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1. BHSEC PHYSICS
THEORY PAPER
December 2018
Marking Scheme

2. i) When a proton and an electron are placed in a uniform electric field, both will
experience forces of
A equal magnitude in the same direction.
B equal magnitude in opposite directions.
C unequal magnitude in the same direction.
D unequal magnitude in opposite directions.
Question 1 (a) MCQ
B equal magnitude in opposite directions.
ii) An alternating emf is given by E = 200 sin (50πt) volt. The rms value of the
emf is
A 282 V.
B 200 V.
C 141 V.
D 50 V.
C 141 V

3. iii) 5 g of oxygen is enclosed in a vessel at pressure P and volume V. If there is
no exchange of heat between the gas and the surrounding, which curve shows
the relation between pressure and volume?
A Curve I
B Curve II
C Curve III
D Curve IV
A Curve I
iv) A liquid does not wet the surface of a solid, if the angle of contact is
A zero.
B right angle.
C acute angle.
D obtuse angle.
D obtuse angle.

4. v) Which of the following equation does NOT represent simple harmonic motion?
A x(t) = A cos (2πft)
B x(t) = A cos (2πft + ϕ)
C x(t) = A sin (2πft) cos (2πft)
D x(t) = A sin (2πft) + B cos (2πft)
C x(t) = A sin (2πft) cos (2πft)
vi) The equivalent capacitance between points X and Y is 1μF. The capacitance
of each identical capacitor is
A 4μF.
B 1μF.
C 0.25μF.
D 0.05μF.
A 4μF

5. vii) When a metallic core is inserted along the axis of a solenoid, its self-
inductance will
A be zero.
B decrease.
C increase.
D remain constant.
C increase
viii) Which of the following produces a virtual image larger than the object?
A concave and convex mirrors
B plane and concave mirrors
C concave mirror
D convex mirror
C concave mirror

6. ix) The instrument used to study the spectra of the galaxies is
A spherometer.
B spectrograph.
C telescope.
D binocular.
B spectrograph.
x) When a photon interacts with matter, it transfers
A momentum.
B electrons.
C intensity.
D protons.
A momentum

7. xi) Two particles execute SHM as shown in the figure below. Which of the
following statement is TRUE for waves A and B?
5
5
time (s)
time (s)
Displ
acem
ent
(m)
Wave A
Wave B
10
10
A Amplitude of wave A is greater than wave B.
B Amplitude of wave A is shorter than wave B.
C Frequency of wave B is greater than wave A.
D Frequency of wave B is less than wave A.
D Frequency of wave B is less than wave A.

8. xii) A proton is an example of
A meson.
B quark.
C baryon.
D lepton
C baryon.
xiii) In the equation given below, identify the hydrogen isotope ‘X’.
A heavy water
B deuteron
C tritium
D proton
2 4 1
1 2 0
H+X He+ n+energy

C tritium

9. xiv) A p-type semiconductor is obtained by doping with
A phosphorus.
B aluminium.
C antimony.
D arsenic
B aluminium
xv) The focal lengths of two double convex lenses are fA and fB respectively.
Their combined focal length will be
A .
B .
C .
D .
A B
A B
f + f
f f
A B
A B
f f
f + f
A B
B
f + f
f
A B
A
f + f
f
B A B
A B
f f
f + f

10. Q1 (b) Fill-in-the-blanks with appropriate words.
i) In ……………………. SHM, the oscillation is reduced due to friction and
its ……………………. always decreases.
damped / amplitude --------------- ½ + ½
ii) In a temperature sensor used to detect low temperatures, the output voltage
is ……………………. in hot condition and ……………………. in cold
condition.
low / high ----------------------½ + ½
iii) In Fresnel’s biprism experiment, ……………………. pattern is produced
with the help of ……………………. sources.
interference / virtual -------------------½ + ½
iv) The top and bottom quarks are known as the ……………………. generation
quarks.
Third -------------------½
v) In an uncharged capacitor, the resistance is always ……………………..
Zero ------------------½
vi) The voltage stabilization on fluctuating loads will be ……………………..
in three phase power systems.
maximum ------------------½

