Exam practice

non-renewable

lakhs (in fact millions, that is many tens of lakhs, of years)

carbon dioxide

saving

alternative (the word “renewable” is not needed)

A shortage of energy, caused by using too much non-renewable energy, is called an energy crisis.
Two efforts (any two, from what you see around you):

1. Using alternative energy: families build gobar gas plants and fit solar panels and solar water heaters, often with a subsidy from the government, so they use less LP gas and kerosene.
2. Using hydroelectricity instead of fuels: more people cook on induction stoves and ride electric scooters, buses and cars, which run on Nepal’s own electricity instead of imported LP gas and petrol.
3. Saving energy: people switch to LED bulbs and switch off lights and machines that are not in use.

Hydroelectricity is the best for the country as a whole (with solar energy and gobar gas for remote villages). Reasons:

1. Nepal has many rivers flowing fast down steep mountains, fed all year by rain and the Himalayan snow, so it can make a lot of hydroelectricity.
2. It is renewable and clean: it gives no smoke.
3. It is Nepal’s own: using it for cooking and transport means buying less LP gas, petrol and diesel from abroad.
4. Extra electricity can even be sold to other countries.

(Answers that choose solar energy or gobar gas are also right, with reasons: solar for remote hilly places where power lines are hard to bring; gobar gas because most families farm and keep cattle.)

To use energy economically means to use only as much as we need, without waste. Every unit of energy we don’t waste is a unit saved: it stays available for later, and less fuel has to be burnt or bought. For example, switching off a fan in an empty room, cooking with a lid on, or using an LED bulb gets the same work done with less energy. So using energy economically is the same as saving (conserving) it. It also saves money, makes non-renewable fuels last longer, and causes less pollution.

Advantages:

1. Easy to get compared with many other sources.
2. Easy to carry from one place to another.
3. Have many uses: they run vehicles and factories and cook food.
4. Give a very large amount of heat when burnt.

Disadvantages:

1. They are non-renewable and may run out because of continuous and heavy use.
2. Burning them pollutes the environment (smoke, harmful gases, carbon dioxide).

Hydroelectricity (and solar energy) could be the best alternative in Nepal, because they are renewable and give no smoke. Suggestions:

1. Make more hydroelectricity, and use it for cooking (induction and rice cookers) and for electric vehicles.
2. Fit solar panels on roofs and use solar water heaters.
3. Build gobar gas plants in villages instead of burning firewood and kerosene.
4. Use public transport, cycle or walk for short trips.

Hydroelectricity can be used in place of non-renewable fuels: electric stoves instead of LP gas, electric vehicles instead of petrol and diesel vehicles, electric lights instead of kerosene lamps. Since it is used to reduce the use of non-renewable energy, it is an alternative source of energy. It is also renewable, because rain and snow keep the rivers flowing.

The heat energy that comes from inside the Earth is called geothermal energy. Deep under the ground the rocks are very hot; in some places this heat warms underground water, which comes up as hot springs (tatopani), as in Myagdi. Some countries use it to make electricity and to heat houses. It is a renewable source.

Coal formed from the remains of plants that were buried millions of years ago and changed by great pressure and heat under the ground. The remains of living things from long ago are fossils, so coal is a fossil fuel.

Petrol, diesel and kerosene are made from crude oil, which took millions of years to form from buried sea creatures. Once we burn them they are gone, and nature can’t make more in a short time, so they are non-renewable.

Heat is a form of energy which gives us the feeling of warmth. Because of heat, the temperature of an object may increase or decrease. When an object is heated, its temperature increases. A thermometer is used to measure body temperature. Celsius, Fahrenheit and kelvin are the scales of temperature. Normal human body temperature is 37 °C. Heat flows from a hotter object to a colder object. At sea level, water freezes at 0 °C and boils at 100 °C.

Heat is a form of energy that gives us the feeling of warmth. It flows from a hotter object to a colder one. Its SI unit is the joule (J).

Draw a long glass tube with a bulb at one end, and label: the bulb (thin glass wall), the mercury or alcohol in it, the capillary tube running up the middle, the stem (thick glass), and the scale from −10 °C to 110 °C, as in the figure.

In winter, the water cools down to the low temperature of its surroundings, while our body stays at about 37 °C. Heat always flows from a hotter object to a colder one, so when we touch the water, heat flows from our hand into the water. Our hand loses heat, and we feel cold.

