# MOECDC Class 8 Energy in Daily Life

## MOECDC Class 8 Energy in Daily Life

MOECDC Class 8 Science and Technology | Energy in Daily Life | Nepal Class 8 | Exercise | Question Answer
MOECDC Class 8 English Medium Science Chapter 7 Energy in Daily Life Exercise Question Answer Solution.

 Board Government of Nepal. Class Eight (8). Subject Science and Technology. Chapter Energy in Daily Life Topic Exercise Solve / Question Answer / Notes.

#### Exercise 1 :

(1) Choose the best alternatives :

(a) What is the process of transmission of heat due to the actual movement of molecules?

(i) conduction

(ii) convection

(iv) temperature

(ii) Convection – is the process of transmission of heat due to the actual movement of molecules.

(b) What is the name of the process of transmission of heat in liquid and gas?

(i) conduction

(ii) convection

(iv) evaporation

(ii) Convection – is the process of transmission of heat in liquid and gas.

(c) How does the heat transmit in a vacuum?

(i) by conduction

(ii) by convection

(iv) by transpiration

The heat transmit in a vacuum – (iii) by radiation.

(d) In which of the following activities is heat transmitted by the radiation process?

(i) spoon kept in sunlight is heated

(ii) spoon immersed in a cup of tea is heated

(iii) spoon kept above the spirit lamp is heated

(iv) water gets heated when the hot iron rod is kept in cold water

(e) What is the average global temperature on the surface of the earth?

(i) 70°F

(ii) 59°F

(iii) 40°F

(iv) 125°F

(ii) 59°F – is the average global temperature on the surface of the earth.

(f) Which one of the following is an example of a mechanical wave?

(iii) sound wave

(ii) gamma-ray

(iv) X-ray

(iii) Sound waves – is an example of a mechanical wave.

(2) Differentiate between :

(a) conductor and insulator of heat

Conductor and insulator of heat: Materials that allow heat to pass through them easily are called good conductors of heat. But, materials that do not allow heat to pass through them are called insulators of heat.

(b) conduction and insulation

Conduction and insulation: The process of heat transmission of heat in a solid without actual movement of the molecules is called conduction. On the other hand, the process of keeping heat from transmission is called insulation.

(c) mechanical waves and electromagnetic wave

Mechanical waves and electromagnetic wave: Those waves which require a material medium for propagation are called mechanical waves. On the other hand, the waves that do not require a medium for propagation are called electromagnetic waves.

(d) sea breeze and land breeze

Sea breeze and land breeze : Sea breeze refers to the flow of cold air from the ocean to the land during the daytime. But, the cold air flows from the land to the sea at night which is termed the land breeze.

(3) Give reasons :

(a) The mud houses are cooler in summer and warmer in winter.

The mud houses are cooler in summer and warmer in winter because during summer, the outside temperature of mud house is more than the inside temperature as mud is poor conductor of heat. On the other hand, during winter, the outside temperature is less than the inside temperature and we feel warm inside the mud house.

(b) A teacup is usually made of porcelain.

A teacup is usually made of porcelain because porcelain is poor conductor of heat, so that they do not burn hands.

(c) We prefer light-coloured clothing in summer.

We prefer light-colored clothing in summer because they absorb less heat.

(d) Ventilation is kept at the upper part of the wall or door.

Ventilation is kept at the upper part of the wall or door so that when the hot air from the bottom of the room rises to the top, expands, cools, and then falls to the bottom again.

(e) Transparent plastic is used to cover the roof and walls of an artificial greenhouse.

Transparent plastic is used to cover the roof and walls of an artificial greenhouse so that maximum solar energy enters through it.

(4) Answer the following questions :

(a) What is the transmission of heat?

The process of transfer of heat energy from one object to another is called transmission of heat.

(b) What is meant by the conduction of heat?

The process of heat transmission of heat in a solid without actual movement of the molecules is called conduction.

(c) How does the wind blow? Explain the process.

During the day time, the earth’s surface is heated due to solar radiation. When the air close to the earth’s surface is heated, hot air becomes lighter and rises, leaving an empty space in its place. Cold air comes from nearby regions to fill this space. In this way, the movement of air takes place due to convection and it is called wind.

(d) What is the convection process? Write down the applications of convection in daily life.

The process of transmission of heat in liquid and gas due to the actual movement of molecules is called convection.

The applications of convection in daily life are,

(i) Convection occurs while boiling water and other liquid.

(ii) Convection is also used in radiators, refrigerators.

(iii) Convection also plays an important role in rainfall, thunderstorms, cloud formation and lightning.

(e) What is the radiation process? Write down the applications of radiation in daily life.

The process of transmission of heat without any material medium is called radiation.

The applications of radiation in daily life are,

(i) The heat radiations from the sun heat the surface of the planet.

(ii) When we sit near the fire, our body receives heat from the fire by the process of radiation.

(iii) White or light-colored clothes are worn in the summer season because they absorb less heat.

