Energy in Daily Life

Many kinds of work go on around us: a fan turns, a bulb glows, a drum booms, a kettle boils, a magnet picks up pins. Each needs energy in a different form. The main forms of energy are:

• mechanical energy (the energy of moving or raised things)
• heat energy
• light energy
• sound energy
• electrical energy
• magnetic energy
• chemical energy (stored in food, fuels and batteries)
• nuclear energy (also called atomic energy, stored in the centers of atoms; it makes the sun shine)

Mechanical energy

Some things around us are moving: a rolling ball, a flying bird, a running bus. Others are still, but kept in a special position: water in a tank on the roof, a stretched catapult. Both kinds have energy.

The energy a body has because of its motion or its position is called mechanical energy. Mechanical energy is of two types: potential energy and kinetic energy.

a. Potential energy

Your arm’s muscles have energy (from your food). When you stretch the catapult, that energy is stored in the stretched rubber. When you let go, the stored energy throws the stone a long way. Energy stored like this is called potential energy. (“Potential” means “ready to be used”.)

A footballer does the same: as he swings his leg back, energy from his muscles is stored in his raised leg and his stretched muscles. As he kicks, it sends the ball rolling far away.

The energy stored in a body because of its position, or a change in its position or shape, is called potential energy.

Potential energy is stored in many things:

• because of their height: water stored in a tank on a roof or behind a dam, a stone or a brick lifted up, a ripe mango high on its tree;
• because their shape is changed: a spring squeezed, stretched, bent or coiled; a stretched rubber band or bow; the wound-up spring of a mechanical (key) watch or a toy car.

b. Kinetic energy

It is easy to catch a ball thrown slowly, but hard to catch one thrown fast. And it is easy to catch a light tennis ball, but hard to catch a heavy cricket ball thrown at the same speed. A moving ball hits your hands with energy, and some balls bring more energy than others.

So everything that is moving has energy. The energy a body has because of its motion is called kinetic energy. (“Kinetic” comes from a Greek word for “moving”.)

A rolling ball, a flying bird, flowing water, blowing wind and a moving bus all have kinetic energy.

Factors affecting kinetic energy

The kinetic energy of a body depends on two things:

1. its mass: the heavier the body, the more kinetic energy it has at the same speed. A cricket ball has more kinetic energy than a tennis ball thrown at the same speed. A motorbike has more than a bicycle going at the same speed.
2. its speed (velocity): the faster it moves, the more kinetic energy it has. A cricket ball thrown fast has more kinetic energy than the same ball thrown slowly.

That is why a crash at high speed does so much more damage than one at low speed.

Transformation of energy

Water stored in a tank, behind a dam, in a lake or in a pond has potential energy. When it flows down, its potential energy changes into kinetic energy. Energy changes from one form to another like this, but it is never destroyed and never made from nothing.

Energy can neither be created nor destroyed; it can only be changed from one form to another. This is called the law of conservation of energy. (“Conserve” means “keep safe”: the total amount of energy is kept the same.)

In daily life we need energy in different forms: sometimes kinetic energy, sometimes light, heat, sound, magnetic or electrical energy. So we use machines and appliances to change the energy we have into the form we need. The change of energy from one form into another is called transformation of energy.

Appliance	Energy it takes in	Energy it gives out
solar panel	light energy (from the sun)	electrical energy
electric bulb	electrical energy	light energy (and some heat)
electric heater	electrical energy	heat energy
torch	chemical energy in its cells → electrical energy	light energy
electric fan	electrical energy	kinetic energy
radio, loudspeaker	electrical energy	sound energy
LP gas stove	chemical energy (in the gas)	heat energy (and some light)

A chain of changes can be shown in a flow chart. In a hydropower plant:

water stored behind a dam (potential energy) → water rushing down from the dam (kinetic energy) → turbine turned by the water, driving a generator (electrical energy) → a bulb lit in your home (light energy)

At Dashain, children swing high on a bamboo ping. As the swing goes down and up again, its energy keeps changing from one form to the other. (A real ping slowly loses its energy to the air, as a little heat and sound, so someone must keep pushing it; the energy still isn’t destroyed.)

1. Fill in the blanks with the right word

kinetic energy, mechanical energy, energy, forest, potential energy

2. Choose the correct answer

3. Answer these questions

In summer we feel hot, and in winter we feel cold. A piece of ice feels cold to touch, and hot water feels warm. What makes us feel hot or cold?

The cause is heat. Heat is a form of energy that gives us the feeling of hotness. Its SI unit is the joule (J), like every form of energy.

Heat always flows from a hotter body to a colder body: from a body at a higher temperature to a body at a lower temperature.

• When you touch something hotter than your skin, heat flows from it into your skin, and you feel hot.
• When you touch something colder than your skin, heat flows from your skin into it, and you feel cold.

