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BHSEC PHYSICS-2017

Q. No. Expected Answers

1.(a)

(i)

Read the following questions carefully. For each question there are four alternatives A, B, C and D.

Choose the correct alternative and write it in the space provided.

In simple harmonic motion, the velocity of a particle is

A constant in all positions.

B minimum at the mean position.

C maximum at the mean position. (Ans.:)

D maximum at the extreme positions.

(ii) The instantaneous current from an AC source is I = 3 sin ωt ampere. The rms value of the current is

A 2 3A .

B 32

A . (Ans.:)

C 32

A .

D 23

A .

(iii) The fluid property due to which mercury does not wet a glass is

A low viscosity.

B high viscosity.

C surface tension. (Ans.:)

D capillary action.

(iv) The electric field of a point charge ‘q’ 30 cm away from it is 5 N/C. The magnitude of the point charge is

A .105 9 C

B .105 10 C

C .105 11C (Ans.:)

D .105 12 C

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(v) The air pressure in tyres slightly increase on driving a car for a long time. This is because the

A temperature of the tyres increase. (Ans.:)

B temperature of the tyres decrease.

C atmospheric pressure on the tyres increase.

D atmospheric pressure on the tyres decrease.

(vi) The current in a capacitor during the charging process

A increases exponentially.

B decreases exponentially. (Ans.:)

C increases linearly.

D decreases linearly.

(vii) The mutual inductance between two coils in a transformer is 1.25 H. To induce an emf of 5 V in the secondary coil, the current in

the primary coil should be changed at the rate of

A 0.25 A/s.

B 1.25 A/s.

C 4.00 A/s. (Ans.:)

D 6.25 A/s.

(viii) A candle is kept away from the focus of a convex lens whose upper half is wrapped in black paper. How will the image of the

candle be affected?

A Full image will be formed. (Ans.:)

B No image will be formed.

C Only the upper half of the image will be formed.

D Only the lower half of the image will be formed.

(ix) A thin layer of oil on water produces beautiful colours when illuminated by white light. This happens because of

A diffraction of light reflected from the lower surface of the thin layer.

B interference of light reflected from the lower surface of the thin layer.

C diffraction of light reflected from the upper and lower surfaces of the thin layer.

D interference of light reflected from the upper and lower surfaces of the thin layer. (Ans.:)

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(x) In the diagram given below, which curve represents the relationship between photoelectric current and intensity of light?

A I

B II (Ans.:)

C III

D IV

(xi) All of the following are examples of lepton EXPECT

A proton. (Ans.:)

B electron.

C neutrino.

D anti-neutrino.

(xii) If a nucleus is split into two smaller nuclei, the average binding energy per nucleon

A increases. (Ans.:)

B decreases.

C remains same.

D increases initially and decreases later.

(xiii) A simple harmonic motion is represented by x (t) = 20 sin (157t + 0.5). The time period of the simple harmonic motion is

A 1.3 × 10-2 s.

B 2.0 × 10-2 s.

C 4.0 × 10-2 s. (Ans.:)

D 4.0 × 10-3 s.

(xiv) n-type semi-conductor is obtained when a pure silicon is doped with

A boron.

B sodium.

C antimony. (Ans.:)

D aluminium.

(xv) Dorji is standing between the focus and centre of curvature of a concave mirror. His image will be

A enlarged and erect.

B diminished and erect.

C enlarged and inverted. (Ans.:)

D diminished and inverted.

Intensity

Curr

ent

I

IV

III

II

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(b)

(i)

(ii)

(iii)

(iv)

(v)

(vi)

(vii)

Fill-in-the-blanks with appropriate words.

