Physics Class 8 Final Exam Paper Analysis

St. Anthony’s School, Adhikari

Final Examination

Session:- 2022-2023

Date:- 16-02-2022

Class – VIII Time:- 2 Hrs

Subject:- Physics Science Paper – 2 Full Marks:- 80

Answers to this Paper must be written on the paper provided separately.

You will not be allowed to write during the first 15 minutes.

This time is to be spent in reading the question paper.

The time given at the head of this paper is the time allowed for writing the answers.

(Q1) Fill in the blanks:- [10]

(i) Light travels slower in a medium with greater refractive index.

(ii) When a light ray passes from an optically rarer medium to an optically denser medium, it bends towards the normal.

(iii) A ray of light coming out of a rectangular glass slab emerges parallel to its original path.

(iv) A rainbow is a circular arc of seven colours formed in the sky.

(v) The increase in the kinetic energy appears as heat of the body.

(vi) When a substance is cooled, the kinetic energy of its molecules decreases.

(vii) Brass expands one and half times more than steel.

(viii) Frequency is measured in Hertz (Hz).

(ix) Loudness of a sound depends on the amplitude of the wave.

(x) The earth wire has no current flowing through it.

(Q2) State whether the following statements are true (T) or false (F). Correct the false statements. [10]

(i) Light always travels in a straight line.

(Ans) True

(ii) As the light enters another medium, its speed changes.

(Ans) True

(iii) The refractive index of a medium is the measure of the frequency of light in that medium with respect to the frequency of light in air.

(Ans) False, Refractive index of a medium is the ratio of the speed of light in vacuum to the speed of light in that medium.

(iv) Swimming pool appears to be shallow because of dispersion.

(Ans) False, Swimming pool appears to be shallow because of refraction of light.

(v) The transfer of heat continues till the two bodies attain the same temperature.

(Ans) True

(vi) The boiling point of a liquid increases with the decrease in atmospheric pressure.

(Ans) False, The boiling point of liquid increases with increase in atmospheric pressure and decreases with a decrease in atmospheric pressure.

(vii) Sound propagates only in front direction of the source.

(Ans) False, Sound propagates in all directions from the source.

(viii) The pitch of a sound determines its shrillness or flatness.

(Ans) True

(ix) The earth and neutral wires have no current flowing through them.

(Ans) False, The earth wire have no current flowing through them.

(x) A neutral atom contains unequal number of protons and electrons.

(Ans) False, A neutral atom contains an equal number of protons and electrons to maintain the electrical neutrality of the atom, as the positive charges are balanced with negative charges.

(Q3) Choose the correct option:- [10]

(i) When a ray of light moves from an optically denser medium to an optically rarer medium ............................ .

(a) it bends towards the normal

(b) it bends away from the normal

(d) none of these

(ii) Which colour suffers maximum bending in the spectrum of white light?

(a) Yellow (b) Green (c) Red (d) Violet

(iii) If the image formed by a concave mirror is virtual, erect and magnified, then the object is placed ...................... .

(a) at focus (b) between pole and focus

(iv) Convex mirrors are used in ..................... .

(a) searchlights (b) vigilance mirrors

(v) The SI unit of heat is ........................ .

(a) joule (b) calorie (c) Kelvin (d) kilocalorie

(vi) 1 joule = ................. calories.

(a) 1000 (b) 4.184 (c) 0.24 (d) 1.24

(vii) The characteristic which determines the energy carried by sound waves is .................... .

(a) loudness (b) pitch (c) quality (d) none of these

(viii) Decibel is the unit used to measure the .................. of sound.

(a) loudness (b) pitch (c) frequency (d) quality

(ix) A fuse wire is made of ....................... .

(a) copper and tin (b) lead and tin

(x) Which of the following is not a method of charging an electrically neutral body?

(a) Friction (b) Conduction (c) Convection (d) Induction

(Q4) Show the following in the form of a flowchart.

(Any 5)[10]

(i) Images formed by a concave mirror

(ii) Image formed by a convex mirror

(iii) Effects of heat

(iv) Change in dimensions of solids, liquids and gases

(v) Characteristics of sound

(vi) Types of musical instruments

(vii) Charging an object

(viii) Charge identification by an electroscope

(Q5) Give reasons for the following statements. [10]

(i) A swimming pool appears shallow than its actual depth.

Refraction of light at the surface of the water makes swimming pools appear shallower than its actual depth when viewed from directly above.