11. vii) The energy of a photon is ……………..proportional to its wavelength.
Inversely ---------------½
Q 1 (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) Simple harmonic motion
(b) Semiconductor
(c) Time constant
(d) Reluctance
(e) Energy
(f) Quarks
(g) Stoke’s law
(h) Unit magnification
(i) Transformer
(j) Resonance
vii. cosine function
x. thermistor
i. RC
ix. AT/Wb
ii work function
iii. hadrons
vi. viscosity
xi. autocollimation
iv. mutual induction
v. maximum amplitude

12. Q 1 (d) Correct the following statements.
i) The average speed of the molecules of an ideal gas is given by .
M
RT
vavg 
The average speed of the molecules of an ideal gas is given by .
πM
RT
vavg
8

ii) The plates of a capacitor are generally separated by a semiconducting material.
The plates of a capacitor are generally separated by an insulating material.
iii) The intensity in a double-slit interference is directly proportional to the
amplitude of the interfering waves.
The intensity in a double-slit interference is directly proportional to square of
the amplitude of the interfering waves.
iv) The electromagnetic force is a universal force and acts between masses.
The gravitational force is a universal force and acts between masses.
v) Solar insolation is a form of disturbance that can affect radio communication.
CME(Coronal mass ejection) is a form of disturbance that can affect radio
communication.

13. Q1 (e) Answer the following questions.
i) Water emerges from a horizontal pipe at a rate of 0.5 × 10-3 m3/s. Find the
velocity of water at a point in the pipe where the area of cross-section is 12.6
m2. [1½]
1 1 2 2
1 1
2
2
3
2
A = A
A
=
A
0.5 10
12.6
v v
v
v
v



Ans:
---------------------------½
05
2
5
3.97 10 /
4 10 /
v m s
OR
m s


 

--------------------½ + unit ---- ½

14. ii) What is oscillatory motion? [1]
Ans: A motion in which a body moves back and forth (or to and fro) repeatedly
about a fixed point in a definite interval of time. --------------- 1
iii) Draw electric field lines due to an isolated negative point charge. [1]
Ans:
Correct diagram ------------------ ½
Correct direction of arrows ------ ½
iv) How does mutual inductance of two closely wound solenoids depend on the
area of each coil? [1]
Mutual inductance of two closely wound solenoids is directly proportional
to the area of each coil. ---------------------------------1

15. v) Which type of electric source, AC or DC is better? Justify your answer. [2]
Ans: AC source, --------------------------------------------- ½
because there is minimum loss of energy and it can be easily converted to
DC. (Any one reason)-----------------------------------------------1½
OR
D.C source, --------------------------------------------½
because it is less dangerous than A.C and most of the electronic appliances use DC.
(Any one reason)--------------1½
(Remark: Mark for the first part will be awarded only if the justification is given.)
vi) When two convex lenses are combined, the magnification of the combination
is found to be greater than the magnification of the individual lenses. What do
you think will happen to the magnification if two concave lenses are combined?
Support your answer with appropriate explanation. [1½]
Ans : The magnification will increase. -----------------------------------------½
Because when lenses are combined, the magnification of the combination is equal
to the product of the magnifications of the individual lenses. ----1

16. vii) Define the term thermal neutron. [1]
Ans: A neutron is said to be thermal neutron when it is in thermal equilibrium with
the environment. ------------------------------ 1
viii) A boy is playing on a swing. After a while, his sister joins him on the
same swing. How will this affect the time period of the swing? Explain. [1]
Ans: The time period is independent of mass OR ----------------½
So the time period remains constant / same. -----------------------------------½
L
T = 2π
g
Question 2
a) An electric shock is felt when we touch a wire connected to a capacitor. The
shock from a 2 μF capacitor is more intense than that from a 0.02 μF capacitor
when connected to the same potential. Why? [2]
Ans: The capacitance of a capacitor is directly proportional to the charge . ---1
Greater the charge, more electric shock is experienced. -------------------- 1
Q
C =
V
 
 
 

17. b) Define solar irradiation. [1]
Ans: It is defined as the instantaneous solar power received per unit area by the
earth’s surface at a particular location and it is variable throughout the day.------1
c) An alternating emf of 100 V – 50 Hz is applied across a capacitor of reactance
318 Ω and a resistor of resistance 100 Ω in series. Calculate the current flowing
through the circuit. (L = 0) [3]
A
I
I
Z
V
I
Z
Z
Z
X
R
Z C
3
.
0
333
100
333
111124
.
318
100 2
2
2
2










