Energy in Daily Life

Look at the pictures of a dam spilling water, the Sun, a solar panel, a car being filled with petrol, and glowing coal, and think about these questions.

1. What types of energy do you see?
2. Can one type of energy do every job?
3. Where does each type of energy come from?

You read, write, play, carry loads and work in the fields. You can’t keep doing any of it for long without food. A bus doesn’t move without diesel, and a fan doesn’t turn without electricity. In each case something gives the ability to do work, and that ability is called energy.

Energy is the capacity (ability) to do work. Its SI unit is the joule (J). An older unit, still widely used for food and heat, is the calorie (cal), the unit of energy in the CGS system:

$$1 \text{ calorie} \approx 4.2 \text{ joules}$$

Energy comes in many forms: mechanical energy, heat energy, light energy, sound energy, magnetic energy, electrical energy, chemical energy and nuclear energy. The places we get these forms of energy from are called sources of energy.

Sources of energy are of two kinds.

• Non-renewable sources can’t be replaced quickly once they are used up: coal, petroleum products (petrol, diesel, kerosene, LPG) and nuclear fuel. Coal and oil took millions of years to form.
• Renewable sources don’t run out however much we use them, because nature keeps renewing them: solar energy, wind energy, the energy of flowing water and of the tides.

Look at these everyday scenes: a frying pan on a gas flame, a pot of water on a stove, hands held out to a fire, clothes drying on a line, and warm air rising over a valley in the sun. How does the heat travel in each one? Does it travel the same way every time?

Heat is a form of energy that gives us the feeling of warmth. Touch something hotter than your body, and heat flows from it into your skin, so it feels hot. Touch something colder, and heat flows from your skin into it, so it feels cold. That is why ice feels cold: it is taking heat out of your hand.

Heat always flows from a hotter body to a colder one, from a higher temperature to a lower temperature, never the other way by itself. The flow of heat energy from one object or place to another is called the transmission of heat (or heat transfer).

Heat travels in three ways, and the rest of this part is about each of them.

Activity 7.1: pins that drop one by one

The rod itself doesn’t move, and none of its particles travel from one end to the other. So how does the heat get along? A solid is made of tiny particles (molecules or atoms) held in place, each vibrating a little about its own fixed position. Heating the end of the rod makes the particles there vibrate harder. They knock into their neighbors and make them vibrate harder, those knock into the next ones, and so the extra vibration, which is heat, is passed along the rod, particle to particle, like a message passed down a line.

Conduction is the transfer of heat through a solid from particle to particle, without the particles moving from their places. It is how heat travels in solids.

Conductors and insulators

Activity 7.2: which things let heat through?

Materials that let heat pass through them easily are good conductors of heat: iron, copper, aluminium, silver, gold and other metals. Materials that hardly let heat pass through them are insulators of heat (bad or poor conductors): wood, plastic, rubber, cloth, wool, paper, porcelain and air.

Thermal conductivity

Some materials conduct heat faster than others. Thermal conductivity tells how quickly or slowly a material conducts heat. A material with a high thermal conductivity carries heat fast; one with a low thermal conductivity carries it slowly. It is a property of the material itself: it depends on what the material is made of (its molecules and how they are arranged), and a little on its temperature.

Material	Thermal conductivity (roughly, W per meter per °C)
diamond	2,000
copper	400
aluminium	240
iron	80
stainless steel	16
glass	1
water	0.6
wood	0.1 to 0.2
wool, cotton wool	0.04
still air	0.025

Most metals have a high thermal conductivity, so they heat up quickly near a flame. That is why cooking pots are made of them. Plastic, cotton, wool, rubber and air have a very low thermal conductivity, and that is useful too: warm clothes keep your body heat in.

Not every metal is equally good: copper conducts about 25 times better than stainless steel. And not every non-metal is poor: diamond conducts heat about five times better than copper, and graphite (the “lead” of a pencil) conducts heat well along its layers.

Uses of conduction in daily life

Why are cooking pots made of copper, steel, iron or aluminium, while their handles are wood or plastic? Why does a jacket keep you warm? Why is a mud house cool in summer and warm in winter? Why do birds look fat and fluffy on very cold days? All of these depend on how well things conduct heat.

1. Cooking pots and kettles are made of metal, which carries heat easily from the flame to the food.
2. The handle of a kettle is made of plastic, a poor conductor, so heat does not reach your hand.
3. Woolen clothes and blankets keep you warm in winter. Wool is a poor conductor, and the air trapped between its fibers, and between the layers of your clothes, is an even poorer one. So your body heat escapes very slowly. Several thin layers trap more air than one thick layer.
4. Teacups and coffee mugs are made of porcelain, an insulator, so the tea stays hot longer and the cup is comfortable to hold.