The reason is that the rays of light reaching us from the object at the bottom of the water do not come straight to us. The light travels straight as long as it is in the water, but if it emerges obliquely from the water into the air it is bent downward toward the surface. This bending is known as refraction, and it occurs whenever light passes from one transparent medium into another of different density. Our eyes do not take refraction into account but judge the position of the object as if the light came in a straight line.
(ii) A rainbow consists of a pattern of seven colours.
A rainbow has seven colors because water droplets in the atmosphere break sunlight into seven colors. A prism similarly divides light into seven colors. When light leaves one medium and enters another, the light changes its propagation direction and bends. This is called refraction.
(iii) Water droplets appear on the outer surface of a glass full of cold water.

Water droplets form on the outer surface of the container containing a cold substance due to Condensation. The temperature of the container is cold enough to cool down the water vapour in the surrounding air, so it turns from gaseous to liquid state.

(iv) One side of the iron girdles of bridge are placed on rollers.

When we have rollers at one end then it is allowed to expand freely without any thermal stress.in summers the bridges may expand due to the increase in the temperature and in winters the bridges may compress due to decrease in the temperature. the rollers help to increase/decrease the length of the bridge without any thermal stress.

(v) As the time period of a wave increases, its frequency decreases.

Frequency is defined as number of repetations per unit time. So when time interval between two waves increases, number of waves per unit time decreases and thus frequency decreases.

(vi) The sound from a violin having thin and tight string is shrill.

The sound from violin having thin and tight string is shrill because thick strings give a low pitch voice. The tightness is also an important factor.

(vii) The valence electrons can be easily knocked off and become free.

Valence electron orbit in shell which are much far than any other shell from nucleus. As the distance increase from nucleus binding force between nucleus and electrons gets decreased so that balance electron experience least amount of force from nucleus that's why valance electron can be knocked of very easily.

(viii) When glass is rubbed with silk, it becomes positively charged.

When a glass rod is rubbed with silk, it acquires a positive charge, means it is deficit of electrons i.e, electrons move from glass to silk. So glass rod becomes positively charged and silk rod is negatively charged.

(ix) Lightning process is an example of electrostatics in nature.

The lightning process is an example of electrostatics in nature because the electrostatic field between the clouds and the ground can produce ions and free electrons in the air.

(x) It is advised to stay away from metallic objects during a thunderstorm.

Lightning is attracted to conductive objects such as metal, especially if the conductive object is connected to the ground where the electricity can discharge or complete its circuit. We are made of 90% water, and water is one of the most conductive elements in the world.

In the case of lightning from a thunderstorm, touching the metal or any other conductive and connected object, it can be electrically charged or very hot. You could burn yourself or become part of the electrical current.

(Q6) Answer in short. (Any 5) [05]

(i) State the laws of refraction.

Refraction is the change in direction of a wave travelling from one medium to another induced by a change in speed.

According to Snell's laws or the law of refraction:

(a) The incident ray, the refracted ray, and the normal at the point of incidence, all lie in the same plane.

(b) The ratio of the sine of the angle of incidence ‘i’ to the sine of the angle of refraction ‘r’ is constant for the pair of given media is always constant.

(ii) With the help of a ray diagram, show the refraction of light from an optically rarer medium to a denser medium.

(iii) Define one kilocalorie of heat.

(Ans) One kilocalorie (1 kcal or 1000 calories) is the amount of heat (energy) needed to raise the temperature of one kg of water by one degree Celsius (°C). The SI standard unit for energy is Joule (J).

(iv) Why do substances expand on heating?

(Ans) (a) When a substance is heated, its molecular movement increases. Due to this, the average distance between the molecules also increases, thereby increasing the volume of the substance, i.e., expanding the substance.

(b) When we heat a substance its molecules gain energy and start vibrating rapidly and start expanding the length, area, and volume of the substance. As a result, a substance expands.

(v) How are time period and frequency of a sound wave related?

(Ans) A Time period (denoted by 'T' ) is the time needed for one complete cycle of vibration to pass a given point. Frequency is the number of occurrences of a repeating event per unit of time. As the frequency of a wave increases, the time period of the wave decreases.

(vi) How does the loudness of a sound wave depend on its amplitude?

(Ans) Loudness is proportional to the square of the amplitude of the sound. Greater the amplitude, the greater the Loudness, and vice versa.

(vii) When does a short circuit occur.

(Ans) If the insulation of the wires used in the circuit is damaged or the appliance used is faulty due to which the live wire and the neutral wire comes in direct contact as a result current in the circuit rises and the short circuit occurs.

(viii) Define power and give its units.

(Ans) We can define power as the rate of doing work, it is the work done in unit time. The SI unit of power is Watt (W) which is joules per second (J/s).

(ix) What is a triboelectric series?

