You write, read, play and run every day. Could you do all this without eating? Not for long. Could a bus, a tractor or a plane run without diesel or petrol? No. Could a phone, a radio or a television work without electricity? No again.
We get energy from our food. Buses and planes get their energy from fuels such as diesel and petrol. A phone, a fan or a computer gets its energy from electricity. Energy is what lets things do work: move, lift, push, heat up, light up or make a sound.
The capacity (ability) to do work is called energy. Energy is measured in joules (J). The joule is the SI unit of energy.
In this unit you will meet the different forms of energy and how one form changes into another (7.1), find out why some things soak up the sun’s heat faster than others (7.2), see that light travels in straight lines, makes shadows and works a camera with no lens (7.3), and find out how sound travels to your ears (7.4).
7.1 Forms of energy
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)
In which form is energy stored in the food you eat and in a battery?
Pick an answer to see why.
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.
Which of these has potential energy?
Pick an answer to see why.
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:
- 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.
- 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.
Which has the most kinetic energy?
Pick an answer to see why.
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.)
What energy change happens in an electric fan?
Pick an answer to see why.
Exercise (7.1 Forms of energy)
1. Fill in the blanks with the right word
kinetic energy, mechanical energy, energy, forest, potential energy
The capacity to do work is called ______.
Answer
The capacity to do work is called energy.
The energy a body has because of its motion or position is called ______.
Answer
The energy a body has because of its motion or position is called mechanical energy.
Water stored in a reservoir has ______.
Answer
Water stored in a reservoir has potential energy. (The word “forest” is not needed.)
A moving vehicle has ______.
Answer
A moving vehicle has kinetic energy.
2. Choose the correct answer
a. Which of these has kinetic energy?
Pick an answer to see why.
b. Which of these has potential energy?
Pick an answer to see why.
c. What is the SI unit of energy?
Pick an answer to see why.
d. Read these statements about energy: (1) Energy cannot be destroyed. (2) Energy cannot be created. (3) Energy cannot be changed from one form to another. Which is correct?
Pick an answer to see why.
3. Answer these questions
What is energy? Write its SI unit.
Answer
The capacity (ability) to do work is called energy. Its SI unit is the joule (J).
Define mechanical energy, and name its types.
Answer
The energy a body has because of its motion or its position is called mechanical energy. It is of two types: potential energy and kinetic energy.
Define kinetic energy and potential energy, with two examples of each.
Answer
Kinetic energy: the energy a body has because of its motion. Examples: a rolling ball, flowing water (also a flying bird, a moving bus).
Potential energy: the energy stored in a body because of its position or a change in its position or shape. Examples: water in a tank on a roof, a stretched catapult (also a squeezed spring, a raised brick).
On which factors does the kinetic energy of a body depend?
Answer
The kinetic energy of a body depends on:
- its mass: the heavier the body, the more kinetic energy at the same speed (a cricket ball has more than a tennis ball thrown at the same speed);
- its speed (velocity): the faster it moves, the more kinetic energy it has (a fast cricket ball has more than a slow one).
If a formula is asked for: kinetic energy = ½ × mass × (velocity)², or KE = ½ m v², where m is the mass and v the velocity. The velocity is squared (multiplied by itself), so twice the speed gives four times the kinetic energy.
Write any two differences between kinetic energy and potential energy.
Answer
| Kinetic energy | Potential energy |
|---|---|
| It is the energy a body has because of its motion. | It is the energy stored in a body because of its position or a change in its position or shape. |
| A body at rest has no kinetic energy. | A body at rest can have potential energy (water in a tank). |
| It depends on the mass and the speed of the body. | It depends on the mass and height of the body, or how much its shape is changed. |
| Examples: a rolling ball, flowing water. | Examples: a stretched catapult, water in a dam. |
What is transformation of energy? What difficulties would we face if energy could not be transformed?
Answer
The change of energy from one form into another is called transformation of energy. For example, a bulb changes electrical energy into light energy.
If energy could not be transformed:
- the electricity from our power stations could not light bulbs, run fans or cook food;
- the energy in food could not move our muscles or keep us warm, so we could not live;
- fuel could not run vehicles, and solar panels and hydropower plants could not make electricity;
- we would have only the forms of energy that happen to be around us, never the form we need.
“The food we eat is also a form of the sun’s energy, transformed.” Explain this statement.
Answer
Green plants take in light energy from the sun and use it to make food in their leaves (photosynthesis). The light energy is changed into chemical energy and stored in the food: in grains, fruit, vegetables and leaves. We eat these plants, or animals that ate plants. So the chemical energy in all our food first came from sunlight that a plant transformed. The statement is true.