(f) What is an electromagnetic wave? Give any three examples of electromagnetic waves.

The waves that do not require a medium for propagation are called electromagnetic waves. Example – light, radio wave, X-ray.

(g) Draw the neat and labeled diagram of athermos flask and label its parts.

(h) What is the greenhouse effect? Write down the applications of greenhouse effects.

The mechanism through which solar radiation is absorbed by greenhouse gases rather than being transmitted into space is known as the greenhouse effect.

The applications of greenhouse effects are,

(i) Greenhouse effects are crucial to keeping our planet’s temperature suitable for the survival of life.

(ii) Off-season vegetables can be grown inside the greenhouse due to the increase in temperature inside it.

(iii) Some plants that have lost their existence in cold temperature can be saved in an artificial greenhouse.

#### Exercise 2 :

(1) Choose the best alternative :

(a) Which type of mirror is used to look at our faces in daily life?

(i) plane mirror

(ii) concave mirror

(iii) convex mirror

(iv) spherical mirror

(ii) Concave mirror – is used to look at our face in daily life.

(b) Which mirror can make a real image of an object?

(i) plane mirror

(ii) concave mirror

(iii) convex mirror

(iv) spherical mirror

(ii) Concave mirror – can make a real image of an object.

(c) What type of image is called a real image?

(i) image that cannot be obtained on the screen

(ii) image that can be obtained on the screen

(iii) an erect image

(iv) an inverted image

(ii) Image that can be obtained in the screen – is called a real image.

(d) What kind of mirror is called a concave mirror?

(i) plane mirror

(ii) diverging mirror

(iii) converging mirror

(iv) simple mirror

(iii) Converging mirror – is called a concave mirror.

(e) The ray of light coming from the centre of curvature in the concave nurror

(i) passes through the focus.

(ii) reflect along the same path.

(iii) passes parallel to the principal axis.

(iv) diverges from the mirror.

The ray of light coming from the center of curvature in the concave mirror – (ii) reflect along the same path.

(2) Differentiate between:

(a) Concave mirror and convex mirror

Concave mirror and convex mirror:

(i) In concave mirror, the middle part is depressed and its edge is continuously raised. But in convex mirror, edge is compressed and the middle part is raised.

(ii) A concave mirror forms an enlarged image of the object if it is kept very close to the concave mirror. On the other hand, the convex mirror forms a diminished image of the larger object lying far from the mirror, so it has a wide field of view.

(b) Real and Virtual images.

Real and virtual images:

(i) Real image is formed in front of the mirror. But virtual image is formed behind the mirror.

(ii) Real image can be obtained on the screen. But virtual image cannot be obtained on the screen.

(3) Define the following vocabulary :

(a) Principal axis

Principal axis – The imaginary straight line of infinite length that passes through the pole of the mirror and center of curvature is called the principal axis.

(c) Centre of curvature

Focus – When the rays of light parallel to the principal axis fall on the concave or convex mirror then after reflection they pass or appear to pass through the fixed point on the principal axis, which is called focus.

(b) Focus

Center of curvature – The center of the sphere from which the mirror is made is called the center of curvature.

(d) Focal length

Focal length – The distance between the pole of the mirror and the principal focus is called the focal length of the mirror.

(4) Give reason :

(a) Concave mirror is also called a converging mirror.

Concave mirror is also called a converging mirror because a concave mirror converges the rays of light falling on it to the principal axis.

(b) Concave mirror is used in solar stoves.

Concave mirror is used in solar stoves to converge solar radiation to cook food.

(c) Convex mirrors are used as looking glass in vehicles.

Convex mirrors are used as looking glass in vehicles to get the rear view, i.e. to look at the traffic behind the vehicle.

(d) Convex mirrors and plane mirrors cannot producethe real image.

Convex mirrors and plane mirrors cannot produce the real image because when an object is kept at any position in front of the convex mirror, the convex mirror always forms a virtual, erect and diminished image of that object.

(5)  Answer the following questions given below :

(a) What is meant by the reflection of light?

The phenomenon of returning light after striking a surface is called the reflection of light.

(b) Which mirror is called a spherical mirror?

Convex mirror is called a spherical mirror.

(c) Write down the factors on which the image formed by the concave mirror depends.

The factors on which the image formed by the concave mirror depends are,

(i) Rays of light parallel to the principal axis, pass through the principal focus after reflection.

(ii) The rays of light coming from the principal focus passes parallel to the principal axis after reflection.

(iii) The rays of light coming from the center of curvature returns through the same path.

(d) Which mirror is suitable to use while shaving a beard and doing make-up, and why?

Concave mirror is suitable to use while shaving a beard and doing makeup to see the enlarged face of the person.

(e) What could be the reason for using a convex mirror in street lamps?

Convex mirror is used in street lamps to scatter the light into a wide region as it is diverging mirror.

(f) Draw the ray diagram of the image formed by a concave mirror when the object is kept

(i) beyond ‘C’

(ii) at infinity

(iii) between ‘C’ and ‘F’

(iii) between ‘C’ and ‘F’

#### Exercise 3 :

(1) Choose the best alternatives:

(a) Which is the time the number of sound waves produced is called wave?