Ans:
-----------------------------½
--------------------------- ½ + ½
----------------------------½
------------------------------½ + ½

18. d) Derive an expression for fringe width in Young’s interference pattern. [2]
max
m D
Let Y =
d

Ans:
d
D
m
Y

)
1
(
1
max



max
1
max Y
Y
y 


 

λD
β =
d
------------------------------½
------------------------------½
-----------------------------½
----------------------------½
e) State any TWO sign conventions for spherical mirrors. [2]
Ans: ANY TWO -------------------------------------------------------------- 1 x 2
 The principal axis of the mirror is taken as x-axis and the pole as the origin.
 The direction of the incident light is taken as positive direction of x-axis.
 The upward direction is taken as positive and the downward direction as
negative.

19. Question 3
a) It is more difficult to drain a cup of honey than a cup of water. Why? [2]
Ans: Honey has higher viscosity than water, due to this honey experiences more
internal frictional force than water. -------------- 1 + 1
b) At what positions will the velocity and acceleration of a pendulum bob be
minimum and maximum when a pendulum bob executes SHM? [2]
Ans: Maximum velocity = mean position,
Minimum velocity = extreme positions, ----------- 4 x ½
Maximum acceleration = extreme positions,
Minimum acceleration = mean position.
c) Can sustained interference take place from two sources of light? Give a
reason to support your answer. [2]
Ans: Yes, two sources of light can produce sustained interference. --------------------½
It will be possible if two sources are coherent sources / in phase with each other. ---1½
OR
No, two sources of light cannot produce sustained interference. --------------------½
If two sources of light are independent sources / polychromatic sources / out of phase
with each other. ----------------------------------------------------------- 1½
(Remark: Mark for the first part will be awarded only if the justification is given.)

20. d) State two properties of a positron. [1]
 Positive charge, --------------- 1 x ½
 Mass is same as the mass of an electron.
e) The current through a 1 mH coil drops from 5 A to 3 A in 1 ×10-3 s. Calculate
the emf developed in the coil. [2]
dI
e = -L
dt
Ans:
-3
-3
2
e = -1 10
10
 
e = - 2 V
--------------------------------½
-------------------------------½
-------------------------------½ + ½
(Remark: Negative sign is not necessary.)
f) Photoelectric emission is not possible at all frequencies. Why? [1]
Ans: If the frequency of the incident light is less than the threshold frequency
then the energy of incident photons (E=hf) will be less than the work function of
the metal and photons will not be emitted. ---------------- 1

21. a) Write the mathematical expression for the ideal gas law. [1]
Question 4
Ans : PV = nRT ----------------------1
b) The electrical conductivity of a pure semiconductor increases on
increasing the temperature. Explain the statement. [2]
Ans: As temperature increases, more electrons in the valence band gain energy,
And more electrons move to the conduction band and hence conductivity increases.
------------------------------------------------------------1+1
OR As temperature increases more covalent bonds break due to agitation,
releasing more free electrons and hence conductivity increases.
c) Electricity can be generated from hydropower plants as well as nuclear power
plants. Which power plant is better for generating electricity and why? [2]
Ans: Hydro power --------------------------------------------------------½
because (ANY ONE) --------------------------------------------------1½
 it is renewable source of energy.
 the source of energy is easily available.
 there is less pollution.

22. OR
Nuclear plant ---------------------------------------------------------------------½
because (ANY ONE) ----------------------------------------------------------1½
 it can be constructed at any place.
 huge amount be of electrical energy can be obtained.
( Remark: Mark for the first part will be awarded only if the justification is given.)
d) Two point charges each of magnitude 1.6 × 10-19 C are separated by a distance
of 3 ×10-15 m. Find the magnitude of the force. [2]
2
2
1
r
q
q
k
F


 2
15
2
19
9
10
3
)
10
6
.
1
(
10
9






F
F= 25.6 N
Ans:
---------------------------------½
---------------------------------½ + ½
---------------------------------½