(Ans) The triboelectric series is a list that ranks various materials according to their tendency to gain or lose electrons. It usually lists materials in order of decreasing tendency to charge positively (lose electrons), and increasing tendency to charge negatively (gain electrons).

Most Positive (+)
Air	+++ +
Human Hands, Skin
Asbestos
Rabbit Fur
Glass
Human Hair
Mica
Nylon
Wool
Lead
Cat Fur
Silk
Aluminum
Paper
Cotton
Steel	- - - -
Wood
Lucite
Sealing Wax
Amber
Rubber Balloon
Hard Rubber
Mylar
Nickel
Copper
Silver
uv Resist
Brass
Synthetic Rubber
Gold, Platinum
Sulfur
Acetate, Rayon
Polyester
Celluloid
Polystyrene
Orlon, Acrylic
Cellophane Tape
Polyvinylidene chloride (Saran)
Polyurethane
Polyethylene
Polypropylene
Polyvinylchloride (Vinyl)
Kel-F (PCTFE)
Silicon
Teflon
Silicone Rubber
Most Negative (-)

(x) How can we charge an object by friction?

(Ans) When two different insulating materials are rubbed, electrons get transferred from one body to another. This method is known as charging by friction. For example, when a plastic rod is rubbed with a piece of cloth, electrons are transferred from the rod to the cloth. As a result, the rod becomes positively charged and the cloth becomes negatively charged.

(Q7) Explain the following terms.(Any 5) [05]

(i) Refraction

(ii) Angle of refraction

(iii) Fusion

(iv) Sublimation

(v) Wavelength

(vi) Pitch

(vii) Electrical power

(viii) Short circuit

(ix) Earthing

(x) Electrostatics

(i) Refraction

(Ans) Refraction is the bending of light (it also happens with sound, water and other waves) as it passes from one transparent substance into another.

(ii) Angle of refraction

(Ans) The angle between the refracted ray and the normal at the point of incidence is called angle of refraction.

(iii) Fusion

(Ans) The process by which a substance changes from a solid state to a liquid state is called fusion.

(iv) Sublimation

(Ans) We can define sublimation as the transition of a substance from the solid phase to the gaseous phase without changing into the liquid phase.

(v) Wavelength

(Ans) Wavelength is the actual distance between the two successive crests that are either troughs or peaks in a wave.

(vi) Pitch

(Ans) Pitch is defined as the characteristic of sound which is used for differentiating between the shrill and flat sound.

(vii) Electrical power

(Ans) Electric power is the rate at which work is done or energy is transformed into an electrical circuit.

(viii) Short circuit

(Ans) A sudden flow of a very large current due to direct contact of a live wire and a neutral wire is called short-circuiting.

(ix) Earthing

(Ans) Earthing is defined as “the process in which the instantaneous discharge of the electrical energy takes place by transferring charges directly to the earth through low resistance wire.”

(x) Electrostatics

Electrostatics is a branch of physics that deals with the phenomena and properties of stationary or slow-moving electric charges.

(Q8) Differentiate between the following. [10]

(i) Reflection and refraction

(ii) Real image and virtual image

(iii) Condensation and solidification

(iv) Loudness and pitch

(v) Live wire and neutral wire

(i) Reflection and Refraction

Reflection	Refraction
This phenomenon usually occurs in mirrors.	This phenomenon usually occurs in lenses.
Reflection can simply be defined as the bouncing back of light when it strikes the medium on a plane.	Refraction can be defined as the process of the shift of light when it passes through a medium leading to the bending of light.
The light entering the medium returns to the same medium.	The light entering the medium travels from one medium to another.
Considering the light waves, they bounce from the plane and change direction.	During refraction, the light waves pass through the surface while simultaneously changing both direction and medium.
The angle of incidence of the light is equal to the angle of reflection.	The angle of incidence is not equal to the angle of refraction.

(ii) Real Image and Virtual Image

Real Image	Virtual Image
Real images are inverted	Virtual images are erect
A real image is always formed by a convex lens.	Virtual images can be formed by concave, convex or plane mirrors.
Real images are formed on the screen	Virtual images appear to be on the lens or the mirror itself
Real images are always formed by a concave mirror	Convex mirror forms a virtual image
Real images are formed due to the actual intersection of light rays	Virtual images are formed due to the imaginary intersection of light rays

(iii) Condensation and solidification

Solidification : The process of changing liquid into a solid state by cooling is known as solidification.

Example : water → Ice.

Condensation : The process of changing a gas or vapour state to a liquid state by cooling is known as condensation.

Example : steam → water.

