# CBSE Sample Paper 2018 Class 10 Physics Solution

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## CBSE Sample Paper 2018 Class 10 Physics Solution

 Board CBSE Topic Sample Paper Solution Year 2018 – 19 Subject Physics Class 10 (Ten Class)

### Solution of sample question paper 2018 / 19

Q.5) A ray of light enters into benzene from air. If the refractive index of benzene is 1.50, by what percent does the speed of light reduce on entering the benzene?

Ans: Data:

vair= 3×108  m/s

vbenzene=? m/s

n=1.50

Refractive index of benzene can be written as,

n = vair / vbenzene

∴1.50 = (3×108) / vbenzene

∴vbenzene=(3×108) / 1.50

∴vbenzene = 2×108 m/s

The percentage decrease in speed of light from air to benzene is then,

= (vair-vbenzene)/vair ×100

= (3×108-2×108)/(3×108)×100

= (3-2)/3×100

= 1/3×100

= 33.33 %

Hence, decrease in the velocity of light from air to benzene is 33.33 %

OR

For the same angle of incidence in media A,B and C, the angles of refraction are 200, 300 and 400 respectively. In which medium will the velocity of light be maximum? Give reason in support of your answer.

Ans: According to law of refraction, we know that the refractive index of material is ratio of sine of angle of incidence to sine of angle of refraction.

Here we have angle of incidence =i

If , nA, nB and nC are refractive indices of medium A, B abd C respectively, then we can have

nA  =sini/sin 200   ………(1)

nB =sini/sin 300  ………(2)

nC =sini/sin 400   ………(1)

Now if we compare equation (1) , (2) and (3) we can say that  nA >nB >nC

The refractive index of medium C is least. i.e. bending of light in medium C will be larger, hence velocity of light in medium C will be maximum as compared to media A and B.

Q.11) The image of an object formed by a mirror is real, inverted and is of magnification -1. If the image is at the distance of 30 cm from the mirror, where is the object placed? Find the position of the image if the object is now moved 20 cm towards the mirror. What is the nature of the image obtained? Justify your answer with the help of ray diagram.

Ans: Magnification formed by mirror is, M= -1, v= -30 cm

Magnification has negative sign which indicates that the image is real but inverted and the mirror must be concave.

∴M=v/u

∴-1=30/u

∴u=-30 cm

Here the distance of object and distance of image from the pole of mirror is same, hence the object is placed at center of curvature of mirror.

From above we can say, R= 30 cm then f= 15 cm

Now the object is moved by distance of 20 cm towards mirror, then Image formed will be virtual and erected.

The following ray diagram shows the given condition. OR

What is meant by power of a lens? You have three lenses L1, L2 and L3 of powers +10D, +5D and -10D respectively. State the nature and focal length of each lens. Explain which of the three lenses will form a virtual and magnified image of an object placed at 15 cm from the lens. Draw the ray diagram in support of your answer.

Ans: Power of lens: The ability of lens to converge or diverge the incident beam of light is known as power of lens. Power of lens is inversely proportional to the focal length; hence reciprocal of focal length is called power of lens, written as SI unit of power of lens is diopter (D)

Convex lens has positive value of focal length; hence its power is also taken as positive.

Concave lens has negative value of focal length; hence its power is also taken as negative.

Now to find nature and focal lengths of lenses L1, L2, and L3 we have, If object is kept at distance of 15 cm from all of the three lenses, lens L2 will form the magnified and virtual image of object. Since for this lens focal length f2 is equal to 0.2 m i.e. 20 cm. Hence when the distance of object is 15 cm, it will between its optical centre and pole. We know that the image formed by convex lens is magnified and virtual when object placed between its optical centre and pole.

Refer the following diagram to understand this in detail. Q.12) Two lamps, one rated 100 W at 220 V and the other 200 W at 220V are connected (i) in series and (ii) in parallel to electric main supply of 220V. Find the current drawn in each case.

Ans: P1= 100 W, P2= 200 W, V=220 V

Let R1 and R2 be resistances of two lamps. Q.13) The figure below shows three cylindrical copper conductors along with their face areas and lengths. Compare the resistance and the resistivity of the three conductors. Justify your answer. Ans: As we know that the resistivity of conductor always remains same irrespective of its length and area. Hence the resistivity in all of these cases will remain same whereas the resistance depend upon length and area of cross-section of conductor. Resistance of conductor is directly proportional to length of conductor and inversely proportional to area of cross-section of conductor.