Activity 7.3: colored water on the move

Here is why the water moves. Water at the bottom of the beaker is heated by the flame. Warm water expands, so it becomes less dense (lighter for its size) than the cooler water above it. The lighter warm water rises, just as a cork rises in water, and the heavier cool water sinks to take its place at the bottom. There it is heated in turn, and rises too. The moving water carries the heat with it, and this goes on until all the water is hot.

Convection is the transfer of heat in a liquid or a gas by the actual movement of its particles (molecules) from a hotter place to a colder place. The loop of moving liquid or gas is a convection current.

Notice the difference from conduction: in conduction, the particles stay in place and pass the heat on; in convection, the particles themselves carry the heat as they move. Liquids and gases (together called fluids) can flow, so they mostly carry heat by convection. Solids can’t flow, so convection can’t happen in a solid.

Convection happens in air in just the same way. Air over a hot surface is heated, becomes less dense and rises; cooler air flows in to take its place.

Wind

During the day, the Sun heats the ground, and the ground heats the air just above it. The warm air expands, becomes lighter and rises, leaving behind air at a lower pressure. Cooler air from nearby flows in to take its place. Moving air is wind, and convection is the main cause of wind.

Sea breeze and land breeze

On a coast, the land and the sea heat up and cool down at different speeds. Water has a much higher specific heat capacity than land: it needs much more heat to warm up by each degree, and gives out much more heat as it cools by each degree. So the sea warms up slowly in the day and cools down slowly at night, while the land changes temperature quickly.

Sea breeze (by day). The Sun heats the land and the sea, but the land gets hotter faster. The air over the land is heated, expands and rises, making an area of low pressure over the land. The air over the cooler sea is denser, at a higher pressure. Air flows from high pressure to low, so cool air blows from the sea to the land. This daytime wind is the sea breeze.

Land breeze (by night). After sunset, the land cools quickly, but the sea stays warm. Now the warm air over the sea rises, making low pressure there, and cool air blows from the land to the sea. This night-time wind is the land breeze.

Natural and forced convection

There are two types of convection.

1. Free or natural convection: the fluid moves by itself, only because the hotter parts are less dense and rise. The air next to a hot object is heated, becomes less dense and rises, and cooler air takes its place: a natural convection current. Water boiling in a pot goes round in this way: warm water rises from the bottom, spreads and cools at the top, and sinks again. Sea breezes, land breezes and the clouds that bring rain are all caused by natural convection.
2. Forced convection: something pushes the fluid along, a fan, a pump or a blower, using outside energy such as electricity. An electric fan, a room heater with a fan, a hair dryer, an air conditioner, and a pump pushing hot water through pipes are examples. Your heart does it too: it pumps warm blood from the warm center of your body out to your hands and feet.

Convection currents and weather

Convection helps us understand the weather and forecast it. The process starts every sunny morning. The Sun heats the ground, and the ground heats the layer of air just above it. Bare surfaces like sand, rock and roads heat up faster than water or ground covered with plants. So the air above different places is heated unevenly. The air above a hot, bare patch becomes warmer and less dense than the air around it, and starts to rise: a rising current of warm air called a thermal. (Eagles and vultures circle in thermals to climb without flapping their wings.)

As the thermal rises, it carries heat and moisture upwards. The hotter the ground, the stronger the convection, which is why it is strongest on very hot days. High up, the rising air cools. If it holds a lot of moisture, clouds form; with strong convection they grow into tall thunderclouds, with lightning, thunder and heavy rain.

Convection also carries extra heat away from the Earth’s surface up into the air. Without it, the ground would be far hotter: by some estimates around 50 to 60 °C (125 to 140 °F), instead of the Earth’s present average surface temperature of about 15 °C (59 °F).

Uses of convection in daily life

Have you seen a hot air balloon rising, or steam curling up from a cup of tea? Both are convection: the transfer of heat from a hotter place to a colder place through a liquid or a gas by the movement of its particles. When a liquid or gas is heated, its particles move faster and spread farther apart, so it becomes less dense and rises.

1. Water and other liquids are heated mainly by convection when they boil in a pot.
2. In warm-blooded animals (mammals and birds), blood carries heat from the warm center of the body to the skin, pumped by the heart (forced convection).
3. Electric fans and air conditioners move air around a room (forced convection).
4. Room heaters and the cooling part of a refrigerator work by setting up convection currents. In many fridges the freezer is at the top: the air it cools sinks through the fridge, and the warmer air rises to be cooled.
5. Convection causes winds, clouds, rain, thunderstorms and lightning.