(iv) Loudness and Pitch

Loudness	Pitch
Loudness is defined as the property of sound which is used for differentiating between the loud and faint sound	Pitch is defined as the characteristic of sound which is used for differentiating between the shrill and flat sound
Loudness is dependent on the energy received by the ear in unit time	Pitch is independent of the energy received by the ear in unit time
Loudness is independent of the change in frequency	Pitch is dependent on the change in the frequency. As the frequency increases, the shrillness of the sound increases

(v) Live Wire and Neutral Wire

Live wire	Neutral Wire
It carries the electric current supplied by the source to the electric appliances.	It completes the circuit by returning the electricity to the source after it has passed through the appliance.
It is given an insulation red or brown in color.	It is identified as blue or black in color.
The live wire is connected directly to the generators of the current supply.	The main switch is connected to the live wires and neutral wires.
It always carries electric current from the supply(transformer) to the appliance.	It is usually at zero volts.

(Q9) Answer in detail. (Any 5) [10]

(i) Explain the unique properties of refraction with the aid of suitable ray diagrams.

(ii) Why do we need ray diagrams? What rules should be followed while drawing ray diagrams?

(iii) Discuss the various factors affecting evaporation.

(iv) Explain the factors affecting boiling.

(v) Discuss the propagation of sound as a longitudinal wave.

(vi) Discuss the factors on which the loudness of sound depends.

(vii) How can we conserve electricity?

(viii) Explain origin of charges.

(i) Explain the unique properties of refraction with the aid of suitable ray diagrams.

(Ans) (a) When a ray of light enters a denser medium (say from air to glass), it always bends towards the normal. So the angle of refraction will be less than the angle of incidence

(∠i ＞∠r).

(b) A ray of light travelling from a denser to a rarer medium always bends away from the normal. In this case, the angle of refraction will be more than the angle of incidence (∠i ＜∠r).

(c) A ray of light travelling along the normal does not suffer any refraction. In this case, both the angle of incidence and the angle of refraction will be equal to zero (∠i =∠r = 0).

(ii) Why do we need ray diagrams? What rules should be followed while drawing ray diagrams?

(Ans) Need for ray diagrams

(a) The image formation by spherical Mirrors can be easily understood with the help of a diagram.

(b) Ray diagrams are used to determine the location and position of the image, formed by convex and concave mirrors.

Rules should be followed while drawing ray diagrams

(a) To draw ray diagram, take at least two rays of convenience, which originate from the same point of the object and whose path can be traced after refraction.

(b) Always keep the object on the left side in front of the reflecting surface.

(iii) Discuss the various factors affecting evaporation.

(Ans) Factors affecting evaporation:

(a) Temperature:

On increasing the temperature the rate of evaporation also increases.

At higher temperatures, the molecules are moving faster; therefore, it is more likely for a molecule to have enough energy to break away from the liquid to become a gas.

(b) Wind speed:

Wind speed and rate of evaporation are directly proportional to each other.
As the wind speed increases, the rate of evaporation also increases.

As the surface area increases, the rate of evaporation also increases.
The more area is exposed to air, allowing water molecules to acquire more heat energy from the surroundings.

(d) Humidity:

Humidity and rate of evaporation are in inverse relation to each other.
As the humidity decreases, the rate of evaporation increases.

(e) Nature of liquid:

Different liquids have different rates of evaporation.

A liquid having weaker interparticle attractive forces evaporate at a faster rate because less energy is required to overcome the attractive forces

(f) Surface area of the liquid:

The evaporation depends upon the surface area. If the surface area is increased, the rate of evaporation increases because the high energy particles from the liquid can go into gas phase only through a surface.

(iv) Explain the factors affecting boiling.

(Ans)

(a) Pressure: Boiling point increases as the pressure increases. If external pressure is lower than the atmospheric pressure then the boiling point is less than the normal boiling point. It increases when the pressure is more than the atmospheric pressure.

(b) Effect of impurities: The boiling point increases with the increase in impurities. Eg:- The boiling point of water increases on adding salt or sugar to it.

(c) Intermolecular forces: If the forces are weak, then the boiling point is also low and if the forces are strong then the boiling is high.

(d) Molecular weight: Boiling increases if the chain length of the molecule increases.

(e) Branching: With the increase in branching, the boiling point decreases.

(f) Nature of the liquid: The boiling point of a liquid is its characteristic property. This means that different liquids boil at different temperatures. For example, under normal conditions water boils at 100 ⁰C.

(v) Discuss the propagation of sound as a longitudinal wave.

(Ans)

(a) A sound wave is a form of energy that flows from one point to another.

(b) This flow of energy is in the form of waves.

(c) A wave can be categorized in two forms on the basis of particle movement i.e. Transverse waves and Longitudinal waves.