23. e) With the help of a ray diagram, derive a mirror equation. [3]
Ans :
Diagram ------------½
From similar triangles OaF and FPd,
o i
h h
-(u-f) -f

u
f
f


o
i
h
h
Or ---------(i) ------------------½
Also from the similar triangles IbF and FPC,
o
i h
-h
-(v-f) -f
 i
o
h f-v
h f

Or ---------(ii) ------------------½
From equation (i) and (ii),

24. 2 2
f f-v
f-u f
f = f + uv - uf - vf
 --------------------------------½
uv = uf + vf
uv
u + v
f

--------------------------------½
Dividing both sides by uv, we get,
1 1 1
f v u
  --------------------------------½

25. a) A simple harmonic motion is represented by an equation: 20y = 0.1 sin50 πt,
where the displacement y is in metre and the time in second. Find the
amplitude and frequency of the particle. [3]
Question 5
--------------------------------------½
t
y 
50
sin
1
.
0
20 
y = 0.005 sin 50 t

a = 0.005 m -------------------------------------½ + ½
--------------------------------------½
f

 2

50 π = 2πf
50
f = = 25 Hz
2
-------------------------------------½ + ½
Ans:
b) Explain Lenz’s law with the help of a diagram. [2]
Ans: Diagram (Any diagram which gives an idea about Lenz’s law) ----1

26. If the north pole of a bar magnet is brought closer to a coil, the coil will act as the
north pole, So the coil opposes the cause which produces it. --------------½+½
OR
Definition and diagram of polarity. ---------------------------------------------- 1 + 1
c) How is sensitivity of a sensor different from the resolution of a sensor? [2]
Ans: The sensitivity of a sensor is a measure of the change in the sensor output that
occurs when the quantity being measured changes by a given amount whereas the
resolution of a sensor is the smallest change in the magnitude of the input signal
that can be detected by a sensor.------------------------------2
d) Differentiate between quark and leptons. [2]
(ANY TWO) ------------ ----------------------- 1 x 2
Quark Leptons
 It is present in neutrons and
protons.
 There is strong attractive force
between quarks.
 It cannot exist freely.
 Examples are electrons, positrons,
neutrino and antineutrino.
 There is no strong interaction
between leptons.
 They can exist freely.

27. e) Explain the variation of velocity of plasma with respect to its mass. [1]
m
kT
2
v 
Ans: -----------------------------------------------------½
The velocity is inversely proportional to mass of plasma
1
V
m

 
 
 
------½
a) Derive an expression for potential energy of a particle in SHM. [2]
Question 6
x
f
m
F
ma
F
x
f
a
2
2
)
2
(
)
2
(







-------------------------½
For small displacement dx, the work done is given by,
Fdx
dW 

2
= m(2 f) d
x x

-------------------------½

28. 2
0
W = (2 ) d
x
m f x x


2
2
0
= m(2 f)
2
x
x

 
 
 
2 2
1
= m(2 f)
2
x
 -------------------------½
2 2
1
W = P.E = m(2 f)
2
x
 -------------------------½
b) Both convex and concave mirrors can be used for shaving. Which mirror is
preferred? Why? [2]
Ans: Concave mirror, -----------------------------------------------½
because the image formed by concave mirror is virtual and magnified whereas
image formed by convex mirror is virtual and diminished, so we can see our
face clearly in concave mirror. ----------------------------------1½
c) In the given electric circuit, use Kirchhoff’s laws to find currents I1 and I2. [3]
A B C
D
E
F
11
12
I = 3A
5 Ω 3 Ω
2 Ω
17 V 21 V

29. Using junction rule at junction E,
1 2
I + 1 = 3A
---------------------------------------------- ½
Using loop rule and going counterclockwise in loop ABEFA,
A
I
I
I
I
1
2
2
0
2
2
0
17
15
2
1
1
1
1







---------------------------------------------- ½
---------------------------------------------- ½
A
I
A
A
I
I
A
I
2
1
3
3
2
2
1
2





---------------------------------------------- ½
---------------------------------------------- ½ + ½
d) When the intensity of light falling on a photocell is increased, what will be its
effect on the:
(i) current flowing out of the cell? [1]
Ans: The current flowing out of the photocell increases. ----------------1
(ii) minimum negative potential required to stop the current? [1]
Ans: The minimum negative potential (stopping potential) required to stop the
current remains constant (or unchanged). ----------------------1