We can work for a short time without eating, but not for a long time. Explain both in terms of energy.
Answer
Our body stores some energy from the food we ate before, as chemical energy (in sugar in the blood and liver, and in fat). For a short time, the body uses this stored energy, so we can keep working without eating.
But the store is small. When it runs low, the body has no energy left for the muscles: we feel weak and tired and cannot work. To keep working, we must eat again to get new energy.
7.2 Heat
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.
You hold a cold glass of water. Which way does heat flow?
Pick an answer to see why.
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.
Which would warm up fastest in the sun?
Pick an answer to see why.
Exercise (7.2 Heat)
1. Fill in the blanks with the right word
less, energy, joule, negligible, more
The SI unit of heat is ______.
Answer
The SI unit of heat is the joule.
Heat is a form of ______.
Answer
Heat is a form of energy.
A larger surface area absorbs ______ heat.
Answer
A larger surface area absorbs more heat.
White objects absorb ______ heat.
Answer
White objects absorb less heat. (“Negligible” is not needed: white things still absorb a little heat.)
2. Write true or false
When we touch an object hotter than our skin, heat flows from the hot object into the skin.
Answer
True.
When we touch an object colder than our skin, heat flows from the cold object into the skin.
Answer
False. Heat flows from the skin into the colder object. That is why it feels cold.
The heat absorbed decreases when the surface area increases.
Answer
False. A larger surface area absorbs more heat.
A black object absorbs less heat.
Answer
False. A black object absorbs more heat than objects of other colors.
3. Choose the correct answer
a. Which is the correct way heat flows?
Pick an answer to see why.
b. Which color absorbs the most heat?
Pick an answer to see why.
c. Why do we feel very hot in black clothes on a strong sunny day?
Pick an answer to see why.
d. Why do we spread out wet or washed clothes in the sun?
Pick an answer to see why.
4. Give reasons
Bedding is spread out in the sun in winter.
Answer
Spread out, the quilts and mattresses have a large surface facing the sun, so they absorb more heat. They become warm and dry, and stay warm for a while at night. (The sunlight also helps kill some germs and dries out damp.)
White clothes are more comfortable than black ones in summer.
Answer
White clothes reflect most of the sunlight and absorb little heat, so they stay cooler. Black clothes absorb a lot of heat and make us feel hotter.
5. Answer these questions
Write the importance of heat in our daily life.
Answer
- We need heat to cook our food and to boil water.
- Heat keeps our bodies and homes warm in winter.
- The sun’s heat dries clothes, grains, and wet things.
- Heat is used to iron clothes and to melt and shape metals (by blacksmiths and goldsmiths).
- Heat makes steam to run some engines, and it bakes bricks and pots.
What is heat absorption? On which factors does it depend?
Answer
The process in which a body takes in heat from hotter things around it is called heat absorption.
It depends mainly on:
- the surface area of the object: a larger surface absorbs more heat;
- the color of the object: a black object absorbs more heat than a white one.
What is the relation between the heat absorbed and the color of an object? Give an example you have seen or know.
Answer
Dark objects absorb more heat and light ones less: black absorbs the most and white the least.
Example: in the sun, a black umbrella, a black car or a black bag gets much hotter than a white one. Solar water heater panels are painted black so that they absorb more of the sun’s heat.
Explain an example from daily life which shows that more surface area absorbs more heat.
Answer
Washed clothes are spread out on a line, not left in a heap. Spread out, a large surface of the cloth faces the sun, so it absorbs more heat, and the water in it dries quickly. A heap of wet clothes, with only its top facing the sun, stays wet for a long time. (Grains spread on a biskun, and fingers spread out at a fire, are other examples.)
What color of clothes would keep you cool in summer, and what color warm in winter?
Answer
In summer, white or light-colored clothes keep you cool, because they reflect most of the sunlight and absorb little heat.
In winter, black or dark clothes keep you warmer when you sit in the sun, because they absorb more of its heat.
7.3 Light
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.
Why can’t you see a candle through a bent pipe?
Pick an answer to see why.
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.
What is the smallest part of a beam of light called?
Pick an answer to see why.
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.
What kind of image does a pinhole camera make?
Pick an answer to see why.
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.
Which of these makes the darkest shadow?
Pick an answer to see why.
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.
Exercise (7.3 Light)
1. Fill in the blanks with the right word
ray, straight, shadow, screen, bending
Light travels only in a ______ line.
Answer
Light travels only in a straight line.
The smallest part of a beam of light is called a ______.
Answer
The smallest part of a beam of light is called a ray.
Light can’t pass through an opaque object, so its ______ is seen behind it.