Why do you feel warm when you sit in the sun, or near a heater? Between the Sun and the Earth, above our air, is almost empty space, with nothing in it to conduct heat, and no air to carry it by convection. (And convection only carries heat upwards, as warm air rises; it could never bring the Sun’s heat down to the ground.) Yet the Sun’s heat reaches us every day. It travels by a third way, which needs no material at all.

The Sun gives out heat and light, and they travel as waves. These waves don’t need any material to travel through, so they can cross empty space. The transfer of heat without any material medium is called radiation. In a vacuum (completely empty space), heat can travel only by radiation. It is also how you feel the warmth of a fire or a heater from across the room.

Activity 7.5: ripples in a pond

The ripples are water waves. The stone disturbs the water where it falls, the water particles there move up and down, and they pull their neighbors up and down a moment later, and so on outwards. So the wave spreads from the point of disturbance to the edge of the pond, but the water itself stays where it is: each particle only moves up and down about its own place. In the same way, if you tie one end of a rope to a tree and jerk the other end up and down, a wave runs along the rope, while each bit of rope only moves up and down. A wave is produced by vibration, and it carries energy from one place to another without carrying the material along.

Mechanical and electromagnetic waves

Some waves need a material to travel through, and some don’t.

• Mechanical waves need a material medium (a solid, liquid or gas) to travel. Sound is a mechanical wave: its energy is passed on by the vibration of the particles of the medium. The material that vibrates as the wave passes is called the medium. As a mechanical wave travels, the medium takes some of its energy, so the wave grows weaker and can only travel a limited distance. Its speed is different in different media: sound travels at about 332 m/s in air (at 0 °C), 1,481 m/s in water and 5,120 m/s in iron.
• Electromagnetic waves don’t need a medium; they can travel through a vacuum. Light, infrared (the heat rays we can’t see), radio waves, microwaves, ultraviolet rays, X-rays and gamma rays are electromagnetic waves. Crossing empty space, they don’t lose energy to any medium. All of them travel through a vacuum at the speed of light, 3 × 10⁸ m/s (300,000 km every second).

Light from the Sun takes about 8 minutes and 20 seconds to reach the Earth. When you see the Sun, you are seeing it as it was 8 minutes ago.

Radiation is the transfer of energy as electromagnetic waves, without any material medium. Heat travels by radiation mainly as infrared rays. This is called thermal radiation, and it is how the Earth gets its heat from the Sun.

The color of an object and radiation

Activity 7.6: which box gets hottest?

Dark, dull surfaces are good absorbers of radiation: they take in most of the heat rays that fall on them. White, pale and shiny surfaces are poor absorbers: they reflect most of it. A surface that is a good absorber is also a good emitter: a hot black object gives out heat rays faster than a hot shiny one at the same temperature.

So:

• White or light-colored clothes are worn in summer and in hot places; dark clothes are better for winter and cold places.
• The inside of a solar water heater or a solar cooker box is painted black, to absorb as much of the Sun’s radiation as possible.
• Cooking pots are often black on the outside, to absorb more heat from the flame, cook faster and save fuel.

Uses of radiation in daily life

Why do a cup of tea and a plate of hot food go cold if you leave them? Anything hotter than its surroundings gives out more heat by radiation than it receives, so it cools down, until it reaches the same temperature as its surroundings: thermal equilibrium. Anything colder than its surroundings absorbs more radiation than it gives out, so it warms up. (The tea also loses heat by convection and evaporation.)

1. Sitting near a fire, your body receives heat from it by radiation.
2. The Sun’s radiation heats the land, the sea and the air.
3. The black grid of pipes (the condenser) at the back of a refrigerator gives off the heat taken from inside the fridge into the room, partly by radiation, which is why it is painted black, and partly by convection.
4. White or light-colored clothes are worn in summer because they absorb less of the Sun’s heat.

Activity 7.7: inside a thermos

A thermos flask (vacuum flask) keeps hot drinks hot and cold drinks cold for many hours. It does this by stopping all three ways that heat can travel: conduction, convection and radiation.