1. It is safer: it has no thin glass that can break and no poisonous mercury that can spill.
2. It is easier and quicker to read accurately: it shows the temperature as numbers on a screen, often beeps when ready, and gives the reading in less time.

$$\frac{C - 0}{100} = \frac{F - 32}{180}$$
or, $\frac{50}{100} = \frac{F - 32}{180}$
or, $F - 32 = \frac{50 \times 180}{100} = 90$
or, F = 90 + 32 = 122 °F

$$\frac{C - 0}{100} = \frac{K - 273}{100}$$
or, 27 = K − 273
or, K = 27 + 273 = 300 K

$$\frac{C - 0}{100} = \frac{K - 273}{100}$$
or, C = K − 273 = 100 − 273 = −173 °C (far colder than anything on the Earth’s surface)

vibration

solid

meter

medium (the word “air” is not needed: sound can travel through solids and liquids too)

frequency

Sound energy is the form of energy that we hear. It is produced when an object vibrates, and it travels through a medium as a longitudinal wave.
Sources (any four): a drum (madal), a guitar or sarangi, a radio or television, a bell, a human voice (the vocal cords).

When we talk, our vocal cords in the throat vibrate. Each time they move forward, they push the air molecules in front of them closer together, making a compression. Each time they move back, they leave a space where the molecules are farther apart, making a rarefaction. The air molecules pass this push and pull on to the molecules next to them, so compressions and rarefactions travel outwards through the air, one after another, as a longitudinal wave. Each molecule only vibrates back and forth in its place. When the wave reaches the listener’s ear, it makes the eardrum vibrate, and the listener hears the sound.

Hitting the drum harder gives its skin more energy, so it moves farther up and down from its rest position. It pushes the air harder, so the air molecules are pushed farther from their mean positions: the amplitude is larger. A larger amplitude means more energy, which we hear as a louder sound.

Draw a wavy line across a straight middle line (the mean position). Mark a high point as the crest and a low point as the trough. Draw a two-headed arrow from one crest to the next and label it wavelength (λ), and a short arrow from the middle line up to the top of a crest and label it amplitude (a), as in the figure.

Compression:

1. A region where the molecules of the medium are crowded close together.
2. The pressure and density of the air there are a little higher than normal.

Rarefaction:

1. A region where the molecules are spread farther apart.
2. The pressure and density there are a little lower than normal.

How they are related: compressions and rarefactions always come one after another, alternately, and travel along together in the direction of the wave. One compression and the rarefaction next to it make one complete wave, so the distance from one compression to the next is one wavelength.

One complete wave. A compression and the rarefaction that follows it together make one complete wave (one wavelength). In the diagram, the stretch from the start of one C to the end of the next R is one complete wave.
(If the question asks from one compression to the third rarefaction after it, the answer is three complete waves: C R, C R, C R.)

A sound wave needs a medium (a solid, liquid or gas) to travel: it travels by making the particles of the medium vibrate. It can’t travel through empty space. A wave that needs a medium is a mechanical wave.

Light does not need a medium: it travels even through empty space, which is how sunlight reaches the Earth across 150 million km of space. A wave that can travel without a medium is an electromagnetic wave.

Plucking makes the string vibrate quickly to and fro. The vibrating string pushes and pulls the air around it, making compressions and rarefactions that travel to our ears as sound. Sound is always produced by vibration.

reflects

parallel

equal

law (the laws of reflection)

perpendicular (the word “refract” is not needed)

(By the first law of reflection, the two are always equal.)

The bouncing back of light after it strikes a surface is called reflection of light.
Laws of reflection:

1. The angle of incidence is always equal to the angle of reflection.
2. The incident ray, the reflected ray and the normal at the point of incidence all lie in the same plane.

When parallel rays of light fall on a rough or uneven surface, the reflected rays are scattered in different directions. This is irregular reflection. Each ray obeys the laws of reflection at the tiny part of the surface it hits, but the tiny parts face different ways, so the rays leave in different directions. It does not form an image, but it lets us see the objects around us from every side.

1. Mirrors on vehicles: the side and rear-view mirrors reflect the road behind, so the driver can see vehicles coming from behind without turning round.
2. Seeing ourselves: a plane mirror forms an image of our face by regular reflection, so we can comb our hair and dress.

(Others: the mirror of a microscope reflects light up through the slide; a periscope lets us see over walls; irregular reflection lets us see everything around us.)

A periscope is a tube with two plane mirrors, one at each end, at 45° and parallel to each other. Light from an object enters the top opening and strikes the top mirror at 45°. It is reflected through 90°, straight down the tube. At the bottom it strikes the second mirror, again at 45°, and is reflected through 90° once more, out of the bottom opening into the eye. So the eye sees the object, which is high up, from a lower place. It works by the laws of reflection of light.

• i.AO is the incident ray and OB is the reflected ray.
• ii.The angle of incidence is ∠AON and the angle of reflection is ∠BON.
• iii.The normal ON makes 90° with the mirror’s surface XY.
• iv.The incident ray makes 30° with the mirror, so ∠AON = 90° − 30° = 60°. Since ∠i = ∠r, ∠BON = 60°.