So cold is not something that flows into your hand from ice. Your hand feels cold because it is losing heat to the ice.

Heat absorption

When we sit in the sun or in front of a heater, or put on clothes dried in the sun, we feel warm. When we touch a pot left in the sun, it feels hot. In each case our body is taking in heat from something hotter than itself.

The process in which a body takes in heat from hotter things around it is called heat absorption (absorption of heat). Not all things absorb heat equally well; it depends on several factors.

Factors affecting the absorption of heat

How well a thing absorbs heat depends mainly on two things: its surface area and its color.

1. Surface area of the object

The more surface a thing spreads out to the heat, the more heat it absorbs. The heat absorbed by an object increases with its surface area, and decreases when its surface area is smaller.

Try both factors in the widget after the next activity.

2. Color of the object

A black object absorbs heat well; a white object absorbs little heat, because it reflects most of the light falling on it. Other colors are in between: darker colors absorb more than lighter ones.

That is why:

• light-colored, white clothes are cooler to wear in the hot season, and black or dark clothes are better in the cold season;
• the collecting surface of a solar water heater is painted black, so that it absorbs as much of the sun’s energy as possible;
• the outer bottom of a cooking pot is often black, so the food cooks faster and fuel is saved.

1. Fill in the blanks with the right word

less, energy, joule, negligible, more

2. Write true or false

3. Choose the correct answer

4. Give reasons

5. Answer these questions

On your way to school you see houses, vehicles, trees, animals and flowers of many colors. How can you see them? What do you need?

In the daytime we see things around us because of the light from the sun; at night, because of the light from lamps and bulbs. Things like the sun, a flame and a glowing bulb give out their own light. Most other things don’t: we see them only when light from a bright source falls on them and bounces back (reflects) into our eyes. In a completely dark room you can see nothing at all. So we need light to see.

Light is a form of energy that helps us see the things around us.

Transmission of light

Have you seen light moving from one place to another? How does sunlight get from the sun to the Earth?

Light travels from its source to other places. Sunlight travels all the way from the sun to the Earth, through space. Light is the fastest thing there is: it travels about three lakh kilometers (300,000 km) in one second. The process in which light travels from one place to another is called transmission of light.

Rectilinear propagation of light

In the morning, watch the sunlight coming into a dark room through a small hole or gap in the window. You can see dust floating in it. Is the beam straight or bent? At night, have you shone a torch up at the sky? The beam is straight. Both show that light travels in straight lines. This is called the rectilinear propagation of light (“rectilinear” means “in a straight line”; “propagation” means “traveling”).

Does this rule hold for the light from the lamps and torches we see every day? Yes. Sometimes the light from a bulb or a torch seems to curve or bend, but that is only an illusion. In air, or in any one clear material, light doesn’t travel in a curved path. It goes out from its source in straight lines, in whichever direction it starts.

Ray of light

When you walk at night with a torch, you can see its light going far ahead. The headlights of vehicles at night shine a long way too. A stream of light like this is called a beam of light. The smallest part of a beam, a single thin line of light, is called a ray of light. A beam is made of many rays.

A ray is drawn as a straight line, because light travels in straight lines, with an arrow on it to show which way the light is going.

Pinhole camera

You can make a camera with no lens at all, just a tiny hole. It is called a pinhole camera.

Where does the light that makes the picture come from? From the object itself. Light from every point of the object travels in straight lines, and the only way in is through the tiny hole. A ray from the top of the object passes through the hole going downwards, and lands at the bottom of the screen. A ray from the bottom passes through going upwards, and lands at the top. Rays from all the other points land in between, each in its own place. So an inverted (upside-down) image forms on the screen. The pinhole camera works only because light travels in straight lines.

The farther the screen is from the hole, the more the rays spread apart before they land, so the bigger the image.

Shadow

On a sunny day, when you stand in an open field, your shadow falls on the ground on the side away from the sun. Houses, trees and animals also cast shadows on the side opposite to the sun. Why does a shadow form, and how? Think about it.

Things are of three kinds, by how much light they let through:

• Transparent things, like clear glass and clean water, let light pass through easily. They make almost no shadow.
• Translucent things, like tracing paper, butter paper and frosted glass, let some light through. They make a faint shadow.
• Opaque things, like wood, a book, a wall or your body, let no light through. They make a dark shadow.

A shadow is the dark area formed behind an opaque object when it blocks the light from a source. To get a shadow, you need three things: a source of light, an opaque object, and a screen (a wall or the ground) for the shadow to fall on. And it all works because light travels in straight lines.

Types of shadow

Look closely at a shadow made by a candle or a big bulb. It has a dark middle and a paler edge. So there are two types of shadow, or two parts of a shadow:

• Umbra: the dark part, where the light is blocked completely: no light from the source reaches it.
• Penumbra: the paler part round the umbra, where the light is blocked only partly: light from part of the source still reaches it.