Part	What it is	How it stops heat
double glass walls	a glass vessel with two thin walls	glass is a poor conductor
vacuum	the space between the walls, with the air pumped out	with no particles in it, heat can’t cross by conduction or convection
silvered surfaces	the outer surface of the inner wall and the inner surface of the outer wall, shiny like a mirror	shiny surfaces are poor emitters and reflect heat rays back, so they reduce radiation
insulating stopper	a cork or plastic stopper in the neck	stops hot air escaping by convection, and stops evaporation
insulating support	cork or rubber pads holding the vessel	stop heat being conducted to the case, and protect the glass from knocks
outer case	metal or plastic	protects the fragile glass from damage

So heat can neither escape from nor enter the flask easily: hot liquids stay hot, and cold liquids stay cold. (A little heat still gets out, mostly by conduction through the stopper and the glass at the neck, which is why tea in a flask is warm, not hot, by the next day.)

Have you seen a greenhouse, or the long plastic tunnels farmers build over vegetables in the hills of Nepal? Why do they build them? And what would happen to the Earth if there were no greenhouse gases in its air?

A greenhouse is a house made of glass or clear plastic on a metal or bamboo frame. It is used mainly to grow off-season vegetables and plants, like tomatoes and cucumbers in winter, or seedlings early in spring.

Sunlight is mostly short-wave radiation (visible light), and it passes easily through glass and clear plastic. Inside, it is absorbed by the soil and the plants, which warm up. Warm things give out their own radiation, but it is long-wave infrared, and glass lets very little of it through (plastic sheet lets more through than glass, but still holds some back). So heat is trapped and the greenhouse warms up. The walls and roof also stop the warm air from rising and blowing away, which matters just as much. Inside, the temperature, light and moisture are kept just right for the plants to grow. The roof of a greenhouse is sloped so that as much sunlight as possible enters through it.

The natural greenhouse effect

The Earth’s air does something similar. The Sun’s short waves pass through the air and warm the ground and the sea. The warm Earth gives out long-wave infrared rays, day and night, back towards space. Some gases in the air, the greenhouse gases, absorb much of this infrared and send part of it back down to the ground. So the Earth’s surface stays warmer than it would otherwise be, warm enough for life.

The greenhouse effect is the warming of the Earth’s surface because greenhouse gases in the air absorb the heat (long-wave radiation) given out by the Earth, instead of letting it all escape into space. It works like a blanket round the Earth, and stops the surface from freezing.

The main greenhouse gases are carbon dioxide (CO₂), water vapor, methane (CH₄), nitrous oxide (N₂O) and chlorofluorocarbons (CFCs). People add more of them to the air by burning coal, oil and gas in factories and vehicles, by cutting down forests (trees take in CO₂), and from farming and rubbish dumps (methane).

Global warming

More factories and vehicles, and fewer forests, mean more greenhouse gases in the air. These trap more heat, so the average temperature of the Earth’s surface rises. This is global warming. In 2024, the warmest year on record (as of 2026), the Earth was about 1.5 °C warmer than in the late 1800s, and the warming has already led to more floods, droughts, heatwaves and other disasters.

Activity 7.8: a model greenhouse

Effects of an increased greenhouse effect

• a.It raises the average temperature of the Earth.
• b.It changes the pattern of the water cycle: rain comes at the wrong times, with more droughts and more floods.
• c.It harms human health: more heatwaves, and diseases spreading to new places, like dengue, carried by mosquitoes, now found in the hills of Nepal.
• d.It lowers farm production and the productivity of the land.
• e.Snow and glaciers in the mountains melt, and glacial lakes fill up. If a lake’s natural dam of rock and ice breaks, it causes a glacial lake outburst flood (GLOF).
• f.Sea levels rise as glaciers melt and warm sea water expands, so low-lying land near the coasts is at risk of going under water.
• g.It reduces biodiversity: plants and animals that can’t move or adapt fast enough die out.
• h.It causes desertification: dry land becomes desert.
• i.It upsets the balance of ecosystems.

Why the greenhouse effect is needed

A natural greenhouse effect is essential. It keeps the Earth’s temperature suitable for life. Without it, the heat given out by the Earth’s surface would escape straight into space, and the Earth’s average temperature would be about −18 °C instead of about 15 °C (some books round this to −20 °C). The oceans would freeze, and life as we know it would hardly be possible.

Artificial greenhouses are useful too. In them:

• off-season vegetables can be grown, which farmers sell at a good price;
• plants that would die in the cold can be kept alive;
• plants from hot places can be grown even in very cold countries.

The trouble comes only when the greenhouse effect becomes too strong: then it has the harmful effects listed above.

Look around: the Sun, a candle, a torch, a bulb, a laser pointer shining on a mirror, a girl looking into a mirror, a big round mirror at a bend in the road. Which of these give out light of their own? What is reflection? Do all objects reflect light in the same way? What kinds of mirrors can you find?