The time period of oscillation of a simple pendulum is ……………………. of the mass of the bob and depends upon the ……………………. of

the pendulum. (Ans.: independent / length)

The resistance of light dependent resistor is …………when it is in light and …………when it is in shade. (Ans.: low / high)

For constructive interference, the ……………………. difference between two waves reaching the point should be zero or an

……………………. multiple of λ (wavelength). (Ans.: path / integral)

The ……………… decay process provides a clear evidence that neutrons and protons are not elementary particles. (Ans.: Beta)

Capacitors store energy in the form of ………………… (Ans.: electric field)

Energy loss in a transformer due to humming sound by loose components is called ………………… (Ans.: magnetostriction)

The quantum of a light wave of frequency ‘f’ has energy equal to……………………. (Ans.: hf)

(c) Match each item of Column A with the most appropriate item of Column B. Rewrite the correct pairs by writing the number and

the corresponding alphabet in the spaces provided. For example, (a) – (xi)

Column A Column B

(a) Angular frequency

(b) Critical temperature

(c) Capacitors in series

(d) Frequency of rotor current

(e) Photocells

(f) Anti-quarks

(g) Turbulent flow

(h) Street lamp

(i) Damped simple harmonic motion

(j) Resonance condition

vi. 2π/T

v. superconductor

i. same charge

viii. f/ = S × f

vii. electric current

iv. opposite charge

ix. critical velocity

ii convex mirror

iii. bvFd

xii. ωd = ω/

(d)

(i)

(ii)

(iii)

(iv)

(v)

Correct the following statements.

At absolute zero temperature, the gas molecules inside a container move randomly.

At absolute zero temperature, the gas molecules inside a container do not move.

When a charged capacitor is filled completely with a metallic slab, its capacitance becomes zero.

When a charged capacitor is filled completely with a metallic slab, its capacitance becomes large or infinity.

Diffraction of visible light is a very faint effect because its amplitude is very small.

Diffraction of visible light is a very faint effect because its wavelength is short or small.

All types of quarks have a topness of +1.

Only the top quarks have a topness of +1.

The northern lights that illuminate the regions near the North Pole are also known as northern stars.

The northern lights that illuminate the regions near the North Pole are also known as Aurora Borealis.

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(e)

(i) Answer the following questions.

The diameter of a ball A is half of that of ball B of the same material. What will be the ratio of their terminal

velocities in water? Ans.: Let radius of B be r

Therefore, radius of A = 2

r

2

T r

2

2

terminal velocity of A 2

terminal velocity of B

r

r

2

2

1

4

r

r

11: 4

4

(ii) What is forced oscillation? Ans.: When a body oscillates with the help of an external periodic force with a frequency which is different from the natural frequency of the

body, such oscillation is known as forced oscillation. (iii)

There are two identical neutral gold coins. How many electrons should be transferred to one of the coins to

make them attract each other? The charge supplied is 3.2 C.

Ans.:

19

Q = ne

Q 3.2n =

e 1.6 10

19n = 2 10

(iv)

State two factors on which permeance depend.

Ans.:

Length of the magnetic path

Cross-sectional area (v)

Dawa removed a wire from a rod heater and another from a computer. The resistance of both wires were the same at room

temperature. When heated the resistance of the heater wire increased and that of the computer decreased. What conclusion can be

drawn regarding the nature of the wires? Support your answer with reasons.

Ans.: Heater wire is a conductor, resistance of a conductor increases with increase in temperature.

Computer wire is a semi-conductor, resistance of a semi-conductor decreases with increase in temperature.

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(vi) Tashi combined a convex lens of focal length 20 cm with a concave lens of focal length 15 cm. Then he tried to

focus the image of a distant tree on a white screen through the combined lenses. Will he be able to observe the

image on the screen? Give a reason.

Ans.: No. It is because the combined lenses behave as a concave lens.

OR

No

21

21

ff

ffF

1520

)15(20

F cmF 60

(vii) The graph given below represents the position of a ball attached to a spring oscillating in simple harmonic

motion. When will the ball have maximum acceleration?

Ans.: 2 sec and 6 sec

(viii)

What is enrichment?

Ans.: Enrichment is the process of increasing the percentage of 235U in a sample of uranium.