Answer
Light can’t pass through an opaque object, so its shadow is seen behind it.
In a pinhole camera, the tracing paper works as a ______.
Answer
In a pinhole camera, the tracing paper works as a screen. (The word “bending” is not needed.)
2. Choose the correct answer
a. How does light travel?
Pick an answer to see why.
b. Which of these is not needed to form a shadow?
Pick an answer to see why.
c. What is the smallest part of a beam of light called?
Pick an answer to see why.
d. What kind of image is seen in a pinhole camera?
Pick an answer to see why.
e. How can the size of a shadow be increased? i. by moving the source closer to the object; ii. by moving the object closer to the source; iii. by moving the screen closer to the object.
Pick an answer to see why.
3. Write the differences
A ray of light and a beam of light
Answer
| Ray of light | Beam of light |
|---|---|
| It is the smallest part of a beam: a single line of light. | It is a stream of many rays together. |
| It is drawn as one straight line with an arrow. | It is drawn as many lines with arrows, side by side, spreading out or coming together. |
| Example: the line drawn to show light going through a pinhole. | Example: the light from a torch or a vehicle’s headlight. |
Umbra and penumbra
Answer
| Umbra | Penumbra |
|---|---|
| It is the dark part of a shadow. | It is the paler part of a shadow, round the umbra. |
| Light from the source is blocked completely. | Light from the source is blocked only partly. |
| No light from the source reaches it. | Light from part of the source reaches it. |
4. Answer these questions
What is a shadow? How many types of shadow are there? Why is a shadow’s length different in the morning, at midday and in the evening?
Answer
A shadow is the dark area formed behind an opaque object when it blocks the light from a source.
There are two types: the umbra (light blocked completely) and the penumbra (light blocked partly).
In the morning and evening the sun is low in the sky. Its light comes in at a slant, so an object blocks light over a long stretch of ground, and its shadow is long. At midday the sun is high. Its light comes down steeply, so the shadow is short. (Because the sun is in the east in the morning and the west in the evening, the shadow also points west in the morning and east in the evening.)
Light travels in a straight line. Explain this with a suitable diagram.
Answer
Stand three cards with small holes in the middle of each on a table, with a burning candle at one end (Figure 7.14). When the three holes are in one straight line, you can see the flame through them. When one card is moved a little to the side, you can’t: its solid part is in the way, and the light can’t bend round it to the next hole. So light travels in a straight line. (A straight pipe and a bent pipe show the same thing.)
Can we make a camera without a lens? What is it called?
Answer
Yes. A camera without a lens is called a pinhole camera. It is a closed box or tube with a tiny pinhole in one end and a screen of tracing paper at the other. Light from an object passes through the pinhole in straight lines and makes an inverted image on the screen.
If light travels in straight lines, why is the image in a pinhole camera upside down?
Answer
It is upside down because light travels in straight lines. A ray from the top of the object travels in a straight line down through the pinhole and lands at the bottom of the screen. A ray from the bottom of the object travels straight up through the pinhole and lands at the top of the screen. The rays cross at the hole, so top and bottom swap, and the image is inverted (Figure 7.15). If light could bend, the rays would not cross neatly, and no clear image would form.
When a bulb is switched on, the light spreads out in all directions. How is this possible if light travels in straight lines? Give your opinion.
Answer
A bulb sends out light from every point of its glowing part, in every direction at once. Each ray goes out in a straight line, but the rays start out in all different directions: upwards, downwards, sideways, in front and behind. So the straight rays spread out like the spokes of a wheel and light up the whole room. Light spreading in all directions doesn’t mean it bends; it means straight rays leave in many directions.
Complete the chart by filling in the boxes.
Light, with five boxes: 1. How does a shadow form? 2. What is it? 3. Why do we need it? 4. In what kind of line does it travel? 5. Which activity proves it?
Answer
- How does a shadow form? When an opaque object blocks light, a dark shadow forms behind it.
- What is it? Light is a form of energy.
- Why do we need it? To see things around us.
- In what kind of line does it travel? In a straight line.
- Which activity proves it? Looking at a candle through a straight pipe and a bent pipe, or through three cards with holes in a line.
7.4 Sound
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.
What is needed to produce sound?
Pick an answer to see why.
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.
In a sound wave, how do the particles of the air move?
Pick an answer to see why.
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.
Two astronauts stand next to each other on the Moon. Why must they use radios to talk?
Pick an answer to see why.
Exercise (7.4 Sound)
1. Fill in the blanks with the right word
solid, vacuum, source of sound, sound, liquid
When a body vibrates, ______ is produced.
Answer
When a body vibrates, sound is produced.