In the day, we see things around us by the light from the Sun. At night we can’t see them, until we switch on a light. Light is a form of energy that lets us see. Things that give out their own light, like the Sun, a candle flame and a glowing bulb, are luminous. We see everything else because it sends some of the light that falls on it into our eyes.

When light falls on an object, some of it is absorbed, some is reflected (sent back), and the rest may be transmitted (passed through it). The bouncing back of light after it strikes a surface is called the reflection of light.

• A smooth, flat, polished surface, like a mirror or still water, reflects light evenly in one direction, so it looks shiny, and you can see images in it. This is regular reflection.
• A rough surface, like paper, cloth or a wall, scatters the light in all directions, so it looks dull, and you see no image. This is diffuse (irregular) reflection.

When you stand in front of a mirror, light from you is reflected by the mirror into your eyes, and you see your image. A mirror is a smooth, polished surface that reflects the light from an object and forms an image of it where the reflected rays meet (or seem to meet). Mirrors are of two main types: plane mirrors (flat, like the one on your wall) and spherical mirrors (curved). This part is about the two kinds of spherical mirror, concave and convex, what images they make, and the difference between real and virtual images.

Activity 7.9: a spoon mirror

A surface that is sunk in (depressed) at the middle and raised at the edges (like the inside of a bowl) is a concave surface. A surface that is raised in the middle and curving away at the edges (like the back of a spoon) is a convex surface. So a spoon is two mirrors in one: concave in front, convex behind.

A spherical mirror is a mirror whose reflecting surface is part of a hollow sphere, a ball. There are two kinds.

Concave mirror

A concave mirror is a spherical mirror whose reflecting surface is curved inwards: the hollow side, sunk in at the middle, reflects. When rays of light parallel to the principal axis (the line through the middle of the mirror, below) fall on a concave mirror, they are reflected and meet at a point on the principal axis, called the principal focus. Because it brings rays together, a concave mirror is also called a converging mirror. When an object is very close to a concave mirror (nearer than its focus), the mirror forms an enlarged image of it, the right way up.

Convex mirror

A convex mirror is a spherical mirror whose reflecting surface bulges outwards: the middle is raised and the edges curve away. A convex mirror spreads out (diverges) the rays falling on it, so it is also called a diverging mirror. It always forms an image that is erect (the right way up) and diminished (smaller than the object). Because it shrinks big, far-away things into a small image, a convex mirror shows a much wider view than a plane mirror of the same size.

To talk about spherical mirrors, and to draw them, we need a few words. Imagine the mirror as a piece cut from a hollow glass ball, silvered on one side.

• a.Pole (P): the middle point of the reflecting surface of the mirror. All distances are measured from the pole.
• b.Center of curvature (C): the center of the hollow sphere that the mirror is a part of.
• c.Radius of curvature (R): the radius of that sphere: the distance from the center of curvature to the pole, CP.
• d.Principal axis: the imaginary straight line passing through the pole (P) and the center of curvature (C).
• e.Principal focus (F): the point on the principal axis where rays parallel to the principal axis meet after reflection from a concave mirror, or from where they seem to come after reflection from a convex mirror. The focus of a concave mirror is in front of the mirror, and the reflected rays really pass through it, so it is a real focus. The focus of a convex mirror is behind the mirror, and the rays only seem to come from it, so it is a virtual focus. The focus lies halfway between the pole and the center of curvature.
• f.Focal length (f): the distance from the pole (P) to the principal focus (F). Its SI unit is the meter (m).

Because F is halfway between P and C, the focal length is half the radius of curvature:

$$f = \frac{R}{2}$$

Rules for drawing rays

To find where a mirror forms an image, we draw a few rays from the top of the object. Three rays are easy to draw, because we know exactly where they go after reflection.

For a concave mirror:

1. A ray parallel to the principal axis passes through the principal focus F after reflection.
2. A ray passing through the principal focus F is reflected parallel to the principal axis.
3. A ray passing through the center of curvature C hits the mirror square on, and is reflected back along the same path.

For a convex mirror:

1. A ray parallel to the principal axis is reflected so that it seems to come from the focus F behind the mirror.
2. A ray heading towards the focus F (behind the mirror) is reflected parallel to the principal axis.
3. A ray heading towards the center of curvature C (behind the mirror) is reflected back along the same path.