2.(a)

(i)

A resistor of 10 Ω, a coil of 5 mH and a capacitor of 10 μF are connected in series. When this combination is

connected to an AC source of appropriate frequency, resonance takes place in the circuit.

Find the resonant frequency. -3

-6 -5

L= 5 mH = 5 10 H

F = 10 μF = 10 10 F or 1 10 F

During resonance

LCf

2

10

530

1011052

1

f = 711.8 Hz OR 712 Hz

(ii) If the resistance is halved, what will be the resonant frequency?

Ans.: Resonant frequency will remain unchanged.

Time (sec) Po

siti

on

(cm

)

8 6 4 2 0

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(b)

When the plates of a charged capacitor are connected to a voltmeter and then separated, what will happen to the

reading of the voltmeter.

Ans.: It will increase because on increasing the plate separation, the capacitance will decrease

d

AC 0

and the potential difference will

increase Q

VC

.

(c)

Pema wants the first dark fringe above and below the central maximum on a screen at an angle of diffraction of 30˚ using a

monochromatic light of wavelength 6200 A . What should be the size of the slit to achieve this pattern?

Ans.:

a sin θ = λ

λa =

sin θ

-7

0

6.2 10a =

sin 30

-6a = 1.24 10 m

(d)

(i)

(ii)

Name the term which refers to

quasi-neutral gas. Ans.: plasma

amount of solar energy. Ans.: Solar insolation

(e)(i)

Name the condition where the object and image formed by a spherical mirror coincide with each other.

Ans.: The condition is called auto-collimation.

(ii) To achieve this condition, where should the object be placed on the principal axis?

Ans.: The object should be placed at the centre of curvature of the mirror or at its pole.

3.(a) How does the motion of planets around the sun differ from the motion of the pendulum of a clock? Explain.

Ans.: The motion of planets is periodic motion which repeats after a definite interval and does not have a fixed point.

Motion of pendulum is an oscillatory motion of a body which moves back and forth repeatedly about a fixed point in definite interval.

(b)

While watering plants, we usually close the opening of the hose with our fingers to increase the speed of the water. Why? Support

your answer with an equation.

Ans.: Because speed of water depends on cross-sectional area through which water flows.

A1V2 = A2V2 ; A1/A2 = V1/V2

(c) Waves from coherent sources intersect at a certain point. Does constructive or destructive interference occur

at this point when one wave travels 2 m and another wave travels 2.5 m? Support your statement.

Ans.: Constructive interference will occur, if the path difference is integral multiple of λ.

OR Destructive interference will occur, if the path difference is odd multiple of 2

.

OR It depends on how the difference in path lengths is related to the wavelength of the waves.

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(d)

What is nanosensor?

Ans.: Nanosensor is a nano device that uses chemical, biological or mechanical sensory points to detect and transmit the

information about the presence of chemical species and nano particles.

OR monitors physical parameters such as temperature, displacement and flow of nano scale.

(e)

(i)

A three-phase induction motor runs at 1500 rpm on no load and at 900 rpm on full load when supplied with

50 Hz three-phase supply. Determine the:

number of poles and

S

120f 120 150P =

N 1500

= 4

(ii) slip at full load.

S

S

N -N 1500-900S=

N 1500 = 0.4

(f)

Compare the photon energies of a radio wave emitted from a radio station and microwave emitted from airport

radar when they fall on an alkali metal.

Ans.: Energy of photons emitted by microwave will be higher than energy if photons emitted by microwaves.

4.(a)

State Charle’s law.

Ans.: Charle’s states that the volume of a gas is directly proportional to the kelvin temperature at constant pressure.

(b)

What are Kirchhoff’s laws generally called? What basic principles are these laws based on?

Ans.: 1st Law: Junction Rule and 2nd Law: Loop Rule

1st Law is based on the conservation of charge or current, 2nd Law is based on the conservation of energy or potential difference.

(c)

Is nuclear fission a boon or a bane for society? Justify with three reasons.