Sound travels fastest in a ______ medium.
Answer
Sound travels fastest in a solid medium.
Sound can’t travel through a ______.
Answer
Sound can’t travel through a vacuum.
An object that produces sound is called a ______.
Answer
An object that produces sound is called a source of sound. (The word “liquid” is not needed.)
2. Choose the correct answer
a. In which medium does sound travel fastest?
Pick an answer to see why.
b. What is the speed of sound in air?
Pick an answer to see why.
c. What is an object that produces sound called?
Pick an answer to see why.
d. Which order of the speed of sound is correct?
Pick an answer to see why.
3. Give reasons
On the Moon, an electronic device (a radio) is needed to talk with a friend.
Answer
Sound needs a medium (a solid, liquid or gas) to travel through. There is no air on the Moon, so the sound of a voice can’t travel from one person to another, even close by. A radio sends the message as radio waves, which can travel through a vacuum, so astronauts must use one.
The speed of sound is highest in solids.
Answer
In a solid, the molecules are very close together and tightly held. When one molecule is pushed, it passes the push on to the next one almost at once, so the sound wave travels very quickly from molecule to molecule.
The speed of sound is lowest in gases.
Answer
In a gas, the molecules are far apart. A pushed molecule has to travel a long way before it bumps into the next, so the push is passed on slowly, and sound travels more slowly than in solids and liquids.
4. Answer these questions
What is a wave? How is it produced?
Answer
When an object vibrates, the process in which its energy is passed on through a medium is called a wave. A wave carries energy from one place to another without carrying the medium along.
A wave is produced by a vibrating (or disturbed) object. For example, a stone dropped in a pond makes ripples, a hand jerking a rope makes a wave run along it, and a vibrating plate makes sound waves in the air.
Explain, with a diagram, how a sound wave is produced and how it travels.
Answer
How it is produced: a sound wave is produced by a vibrating body, such as a plate struck with a spoon, a drum skin or a loudspeaker. As it vibrates, it pushes the air next to it forward and then pulls back, again and again.
How it travels: each push squeezes the air molecules together into a compression (C); each pull-back leaves them spread out in a rarefaction (R). Each molecule pushes the next one and moves back, so the compressions and rarefactions travel outwards through the medium to our ears (Figure 7.21). The molecules only vibrate to and fro, in the same direction as the sound travels: sound is a longitudinal wave. It needs a medium (a solid, liquid or gas) and can’t travel through a vacuum.
What are the similarities and differences when sound travels through solids, liquids and gases?
Answer
Similarities:
- In all three, sound travels as a longitudinal wave, as compressions and rarefactions.
- In all three, the molecules of the medium vibrate to and fro and pass the push on to their neighbors; the medium itself doesn’t travel.
Differences:
| Solid | Liquid | Gas |
|---|---|---|
| molecules very close together | molecules a little farther apart | molecules far apart |
| sound travels fastest (steel about 5200 m/s) | sound travels more slowly (water about 1500 m/s) | sound travels slowest (air about 332 m/s) |
We see the lightning in the sky first and hear the thunder later. Why?
Answer
The lightning and the thunder happen at the same time, but light travels much faster than sound. Light travels about 3 lakh km in a second, so the flash reaches us almost at once. Sound travels only about 332–340 m in a second in air, so the thunder takes several seconds to reach us. That is why we hear it later.
You put your ear to one end of the longest wall of your classroom, and a friend scratches the other end of the wall gently with a nail. Then you try to hear the scratch without your ear on the wall. In which case do you hear the sound, and why? Explain in terms of how sound travels.
Answer
You hear the scratch clearly with your ear on the wall, but hardly at all (or not at all) without it.
With your ear on the wall, the sound travels to you through the solid wall. In a solid, the molecules are close together, so sound travels fast and loses very little of its energy on the way.
Without your ear on the wall, the sound must travel through the air. In a gas, the molecules are far apart, and the faint sound spreads out in all directions and loses its energy, so very little reaches your ear.
Complete the chart by filling in the boxes.
Sound, with four boxes: 1. What is it? 2. Sources 3. Media it travels through 4. How it travels
Answer
- What is it? A form of energy produced by vibrating bodies.
- Sources: madal, guitar, bell, bansuri (flute), our vocal cords.
- Media it travels through: solids, liquids and gases (not a vacuum).
- How it travels: as a longitudinal wave, by compressions and rarefactions of the molecules of the medium.
Worksheet: Energy in Daily Life: forms of energy and heat (PDF, 4 pages)
Worksheet: Energy in Daily Life: light and sound (PDF, 3 pages)
Practice every question in the book, and keep track of the ones you miss →