Activity 7.11: catching an image on a card

An image that can be caught on a screen is a real image. It forms where the reflected rays actually meet. An image that can’t be caught on a screen is a virtual image. It forms where the reflected rays only seem to meet, when you trace them backwards behind the mirror; they never really meet there. Your image in a plane mirror is virtual: there is no light behind the mirror.

In general, a concave mirror can form a real image (except when the object is very close to it), while a plane mirror and a convex mirror always form virtual images.

Real image	Virtual image
Formed in front of the mirror.	Formed behind the mirror.
Can be obtained on a screen.	Can’t be obtained on a screen.
Formed where the reflected rays actually meet.	Formed where the reflected rays appear to meet, behind the mirror.
Inverted (upside down).	Erect (the right way up).
Formed by a concave mirror.	Formed by plane and convex mirrors, and by a concave mirror when the object is very close.

Activity 7.12: drawing a ray diagram

Images formed by a concave mirror

Where the image forms, how big it is, and whether it is real or virtual all depend on where the object is: its distance from the mirror compared with the mirror’s focal length. Here are the six positions.

1. Object at infinity (very far away): the image forms at the focus F. It is real, inverted and highly diminished (a point).
2. Object beyond C: the image forms between F and C. It is real, inverted and diminished.
3. Object at C: the image forms at C. It is real, inverted and the same size as the object.
4. Object between C and F: the image forms beyond C. It is real, inverted and magnified (bigger than the object).
5. Object at F: the reflected rays are parallel, so the image forms at infinity. It is real, inverted and highly magnified.
6. Object between F and P: the image forms behind the mirror. It is virtual, erect and magnified.

Position of the object	Position of the image	Nature and size of the image
at infinity	at F	real, inverted, highly diminished (a point)
beyond C	between F and C	real, inverted, diminished
at C	at C	real, inverted, same size
between C and F	beyond C	real, inverted, magnified
at F	at infinity	real, inverted, highly magnified
between F and P	behind the mirror	virtual, erect, magnified

Images formed by a convex mirror

Wherever an object is in front of a convex mirror, the image forms behind the mirror, and it is always virtual, erect and diminished.

1. Object at infinity: a point image forms at the focus F, behind the mirror. It is virtual, erect and highly diminished.
2. Object anywhere between infinity and P: the image forms behind the mirror, between P and F. It is virtual, erect and diminished. The closer the object, the bigger the image, but it is always smaller than the object.

Uses of spherical mirrors

Concave mirrors are used:

1. in torches, searchlights and vehicle headlights, with the bulb at the focus, to send light out in a nearly parallel beam that reaches far;
2. by doctors (ear, nose and throat doctors and dentists), to see a magnified view of the inside of the ear, nose, mouth or throat, and to shine light into it;
3. as shaving and make-up mirrors, held close (within the focal length) to give an enlarged, erect image of the face;
4. in reflecting telescopes, to collect the faint light of stars and planets;
5. in solar cookers: as a converging mirror, it gathers the Sun’s rays to a focus where the pot sits.

Convex mirrors are used:

1. as rear-view and side mirrors of vehicles, to see a wide view of the traffic behind;
2. in street lamps, to spread light over a wide area;
3. at blind bends on roads, and in shops and car parks, to see round corners.

You hear people talking, birds singing, temple bells, a guitar, a drum, the horn of a bus, the radio. Anything that produces sound is a source of sound. Sound is a form of energy that gives us the sensation of hearing, and it is produced when an object vibrates. The sound travels from its source through a material medium, a solid, a liquid or a gas, to our ears. Sound can’t travel through a vacuum, where there is no material at all.

Sounds are different. A guitar’s note is sharp and high; a drum’s is deep and low. Some sounds are loud, some faint. This part is about what a sound wave is, how fast it travels, the kinds of sound by frequency (infrasound, audible sound and ultrasound), how strong a sound is (its intensity), and the causes of noise pollution and how to reduce it.

Activity 7.13: feel a sound

How sound travels

When an object vibrates, it pushes the particles of air next to it back and forth. They push the next particles, and these push the next, so the vibration is passed on through the air as a sound wave, carrying energy from the source to your ear. Sound needs a material medium to travel through, so sound is a mechanical wave, and it can’t travel through a vacuum.

As a sound wave passes, each particle of the medium vibrates to and fro, along the direction the sound is traveling. A wave in which the particles of the medium vibrate along the direction in which the wave travels is called a longitudinal wave. So sound is a longitudinal wave. Where the particles are pushed close together, the air is squeezed: a compression. Where they are pulled apart, the air is stretched: a rarefaction.

A sound wave is described by four quantities: its frequency, time period, wavelength and amplitude.