Ans.: It’s a boon. Nuclear fission is a clean form of energy that can be utilised in place of fossil fuel to generate electricity.

Nuclear fission isotopes is used to produce radio isotopes which are used in the medical field for treatment of many diseases.

OR It’s a bane because it can be used to make destructive weapons like atom bomb and hydrogen bomb. It is dangerous for the workers due to

radiation. Nuclear power plant accidents possess a greater danger.

(d) Dema rubbed a piece of glass on a silk cloth and kept them apart at a distance of 0.01 m. Using a special instrument she found that

the force of attraction between them was 20 N. What was the charge developed on each piece?

Ans.: 2

1 2

2

q q FrF = k q=

r k

1 2q =q = q = -q (charges developed on each piece will be equal but opposite)

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(e)

A shaving mirror has a radius of curvature of 35.0 cm. It is so arranged that the upright image of a man’s face

is 2.50 times the actual size of the face. How far is the mirror from the face?

Ans.: R 35

f = 17.5 cm2 2

ii 0

0

hGiven: h = 2.5h 2.5

h ;

uv

h

h

u

v

o

i

5.2

5.2

;

uvf

111;

1 1 1= -

-17.5 2.5 u u

26.25u =

2.5

; u = 10.5 cm

5.(a)

(i)

A particle executes linear simple harmonic motion of amplitude A.

What is the ratio of the kinetic energy and total energy when the displacement is half of the amplitude A?

Ans.: A

y = 2

; 22

222

8

3

)(2

1

Am

yAmEk

; 22

2

1AmET ; 4:3

4

3

218

3

22

22

Am

Am

E

E

T

K

(ii)

At what displacement will the kinetic energy be equal to the potential energy?

Ans.: K.E = P.E; 22222

2

1)(

2

1ymyAm ;

2 2 2

2 2

A - y y

A 2y

;

2A Ay =

2 2

(b)

Explain how Lenz’s law is in accordance with the principle of conservation of energy.

Ans.: When a magnet is brought towards a coil, current is induced in the coil which opposes the movement of magnet towards the coil.

Therefore, work is done against this force to bring the magnet closer. This mechanical energy (work) is converted into electrical energy in the

coil.

(c)

During long distance travel, the heat indicator on the dashboard of the car glows. How does such an indicator work?

Ans.: Thermistor is connected to indicator which is used to find temperature of coolant and oil by which one is able to know if the vehicle is

over heating or not. It gathers information about the level of heating in device and acts as temperature sensor.

(d)

Give two differences between decay and decay.

decay decay

A proton transforms into a neutron.

A positron and a neutrino are emitted.

An up quark in the proton transforms into a down

quark.

A neutron transforms into a proton.

An electron and an anti-neutrino are emitted

A down quark in the neutron transforms into an up quark.

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e)

The graph given below represents the annual variations in day length for locations at the equator, 30˚, 60˚ and 90˚ North Latitude.

What are the locations on earth that receive maximum insolation in the month of November and June?

Ans.: November: equator; June: 90˚N

6.(a) If you are given a concave or convex mirrors, which one will you prefer to use as a rear mirror in your car. Why?

Ans.: Convex mirror, because it gives erect and small image of all objects at different distances thereby giving a wider field view of the traffic

at the back.

(b) (i)

Derive an expression for acceleration of a particle executing simple harmonic motion.

)()2cos()( iftAtx

Differentiating eqn. (i), we get

)()2sin()2()(

)2cos()(

iiftAftv

ftAdt

d

dt

dxtv

Differentiating eqn. (ii), we get

dv d

a(t) = = -(2 f) A sin (2 ft)dt dt

2a(t) = (2 f) A sin (2 ft)

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(c)

(i)

An LCR circuit has the following information: R = 30 Ω, reactances XL = 40 Ω, XC = 0 Ω and emf of AC

source = 220 V. Calculate the:

impedance

2 2

LZ = R + X

2 2= 30 + 40 50

(ii)

phase difference between emf ad current

LXtan =

R

-1

0

4 = tan

3

53

(d)(i)

What are matter waves?