Let, ρ = resistivity of copper  Q.14) What is biogas? Describe the steps involved in obtaining biogas.

Ans: The fuel in the form of gas that is generated from plant and animal products (waste) is known as Bio-gas. Eg. Cow-dung cake, animal waste etc. As per shown in figure above, following are the steps involved in formation of bio-gas:

• Slurry of cow dung and other waste is added in mixing tank.
• This slurry is then sent to digester tank where slurry is allowed to get decompose for 10/15 days and then produces gases like methane, hydrogen sulphide and CO2.
• The bio gas is stored in a tank mounted above digester tank and can be used via outlet provided with tap.
• Slurry left behind contains nitrogen and phosphorus is used as fertilizers. The bio- gas is 100 % safe and cost free mode of fuel.

Q.16) a) How will you show experimentally that metals are good conductors of heat.

Ans: We know that metals consist of one or more than one loosely bounded electron in its last orbit. These free electrons are available for carrying energy through metals. Due to this metals are considered as the good conductors of heat and electricity.

The small experiment explained below can easily demonstrate that the metals are good conductors of heat. Follow the given procedure,

• Take three rods respectively of wood, plastic and copper of same length and thickness. Measure their initial temperatures.
• Heat the water till it boils and pours it in to beaker.
• Now dip all the rods in to the beaker, wait for 2/3 minutes and take the rod out of the beaker one by one.
• Note down the temperature of all three rods.

Here we will find that the temperature of rods made of wood and a plastic doesn’t changes whereas the temperature of copper rod is increase. This indicates that the heat of water is transferred from one end of copper to its other end, hence metals are good conductors of heat.

Q.20) (a)What is meant by the term ‘power of accommodation’? Name the component of eye that is responsible for the power of accommodation.

(b) A student sitting at the back bench in a class has difficulty in reading. What could be his defect of vision? Draw ray diagrams to illustrate the image formation of the blackboard when he is seated at the (i) back seat (ii) front seat. State two possible causes of this defect. Explain the method of correcting this defect with the help of a ray diagram.

Ans (a): Power of accommodation: The ability of human eye lens to contract or expand so that the focal length of eye can increase or decrease as per the need is called as Power of accommodation.

Ciliary glands are present around the lens which helps eyes to change its focal length.

• When we want to see nearby object, focal length of eye lens should be small, to achieve this ciliary muscles pull the lens, makes it thicker so that provide us ability to see nearby object.
• If we want to see distinct objects, ciliary muscles contracts the eye lens, due to this the focal length of lens is increased and we can see distinct objects clearly. This ability plays very vital role in our visual ability. Note that, the focal length cannot be increased or decreased beyond certain limited value.

Ans (b): A student sitting at the back bench in a class has difficulty in reading; this indicates that he/she is suffering with the disease of myopia or nearsightedness. It is the defect in human eye in which one can see nearby objects clearly, but cannot see distinct object clearly is known as myopia or near sightedness.

Following are the ray diagrams which illustrate the given conditions This defect is mainly cause due to,

• ciliary muscles expands the lens, but doesn’t relaxed, so that the focal length of eye lens decreases, eyeball gets elongated.
• Image of object converges before retina, so that one can see nearby objects Cleary but can’t see distinct objects.

A concave lens of suitable focal length or power is prescribed by doctors to correct this defect. Concave lens diverges the rays of light before falling on eye lens, and then converges to retina so that a person can see clear object of distinct object. Refer the following diagram, Q.21) (i) With the help of an activity, explain the method of inducing electric current in a coil with a moving magnet. State the rule used to find the direction of electric current thus generated in the coil. (ii) Two circular coils-1 and coil-2 are kept close to each other as shown in the diagram.Coil-1 is connected to a battery and key and coil-2with a galvanometer. State your observation in the galvanometer. Ans (i): According to Faraday’s law of electromagnetic induction, if the magnetic field around the conductor changes, a current is induced in the conductor.