Frequency

The number of complete waves (vibrations) produced in one second is called the frequency. It is written f and its SI unit is the hertz (Hz): 1 Hz is one wave per second. Bigger units are the kilohertz (1 kHz = 1,000 Hz), megahertz (1 MHz = 1,000,000 Hz) and gigahertz (1 GHz = 1,000,000,000 Hz). A high frequency sounds high-pitched (sharp, shrill); a low frequency sounds low-pitched (flat, deep).

Time period

The time taken to produce one complete wave is called the time period. It is written T and its SI unit is the second (s). If f waves are made in 1 second, then one wave takes 1/f seconds:

$$\text{time period} = \frac{1}{\text{frequency}} \qquad T = \frac{1}{f}$$

Wavelength

The distance a sound wave travels during one complete vibration is called its wavelength. It is also the distance between two neighboring compressions (or two neighboring rarefactions), or, on the graph, between two neighboring crests or troughs. It is written with the Greek letter λ (lambda) and its SI unit is the meter (m).

Amplitude

The largest distance the particles of the medium move from their resting (mean) position as the wave passes is called the amplitude. It is written A and its SI unit is the meter (m). A bigger amplitude means a louder sound.

The graph in Figure 7.23 shows the air pressure, not how far the particles move. The two go together: when the particles swing farther (a bigger amplitude), the air is squeezed harder in each compression, so the pressure crests are higher too. A louder sound has both a bigger particle movement and a bigger pressure swing.

Speed of a sound wave

The distance traveled by sound in one second is called the speed of sound. In a given medium (at a given temperature), the speed of sound is fixed, but it is different in different media: about 332 m/s in air at 0 °C, faster in water and much faster in solids like iron.

In one time period, the wave moves forward by one wavelength. In one second there are f waves, so the wave moves forward f wavelengths:

$$\text{speed} = \text{frequency} \times \text{wavelength} \qquad v = f \lambda$$

With the wavelength in meters and the frequency in hertz (per second), the speed comes out in m/s.

The speed of sound in a medium stays the same whatever its frequency or wavelength: high notes and low notes from a band reach you at the same moment. Since f × λ always equals the same v, if the frequency goes up, the wavelength must go down, and if the frequency goes down, the wavelength goes up. (They are inversely proportional.)

Different sources give out sounds of different frequencies, even in the same medium. Sound waves can have frequencies from less than 1 Hz to many millions of hertz. Children’s and most women’s voices have higher frequencies than men’s, so they sound sharper. As boys grow up, their vocal cords grow longer and thicker, and the voice “breaks” and becomes deep (dhodro). A man’s speaking voice is roughly 85–180 Hz; a woman’s, roughly 165–255 Hz.

We can’t hear every frequency. A healthy human ear can hear frequencies from about 20 Hz to 20,000 Hz (20 kHz). By frequency, sounds are sorted into three types.

a. Infrasonic sound (infrasound)

Sound with a frequency below 20 Hz is infrasound (or infrasonic sound; some books call it subsonic). Humans can’t hear it, but very strong infrasound can be felt as a shaking or pressure. It is produced by earthquakes, volcanic eruptions, storms and big waves. Elephants, whales and rhinoceroses make infrasonic calls; an elephant’s low rumble can be heard by other elephants several kilometers away.

b. Audible sound

Sound with a frequency from 20 Hz to 20 kHz is audible sound: humans can hear it. Speaking, singing, tuning a guitar, and music on the radio are examples.

c. Ultrasonic sound (ultrasound)

Sound with a frequency above 20 kHz is ultrasound (ultrasonic sound). Humans can’t hear it. Bats, dolphins, rats and mice, and some insects can make and hear it; dogs can hear it too (a dog whistle is ultrasonic). Because its wavelength is very short, ultrasound can be sent out in a narrow beam and reflects even from small objects, which makes it useful for finding things and for seeing inside the body.

Uses of ultrasound

Ultrasound has frequencies above 20 kHz, and so very short wavelengths. A beam of ultrasound spreads out very little, so it can be aimed precisely, and it bounces back (echoes) from the boundaries between different materials, even small ones. That makes it useful in many ways:

• Ultrasonography (USG): an ultrasound scan, to check how a baby is growing in the mother’s womb, and to find stones, tumors and diseases in the liver, kidneys, heart and other internal organs, without cutting the body and without harmful rays.
• Treatment without cutting: focused beams of strong ultrasound can heat and destroy some tumors, and break up kidney stones, without cutting the skin, so there is no bleeding.
• Cleaning and killing bacteria: in ultrasonic cleaners, ultrasound shakes dirt and germs off medical instruments, jewelry and machine parts.
• SONAR (sound navigation and ranging): to find the depth of the sea, and to find fish shoals, wrecks and submarines. A ship sends a pulse of ultrasound down to the sea bed and times its echo. Knowing the speed of sound in sea water, it calculates the depth:

$$\text{depth} = \frac{\text{speed of sound} \times \text{time for the echo}}{2}$$

(divided by 2 because the sound goes down and back up).