Ans.: Waves associated with moving particles of matter.

(ii)

How are wavelength and momentum of matter waves related?

Ans.: Wavelength and momentum are inversely related or proportional.

h=

P

(e)

State Bernoulli’s principle.

Ans.: For a streamlined flow of an ideal liquid, the total energy per unit mass remains constant at every cross-section throughout the liquid

flow.

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7.(a)

With the help of a diagram, deduce an expression for the height of water in a capillary tube.

Ans.: From the diagram, pressure difference between two sides of the top surfaces is

i 0

i 0

2SP - P =

r

ar=

cosθ

2SCosθP - P = (i)

a

Consider two points A and B which have same pressure

(ii)-------0

0

ghPP

PPghP

i

ai

(i) = (ii)

2SCos

a

2SCosh =

h g

h g

(b) What is pair annihilation? Write an equation to represent it.

Ans.: When a particle and its anti-particle collide with each other, they annihilate and release a large amount of energy in the form of

radiation. 2 ee

(c)

(i)

(ii)

When a radiation produced by hydrogen gas is passed through a prism,

what kind of spectrum is formed? Ans.: Emission spectrum is produced when passed through a prism.

name the spectrum produced when this radiation is passed through cold gas. Ans.: Absorption spectrum is produced when passed

through cold gas.

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(d)

Determine the equivalent capacitance between points A and B of the

combination of capacitors shown in the

figure below.

1

1

1 1 1 1 3+ =

C 3 3 3 3

C =1μF

2C =5 + 1 = 6μF

3

3

1 1 1 1 5+ =

C 3 6 3 6

6C = =1.2μF

5

(e)

During an earthquake, it was observed that most of the intermediate height buildings collapsed while taller and

shorter buildings remained erect. What conclusion can be drawn from this observation?

Ans.: The angular frequency of the seismic waves was more close to the natural frequency of the intermediate buildings. The taller buildings

had lower resonant angular frequency and shorter buildings had higher resonant angular frequency.

(f)

What is Sunspot cycle?

Ans.: The total number of sunspots has long been known to vary with an 11-year period known as sunspot cycle.

8.(a) State any two characteristics of oscillatory motion.

Ans.:

When particle in equilibrium is disturbed, it has a tendency to return to its equilibrium.

Force is always opposite to the displacement vector of the particle with respect to mean position.

Energy is also conserved in the motion. If energy is not conserved then the particle will not be able to repeat the parameters of the

motion.

(b)

Write down two properties of electric charges.

Ans.: Additive nature of electric charge, Conservation of electric charge, Quantization of charge.

(c) Calculate the disintegration energy Q of a uranium 235

95 U nucleus. Mass of uranium nucleus is 235.04 u.

P Pm = Zm + Nm M

= (95 × 1.007276 + 140 × 1.008665 – 235.04) = 1.86432 u

Q = Δm × 931 MeV = 1735.68 MeV

B

5μF

D

C A

3μF

3μF

3μF 3μF

3μF

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(d) Differentiate between step-up and step-down transformer in terms of current and voltage in the table given

below.

Step-up transformer Step-down transformer

Output current is greater than the input current. Output current is less than the input current.

Output voltage is less than the input voltage. Output voltage is greater than the input voltage.

(e)

If the pressure increases to 500 mm Hg, what will happen to the volume of a balloon filled with 0.5 litre of

oxygen gas collected at 450 mm Hg?

Ans.:

1 1 2 2

1 12

2

P V = P V

P VV =

P

; 450 0.5

= 0.45 litres500

so it will decrease.

(f)

The figure given below represents a light ray undergoing multiple reflections from a mirrored corridor. If the

initial angle of incidence is 60˚, what will be the angle of reflection when the ray makes its final reflection?

Ans.: 30˚