30. e) Define terminal velocity. [1]
Ans: The maximum constant velocity acquired by a freely falling body in a
viscous medium. ----------------------------------1
Question 7
a) A glass capillary tube of diameter 2 × 10-4 m is dipped in mercury whose
surface tension is 0.547 N/m and density is 13500 kg/m3. By how much will
the mercury be depressed in the capillary tube, if the angle of contact of
mercury with glass is 135˚? [2]
2Scos
h
a g


 -----------------------------------------½
4
2 0.547 cos135
1 10 13500 9.8
h 
 

  
2
5.8 10
h m

 
-----------------------------------------½
-----------------------------------------½ + ½
Ans:

31. b) Give four applications of nanotechnology. [2]
 Carbon nanotubes (multi walled carbon nanotubes) ---------- 4 x ½ = 2
 Nano electronics (computer chip / television)
 Nano sensors (pollution monitoring / monitoring electrical change)
 Nano textiles (stain and wrinkle proof textiles / anti-bacterial or antifungal
socks)
c) Obtain an expression, for energy stored in a charged capacitor? [2]
2
2
1
CV
E 
dq
C
q
dq
v
'
'
'
dW 



q
dq
C
q
0
'
W
C
q
W
2
2

------------------------------------- ½
------------------------------------- ½
------------------------------------- ½
Using q = CV and W = E,
2
2
1
CV
E  ------------------------------------- ½

32. d) Study the diagram given below: [1]
Name the phenomenon shown in the experimental set-up given above
Ans: Electron diffraction-----------------------------1

33. e) Which of the following examples represent SHM and periodic motion? [2]
i) Rotation of the earth about its axis. = Periodic motion ------- 4 x ½ = 2
ii) Motion of a bob in a pendulum. = SHM
Motion of a ball bearing inside a smooth curved bowl. = SHM
iii) Motion of the hands of a clock. = Periodic motion
f) Explain the term radio blackout. [1]
Ans: Radio blackouts occur when the strong, sudden burst of X-rays from a
solar flare hit the earth’s atmosphere, blocking both high and low frequency
radio signals. This disturbs the radio propagation / communication. -------1
Question 8
a) i) Explain the term forced oscillation. [1]
Ans: When a body oscillates with the help of an external periodic force with a
frequency different from the natural frequency of the body, it is called forced
oscillation. ------------------------1

34. ii) State the condition which leads to resonance in oscillating bodies. [1]
Ans: It leads to resonance when the frequency of applied periodic force is equal to
or very close to the natural frequency of the oscillating body.----------------1
b) What type of path is followed by a moving charged particle in uniform
electric field? [1]
Ans: It follows parabolic path. ----------------------------1
c) A stationary conducting loop is held perpendicular to a field between the
pole pieces of a fixed permanent magnet. Is the current induced in the loop?
Why? [2]
Ans: No, current cannot be induced, ---------------------------------- ½
because current is induced in the loop only when there is a change in the
magnetic flux linked with the loop. -------------------------------------- 1½
d) Obtain the average kinetic energy equation, [2]
kT
2
3
Kavg 
.
2
2
1
rms
avg mv
K 
avg
1 3RT
K = m
2 M
  
  
  
---------------------------------- ½
---------------------------------- ½

35. A
N
RT
2
3
Kavg 
kT
2
3
Kavg 
---------------------------------- ½
---------------------------------- ½
e) Draw a ray diagram to show the virtual focus of a convex mirror. [1]
Ans: Outline diagram -------------------------------------------½
Labelling and arrows --------------------------------------------½

36. f) Calculate the mass defect of helium nucleus and its binding energy
in MeV. [2]
4
2 He
 
 
Ans: Mass defect (Δm) = mass of nucleons – mass of nucleus
= (mass of 2 protons + mass of 2 neutrons) – mass of nucleus -----½
= [2 × 1.007276 + 2 × 1.008665 – 4.001506]u
= 0.030376 u --------------------------------------½
Binding energy = Δm × 931.5 MeV --------------------------------------½
= 0.030376 × 931.5
= 28.3 MeV -----------------------------------------½
[½ mark may be deducted if units are missing.]