Sound waves carry energy. The amount of sound energy passing through a unit area (1 m²) every second is called the intensity of sound. A louder sound has a higher intensity.

The loudness of a sound, its sound level, is measured in decibels (dB). The faintest sound a healthy ear can hear is about 0 dB. The scale goes up in a special way: each 10 dB more means ten times the intensity, and sounds about twice as loud to us.

A person who can’t hear sounds quieter than about 45 to 60 dB has lost part of their hearing. Someone who can’t hear sounds even as loud as 80 dB is called deaf. Very intense sounds can damage hearing for ever.

Sound	Sound level (dB, roughly)
leaves rustling, quiet breathing	10–20
a whisper	20–30
a quiet library	30–40
normal conversation	50–60
busy traffic	70–85
a printing press, factory machines	80–90
a railway station with trains	85–110
a motorbike or a bus horn close by	100–115
a loud music concert, thunder overhead	110–120
threshold of pain: sound that hurts	120–140
a jet engine close by, firecrackers next to you	140–160

Measuring the intensity of sound

The intensity of sound at a point is the rate at which sound energy passes through a unit area, held square to the direction the sound is traveling. It is written I and its SI unit is the watt per square meter (W/m²):

$$\text{intensity} = \frac{\text{power of the sound}}{\text{area}} \qquad I = \frac{P}{A}$$

where P is the power of the sound (the sound energy passing per second, in watts) and A is the area (in m²).

What the intensity of sound depends on

1. Amplitude: the intensity is proportional to the square of the amplitude ($I \propto A^2$). Double the amplitude, and the intensity becomes four times as much.
2. Distance from the source: the intensity is greatest near the source and decreases as you move away. The sound spreads out over a bigger and bigger area, and as it travels through the air, some of its energy is absorbed by the air, some is reflected, and the rest goes on. So a sound grows fainter with distance.
3. Density of the medium: for the same vibration, sound is more intense in a denser medium. Cold air is denser than hot air, so the same vibration gives a slightly more intense sound in cold air than in hot air.
4. Area of the vibrating body: the larger the vibrating surface, the more air it pushes, and the more intense the sound: a big temple bell or a big drum is louder than a small one.
5. Frequency: for the same amplitude, a higher frequency gives a higher intensity, so a sharp, shrill sound carries more energy than a deep one of the same amplitude.

Sound pollution (noise pollution) is unwanted or too much sound that harms the health of people and animals and the quality of the environment. Noise is any unwanted sound that is unpleasant, too loud or disturbing. Noise in the environment causes sound pollution.

Causes of sound pollution

• Loud and unnecessary sounds in markets, crowded towns and heavy traffic, especially vehicle horns.
• Construction sites: drills, hammers, mixers and generators.
• Televisions, radios and music played very loudly, and loudspeakers at events.
• Machines running in factories and workshops.
• Crowds shouting in markets, and announcements over loudspeakers.

Effects of sound pollution

• It weakens hearing, and loud noise can burst the eardrum and cause deafness.
• It raises blood pressure, and can lead to heart disease.
• It causes headaches, stress, lack of sleep, and problems in digestion.
• It makes it hard to concentrate on reading, writing or any other work; children in noisy schools learn more slowly.
• It disturbs animals and birds, which use sound to find mates, food and each other.

Ways to reduce noise pollution

• Plant trees along roads and around factories and industrial areas: leaves and branches absorb and scatter sound.
• Fit silencers on vehicles and keep engines in good order; don’t use the horn without need.
• Play music and the television softly, and don’t disturb others while practicing an instrument or listening to music.
• Keep noisy factories away from homes, schools and hospitals, and give workers ear protection.
• Run awareness programs in the community about the harm noise does.

7.1 Heat

1. Choose the best answer

2. Differentiate between

3. Give reasons

4. Answer these questions

7.2 Light

1. Choose the best answer

2. Differentiate between

3. Define these terms

4. Give reasons

5. Answer these questions

7.3 Sound

1. Choose the best answer

2. Differentiate between

3. Answer these questions

4. Solve these numerical problems