(d) On the basis of medium requirement, it is categorized into two parts i.e. mechanical and non-mechanical waves.

(e) Mechanical waves are waves that require a medium to travel.

(f) Longitudinal waves are the waves in which particle velocity of the medium is along the line of flow of the wave.

(g) Sound wave is a longitudinal wave in which particles of the medium vibrate along the direction of wave motion.

(h) Flow of sound waves is in the form of compression and rarefaction.

(i) Compression- It is a region of high pressure and high density in a wave motion.

(j) Rarefaction- It is a region of low pressure and low density in a wave motion.

Longitudinal waves are waves in which the motion of the individual particles of the medium is in a direction that is parallel to the direction of energy transport. A longitudinal wave can be created in a slinky if the slinky is stretched out in a horizontal direction and the first coils of the slinky are vibrated horizontally. In such a case, each individual coil of the medium is set into vibrational motion in directions parallel to the direction that the energy is transported.

Sound waves in air (and any fluid medium) are longitudinal waves because particles of the medium through which the sound is transported vibrate parallel to the direction that the sound wave moves. A vibrating string can create longitudinal waves as depicted in the animation below. As the vibrating string moves in the forward direction, it begins to push upon surrounding air molecules, moving them to the right towards their nearest neighbor. This causes the air molecules to the right of the string to be compressed into a small region of space. As the vibrating string moves in the reverse direction (leftward), it lowers the pressure of the air immediately to its right, thus causing air molecules to move back leftward. The lower pressure to the right of the string causes air molecules in that region immediately to the right of the string to expand into a large region of space. The back and forth vibration of the string causes individual air molecules (or a layer of air molecules) in the region immediately to the right of the string to continually vibrate back and forth horizontally. The molecules move rightward as the string moves rightward and then leftward as the string moves leftward. These back and forth vibrations are imparted to adjacent neighbors by particle-to-particle interaction. Other surrounding particles begin to move rightward and leftward, thus sending a wave to the right. Since air molecules (the particles of the medium) are moving in a direction that is parallel to the direction that the wave moves, the sound wave is referred to as a longitudinal wave. The result of such longitudinal vibrations is the creation of compressions and rarefactions within the air.

(vi) Discuss the factors on which the loudness of sound depends.

(Ans)

(a) Amplitude of sound wave- The amplitude of a sound wave is the measure of the height of the wave. The amplitude of a sound wave can be defined as the loudness or the amount of maximum displacement of vibrating particles of the medium from their mean position when the sound is produced.

Loudness ∝ amplitude²

(b) Distance from sound source- Less the distance from the source of sound more the loudness of sound, therefore loudness of sound is inversely proportional to the distance from the source.

(c) Density of medium- More the density of medium more the loudness. For example- the loudness of the sound of the train will be more on a railway track as compared to in air as the density of iron is more than air.

(d) Surface area of vibrating bodies- More the surface area of vibrating bodies, the more will be the loudness of sound. For example - the loudness of the sound of beating a stick on a drum with less surface area will be less than beating it on a drum with more surface area.

(vii) How can we conserve electricity?

(Ans) We can conserve electricity by using following ways:-

At Home:-

(a) Turn off the appliances when not in use.

(b) Close doors when going out of the room or coming in when air conditioner is on.

In School

(a) Make sure that computer monitors are turned off when you leave the computer room.

(b) Turn off the lights and fans when you leave the classroom.

(viii) Explain origin of charges.

(Ans) We know that every substance consists of atoms. Each atom has a nucleus at its center, which consists of the positively charged particles protons and the null charged particles neutrons.

Negatively charged particles Electrons are always spinning around the nucleus of an atom. An electron has as much negative charge as a proton has a positive charge.

The atom has an equal number of electrons and protons. Therefore, the atom is electrically neutral.

Those electrons who rotate near of nucleus of the atom called Bound Electrons because these electrons are bound with the nucleus with intense attraction. But the electrons who far from the nucleus compared to bound electrons have a low attraction to the nucleus.

Electrons orbiting away from the nucleus can separate from the atom by obtaining less energy, this type of electrons called Free Electrons.

Whenever any two substances were rubbed together, so electrons come out of one substance and move to another (in which electrons are bound by less force than other matter). So Due to this phenomenon, the negatively charged particles are reduced in any one substance, due to which that substance becomes positively charged

Example –

When the glass rod is rubbed with silk, the electrons from the glass rod go out into the silk, due to which the glass rod is positively charged due to lack of electrons, and silk becomes negatively charged due to excess of electrons.

Physics Class 8 Final Exam Paper Analysis

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