This can be explained with the following activity along with the diagram as below • Take a small piece of plastic pipe and wound an insulated copper wire of suitable thickness over the pipe.
• Connect the terminals of wires to sensitive galvanometer.
• Put a strong bar magnet in to the pipe.
• Close both the open ends of pipe with fingers and shake the pipe such that the magnet moves to and fro inside pipe.
• Observe the pointer of galvanometer.

Here we can observe that the pointer of galvanometer deflects when magnet inside pipe moves to and fro. This is because when magnet moves in pipe, magnetic field around the wire changes which induces current in the conductor due to electromagnetic induction.

Ans (ii): In diagram, two circular coils-1 and coil-2 are kept close to each other as Coil-1 is connected to a battery and key and coil-2with a galvanometer.

When key is close, current flows through coil-1, which produces magnetic field around it. The magnetic field around coil-1 is directed towards the coil-2 (since coil-1 behaves as solenoid, whose magnetic field behaves as bar magnet). During this current is induced in the coil-2 by Faraday’s law i.e. due to electromagnetic induction and pointer of galvanometer gets deflected. When key is open magnetic field decrease, so that the pointer of galvanometer moves in opposite direction.

OR

Q.21) Name a device which converts mechanical energy into electrical energy. Explain the underlying principle and working of this device with the help of a labelled diagram.

Ans: The device which converts mechanical energy into electrical energy is known as electric generator.

The following diagram shows the different parts used for construction of electric motor Principle of working:

It works on principle of electromagnetic such that the coil rotated in uniform magnetic field, induces current in it. According to Faraday’s law of electromagnetic induction, whenever there in change in magnetic field linked with coil, current is induced in the coil. Using this phenomena an armature is rotated mechanically in strong U shaped magnet, and due to electromagnetic induction mechanical energy is converted into electrical energy.

Q.26) The current flowing through a resistor connected in an electrical circuit and the potential difference developed across its ends are shown in the given ammeter and voltmeter. Find the least count of the voltmeter and ammeter .What is the voltage and the current across the given resistor? Ans: In ammeter shown in diagram above, the range of ammeter is 1 A.

Now note that there are 10 divisions between 0 to 0.2 A. Then value of smallest division i.e. least count of ammeter is,  Q.27) Consider the path of a ray of light passing through a rectangular glass slab for different angles of incidence. (i) Which one is greater: angle of incidence or angle of refraction? (ii) What happens to the emergent angle on increasing the incident angle at air-glass interface? (iii) State the conditions when no bending occurs.

Ans: Figure shown below explains the refraction of light through a glass slab. Since the light is travelling from air to glass, i.e. from rarer to denser medium, the refracted ray bends towards normal. In such case angle of incidence is greater than angle of refraction.

• Since the light is travelling from rarer to denser medium, refracted ray bends towards the normal therefore angle of incidence is greater than angle of refraction.∴ ∠i> ∠r
• The magnitude of angle of emergent also increases with increase in angle of incidence, since the light from slab emerges out in rarer medium. It found that angle of incident is equal to angle of emergent.∴ ∠i= ∠e
• When light incident at right angles to the surface of glass slab, angle of incidence will be 00, then no bending of light is observed.

OR

Q.27) Sunita takes a mirror which is depressed at the centre and mounts it on a mirror stand. An erect and enlarged image of her face is formed. She places the mirror on a stand along a meter scale at 15 cm mark. In front of this mirror, she mounts a white screen and moves it back and forth along the meter scale till a sharp, well-defined inverted image of a distant tree is formed on the screen at 35 cm mark. (i) Name the mirror and find its focal length. (ii) Why does Sunita get sharp image of the distant building at 35 cm mark

Ans: Sunita takes a mirror which is depressed at the centre i.e. concave mirror. The arrangement of made by Sunita can be explained with the help of following diagram, • Since the mirror is depressed at center, it is concave mirror. The position of tree is at infinite distance from the mirror which mounted on stand at mark of 15 cm. The sharp but inverted image of tree forms at mark of 35 cm, where the convergence of reflected rays is takes place. Hence focal length according to sign convections will be, f = 15-35 = – 20 cm
• We know that the concave mirror has ability to produce convergence of all the incident rays falling parallel on it at its focus. After reflecting the rays from mirror forms inverted image at focus.

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Updated: August 24, 2021 — 2:58 pm