricity-making machine called a turbine. The steam turns wheels in the turbine, which helps a generator to make electricity.

Movement in the oceans also releases huge amounts of energy. People have invented several ways to use this energy. One way uses the power of the rising and falling tides. To capture this energy, a dam is built across the opening to a bay. At high tide, the dam is closed to hold water in the bay. At low tide, the water can be released. The flowing water spins wheels inside a turbine to help make electricity.

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Snorkel and Scorch are terribly sad. For 200 years they worked as palace dragons. They carried wood for fires, huffed flames into fireplaces, and lit candles in the palace halls. But now they are out of work. It is not their fault. Things at the palace have changed. Now the royal halls are lit with light bulbs, and the rooms are heated with a boiler. There is even a brand-new stove in the kitchen.

Electricity is doing the work that Snorkel and Scorch used to do. Electricity makes one kind of energy. When electricity flows through a wire, much as water flows through a hose, it is called an electric current. An electric current can do the same kind of work a fuel can do. It can make light and heat. And it can make the push or pull that runs all kinds of machines.

So Snorkel and Scorch do not have to carry wood, light fires, or keep the candles burning any more. All they have to do is push the switches that turn the electric current on and off. Life will be much easier from now on, and Snorkel and Scorch will have enough free time to help out at palace barbecues.

The word electricity comes from the Greek word elektron, which means “amber”. Amber is a yellow stone that gets an electric charge when it is rubbed with a cloth.

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All things - even you - are made up of billions and billions of “bits” that are smaller than anything you can imagine. These “bits” are called atoms. Atoms are the tiniest parts that something can be broken into and still be the same stuff. For example, one atom of gold is the tiniest bit of gold possible.

Special machines can split certain kinds of atoms into even smaller parts. When atoms split, they give off energy. The part of the atom that splits is called the nucleus, so the energy is called nuclear energy.

The energy given off by atoms when they are split is in the form of heat. So a machine that splits atoms can be used the same way a fuel-burning engine is used. The machine makes heat. The heat can be used to run generators that make electricity and to run other machines, too.

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You have seen candles burning. An orange flame dances around the wick, while the wax melts underneath and drips down. The candle gets shorter as the wax melts.

The candle wax has a kind of stored-up energy. It is a fuel. A fuel is something that is burned to make light, heat, or a push that makes things move. The coal, oil, and gas used to heat homes and other buildings and to cook food are fuels. The petrol in a car and the wood in a fireplace are fuels, too.

The same thing happens to every kind of fuel when it burns. When it starts burning, the heat makes it break down and change to other things, such as ashes. As the fuel breaks down, it gives off energy. Some of the energy is light, and some is heat.

When petrol is burned in a car engine, the heat energy makes the engine push. The push from the engine makes the car run.

The energy stored in a fuel is called chemical energy.

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A push from the wind can make a kite fly. It can make a sailing boat speed across the water. It can even light lamps and pump water.

People use wind energy to pump water and make electricity for their homes and other buildings. They do it with the help of a machine called a windmill. Windmills come in different shapes and sizes, but they are all alike in some ways.

They are tall enough to catch the strong winds that blow high above the ground. They also have a wheel, which is the part that spins. The wheel has paddles, sails, or blades for the wind to push against. When the wind blows, the wheel spins. This makes a push that runs a water pump or an electricity-making machine called a generator.

Of course, the wind doesn’t always blow, so the windmill doesn’t always run. But water pumped by the windmill can be stored in tanks. And electricity made when the windmill runs can be stored in batteries. Until the windmill goes back to work, people can use stored-up water and stored-up electricity in their homes. There are even “wind farms”, where many windmills make electricity for whole communities.

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Someday you may live in a house that keeps itself warm with a tankful of “sunshine”. The heat for such a house comes from solar energy, or energy from the sun.

The sun’s energy gives power to everything on the earth. Using the sun’s energy, plants make food for animals and people.

A house that uses the sun’s energy is a solar house - a “sunshine house”. A solar house has special collectors to capture the sun’s heat. Usually the collectors are on the roof or on the sunniest side of the house.

How is the sun’s heat stored? In one kind of solar house, water or another liquid is pumped through collectors on sunny days to pick up heat. Next, the hot water is pumped to a huge tank filled with sand or gravel. Bit by bit, it heats the whole tank. Sand or gravel can hold heat longer than water does, so heat stays in the tank long after the sun goes down.

When the house is chilly, you just press a button that turns on a fan beside the tank. The fan pulls the cool air into the hot tank. When the air gets warm, it blows out of the tank and through the house. So the house gets warm, and you do, too - with a tankful of stored-up heat from the sun.

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Opening doors, playing football, running a race - you really keep moving. What gives you the power to move?

The answer is energy! Your body gets its energy from food. The energy you get makes the pushes and pulls that keep you moving.

A car engine burns petrol. As the petrol burns, it gives off energy. That energy makes the pushes and pulls that turn the wheels.

There are many kinds of energy. Things that move, such as water and wind, have energy. Things that burn, such as fire, have energy. But the most important source of energy is the sun. It keeps everything on the earth on the move.

Energy, in physics, the capacity for doing work. It may exist in potential, kinetic, thermal, electrical, chemical, nuclear, or other various forms. There are, moreover, heat and work - i.e., energy in the process of transfer from one body to another. After it has been transferred, energy is always designated according to its nature. Hence, heat transferred may become thermal energy, while work done may manifest itself in the form of mechanical energy.

Energy can be neither created nor destroyed but only changed from one form to another. This principle is known as the conservation of energy or the first law of thermodynamics. For example, when a box slides down a hill, the potential energy that the box has from being located high up on the slope is converted to kinetic energy, energy of motion. As the box slows to a stop through friction, the kinetic energy from the box’s motion is converted to thermal energy that heats the box and the slope.

Energy can be converted from one form to another in various other ways. Usable mechanical or electrical energy is, for instance, produced by many kinds of devices, including fuel-burning heat engines, generators, batteries, fuel cells, and magneto-hydrodynamic systems.