A penumbra forms because a real source of light, like a flame, a bulb or the sun, has a size: it isn’t just a point.

The size of a shadow depends on the distances between the source of light, the object and the screen:

• If the object is moved closer to the source (or the source closer to the object), the shadow gets bigger.
• If the source is moved farther from the object, the shadow gets smaller.
• If the screen is moved closer to the object (or the object closer to the screen), the shadow gets smaller and sharper.

Try it with your hand and a torch: hold your hand close to the torch, and a huge hand-shadow fills the wall.

Sundial

Did you know how people told the time before clocks and watches were invented? Our ancestors used simple instruments. One was the sundial: a stick or a slanting blade that casts a shadow on a flat dial marked with the hours. The direction and the length of its shadow tell the time.

1. Fill in the blanks with the right word

ray, straight, shadow, screen, bending

2. Choose the correct answer

3. Write the differences

4. Answer these questions

What is sound? How is it made? How does it reach our ears? Let’s find out.

Sound is a form of energy. It is produced when things vibrate (shake quickly to and fro), and it travels as a wave to our ears. We hear many kinds of sound every day. Some are useful and pleasant, like music, a friend’s voice or a warning horn. Others are unwanted and unpleasant, like the noise of traffic and machines; unwanted sound is called noise.

Wave

Drop a stone into the still water of a pond. Ripples move out towards the edge, going up and down as they go. These are waves. They spread out in all directions. But the water itself doesn’t flow to the edge: a leaf floating on it just bobs up and down in its place. A wave on a stretched rope is the same: humps run from one end to the other, while each bit of the rope only moves up and down.

So a wave carries energy from one place to another, but not the water or the rope. When an object vibrates, the process in which its energy is passed on through a medium (the water, the rope, the air) is called a wave.

Sources of sound

The vibrating plate pushes on the air around it and makes waves in the air. These waves reach our ears as sound. So the main thing needed to make sound is a vibrating body.

An object that produces sound is called a source of sound. A madal, a guitar, a bell, a flute (bansuri), your own throat and a loudspeaker are all sources of sound.

Origin of sound waves

As we saw in activity 7.21, when the plate vibrates, it pushes the air around it forward and back (to and fro). This disturbance is passed on through the air from one particle to the next, until it reaches our ears. Then we hear the sound.

So to make a sound wave, two things are needed: a vibrating body, and a material medium (a solid, a liquid or a gas) around it. Even if a body is vibrating, if there is no medium around it, no sound wave is made, and we can’t hear it.

Propagation of sound

On the slinky, each ring moves to and fro in the same direction as the wave travels. A wave like this, in which the particles of the medium vibrate in the same direction as the wave travels, is called a longitudinal wave.

Sound is a longitudinal wave. It travels from place to place through the particles of solids, liquids and gases, and the particles of the medium vibrate to and fro in the same direction as the sound travels. Sound spreads out in all directions around its source.

Process of propagation of sound wave

Sound needs a material medium: a solid, a liquid or a gas. It can’t travel through a vacuum (completely empty space).

In a solid, a liquid or a gas, the particles (molecules) are lined up next to one another. When a body vibrates, it pushes the molecules next to it; they push the next ones, and so on. As they push one another, the molecules move to and fro. Where they are pushed together, they are crowded: a compression (C). Where they move apart, they are spread out: a rarefaction (R). So sound travels as a series of compressions and rarefactions of the molecules of the medium.

Propagation of sound in different media

Sound needs a solid, liquid or gas to travel through, but its speed is different in each.

In a solid

In a solid, the molecules are very close together and held tightly to each other. When one is pushed, it pushes its neighbors almost at once. So the wave passes from molecule to molecule very quickly: sound travels fastest in solids.

In a liquid

In a liquid, the molecules are a little farther apart and less tightly held than in a solid. A pushed molecule takes a little longer to pass on the push. So sound travels slower in liquids than in solids.

In a gas

In a gas, the molecules are far apart from one another. A pushed molecule has a long way to go before it bumps into the next one. So sound travels slowest in gases, much slower than in solids and liquids.

Speed of sound: solid > liquid > gas.

Medium	Speed of sound (roughly)
steel	5200 m/s
water	1500 m/s
air	332 m/s at 0 °C (about 340 m/s on a warm day)

Does sound travel in a vacuum?

Sound needs a solid, liquid or gas to travel through. There is no air in the huge space between the sun and the Earth: it is a vacuum. So the sound of the giant explosions on the sun never reaches us, though its light and heat do. In the same way, there is no air on the Moon, so astronauts there can’t talk to each other directly, even standing side by side. They talk by radio, through special electronic sets in their helmets. Radio waves, like light, can travel through a vacuum.

1. Fill in the blanks with the right word

solid, vacuum, source of sound, sound, liquid

2. Choose the correct answer

3. Give reasons

4. Answer these questions