(i) 1 second

(ii) 2 seconds

(iii) 3 seconds

(iv) 4 seconds

(i) 1 second – is the time the number of sound waves produced is called wave.

(b) What is the range of frequency of audible sound?

(i) 2 Hz to 20 Hz

(ii) 2 Hz to 20 kHz

(iii) 2 Hz to 200 kHz

(iv) 2 Hz to 200 Hz

(c) What is the range of intensity of audible sound?

(i) 2 Hz to 20 dB (iii) 2 Hz to 200 dB

(ii) 2 Hz to 40 dB

(iv) 2 Hz to 200 dB

(d) What is the intensity of sound having a higher amplitude?

(i) more

(ii) Less

(iii) remains the same

(iv) more or less

The intensity of sound having a higher amplitude is – (i) more.

(e) What is the threshold of hearing for deafness?

(i) 90 dB

(ii) 80 dB

(iii) 60 dB

(iv) 55 dB

The threshold of hearing for deafness is – (ii) 80dB.

(f) Which of the following animal can produce and hear ultrasound?

(i) elephant

(ii) bat

(iii) crocodile

(iv) whale

(ii) Bat – can produce and hear ultrasound.

(2) Differentiate between :

(a) Wave length and amplitude

Wave length and amplitude: The distance travelled by a sound wave in one complete vibration is called wave length. On the other side, the maximum displacement of particles of the medium above or below the mean position in a wave is called the amplitude of the wave.

(c) infrasound and ultrasound

Infrasound and audible sound:

(i) The frequency of infrasound is 20Hz but the frequency range of audible sound lies between 20 Hz to 20 kHz.

(ii) Infrasound cannot be heard by humans but can be felt by the sensation of touch. But audible sound can be perceived by the human ears.

(b) infrasound and audible sound

Infrasound and ultrasound: The frequency of infrasound is 20Hz but the frequency of ultrasound is more than 20000 Hz.

(3) Answer the following questions given below :

(a) The speed of sound remains the same on changing the frequency or wavelength of sound, why?

The speed of sound remains the same on changing the frequency or wavelength of sound because the speed of sound wave is constant in a particular medium.

(b) Write the range of frequency of sound that can be heard by human beings.

The range of frequency of sound that can be heard by human beings lies between 20 Hz to 20 kHz.

(c) What is the relationship betweenthe wavelength, frequency and speed of the sound wave?

Speed of sound wave = Frequency * Wave length.

(d) Draw a neat and clean diagram of the sound wave.

(e) What is an ultrasonic wave? Write down the name of any two animals that can produce and hear ultrasound.

The sound which has a frequency of more than 20 kHz is called ultrasonic sound. Bats and rats can produce and hear ultrasonic sound.

(f) Write down any three applications of ultrasound.

Three application of ultrasonic sound are,

(i) It is used in UGC to find the growth condition of the baby inside the mother’s womb.

(ii) It is used to perform bloodless operations.

(iii) It is used to kill bacteria.

(g) The sound is heard fainter when the listener is moving away from the source of the sound, why?

The sound is heard fainter when the listener is moving away from the source of the sound because the intensity of the sound is decreases as the distance of the listener from the source increases.

(h) Write down any two effects of sound pollution and any two ways to minimize its effect.

Two effects of sound pollution:

(i) The weakening of sensation of hearing.

(ii) The blood pressure of a person increases.

Two ways to minimize its effects:

(i) Afforestation along the side of the road and near industrial zones can reduce noise pollution.

(ii) Noise pollution can be minimize by using silencers in vehicles.

(4) Solve these numerical problems :

(a) Calculate the wavelength of the sound wave if the speed of sound in air is 332 m/s and its frequency is 10 Hz.

Speed of sound in air (v) = 332 m/s

Frequency of sound (f) = 10 Hz

Wavelength of the sound (ƛ) = v / f = 332 / 10 = 33.2 m        [v = f * ƛ]

(b) Calculate the speed of the sound wave if the wavelength is 22 m and its frequency is 10 Hz.

Wavelength (ƛ) = 22 m

Frequency (f) = 10 Hz

Speed of the sound wave (v) = f * ƛ = 10 * 22 = 220 m/s

(c) If the speed of sound in air is 332m/s then calculate the wavelength of the shortest and longest wave that can be heard by human beings.

Speed of sound in air (v) = 332 m/s

Frequency of longest wave (f) = 20 Hz

Wavelength of longest wave (ƛ) = v / f = 332 / 20 = 16.6 m            [v = f * ƛ]

Frequency of shortest wave (f) = 20000 Hz

Wavelength of shortest wave (ƛ) = v / f = 332 / 20000 = 0.0166 m             [v = f * ƛ]

(d) If the 2×10-5 W/m² intensity of sound spreads out in an area of 6 m2 in 1 second, then calculate the energy carried by the sound wave.