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Sample Paper (CBSE)

Series PHY/SP/1B Code No. SP/1-B

SP/1-B ©Educomp Solutions Ltd. 2014-15

PHYSICS (Theory)

Time Allowed: 3 hours Maximum Marks: 70

General Instructions:

(i) All questions are compulsory.

(ii) This question paper has five sections: Section A, Section B, Section C, Section

D and Section E.

(iii) Section A contains five questions of one mark each, Section B contains five

questions of two marks each, Section C contains twelve questions of three

marks each, Section D contains one value based question of four marks and

Section E contains three questions of five marks each.

(iv) There is no overall choice. However, an internal choice has been provided in

one question of two marks, one question of three marks and all three questions

of five marks each weightage. You have to attempt only one of the choices in

such questions.

(v) You may use the following values of physical constants wherever necessary:

c = 3 × 108 m/s

h = 6.63 × 10-34 Js

e = 1.6 × 10-19 C

μo = 4 × 10-7

T mA-1

1

4 o = 9 × 109 N m2 C-2

me = 9.1 × 10-31 kg

Mass of Neutrons = 1.675 × 10-27 kg

Mass of proton = 1.673 × 10-27 kg

Avogadro’s number = 6.023 × 1023 per gram mole

Boltzmann constant = 1.38 × 10-23 JK-1


Section A

1. How can a galvanometer be converted into voltmeter?

2. What happens to then drift velocity of electrons, if length of a

conductor is doubled keeping the potential difference constant?

3. The electric field part of an electromagnetic wave in a medium

is represented by

Calculate the wavelength of the wave.

4. What is the rest mass of photon?

5. Which type of biasing gives a semiconductor diode very high

resistance?

Section B

6. Draw a plot of the binding energy per nucleon as a function of

mass number for large number of nuclei, 2 ≤A≤240. How do

you explain the constancy of binding energy per nucleon in the

range 30 ≤A≤170 using the property that nuclear force is short

ranged?

7. Give the direction in which the induced current flows in the

coil mounted on an insulating stand, when a bar magnet is

1

1

1

1

1

2


quickly moved along the axis of the coil from one side to the

other as shown in the figure.

8. Draw the graph showing the variation of reactance of a

capacitor and an inductor with the frequency of an a.c. circuit.

9. An object is kept in front of a concave mirror of focal length 15

cm. The image formed is three times the size of the object. Find

two possible distances of the object from the mirror.

OR

What is the polarizing angle of a medium of refractive index

.

10. In a meter bridge, the balance point is found to be at 39.5 cm

from the end A, when the resistor Y is of 12.5 ohm as shown in

figure. Determine the resistance of X. What happens if the

galvanometer and cell are interchanged at the balance point of

the bridge?

2

2

2

2


Section C

11. a. Explain, whether an induced current will be developed in a

conductor, if it is moved in a direction parallel to magnetic

field.

b. An electron in an atom revolves around the nucleus in an

orbit of radius 0.53 Å. Calculate the equivalent magnetic

moment, if the frequency of revolution of electron is 6.8 ×

109 MHz.

12. a. Two charges of magnitudes + 4Q and –Q are located at

points (a, 0) and (–3a, 0) respectively. What is the electric

flux due to these charges through a sphere of radius '2a'

with its centre at the origin?

b. Two point electric charges of unknown magnitude and sign

are placed at a distance apart. The electric field intensity is

zero at a point not between the charges but on the line

joining them. Write two essential conditions for this to

happen.

13. a. How is the focal length of a spherical mirror affected when

2

3

3


the wavelength of the light used is increased?

b. If ε0 and μ0 are the electric permittivity and magnetic

permeability in free space, ε and μ are the corresponding

quantities in a medium, then calculate the index of

refraction of the medium.

14. a. Define input resistance of a transistor used in its common

emitter configuration.

b. Draw the truth table of the following combination of gates:

15. By what percentage will the transmission range of a TV tower

be affected, when the height of the tower is increased by 21 %?

16. Two cells of e.m.f. 1.5 V and 2 V and internal resistances 1 Ω

and 2 Ω respectively are connected in parallel so as to send

current in the same direction through an external resistance of

5 Ω. Draw the circuit diagram. Using Kirchhoff’s laws,

calculate the current through each branch of the circuit and

potential across the 5 Ω resistance.

17. Derive an expression for the average power consumed in a

series LCR circuit connected to a.c. source in which the phase

difference between voltage and current in the circuit is φ.

3

3

3

3


Define the quality factor in an a.c. circuit.

18. A convex lens made up of glass of refractive index 1.5 is

dipped, in turn, in

a. medium A of refractive index 1.65

b. medium B of refractive index 1.33.

Explain giving reason whether it will behave as a converging

lens or a diverging lens in each of these two media.

OR

What is the effect on the interference fringes in a Young’s

double slit experiment due to each of the following operations:

a. The screen is moved away from the plane of the slits

b. The separation between the two slits is decreased

c. The monochromatic source is replaced by a source of white

light.

19. Define resolving power of a compound microscope. How does it

change on:

a. decreasing the wavelength of light used and

b. decreasing the diameter of its object lens?

20. Figure shows the variation of stopping potential V0 with the

frequency of incident radiation for two photosensitive metals A

and B.

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3

3

3


a. Which of the metals has larger threshold wavelength?

Give reason.

b. Which metal gives out electrons having larger kinetic

energy for the same wavelength of incident radiation?

Explain why?

c. If the distance between the light source and metal A is

halved, how will the kinetic energy of electrons emitted

change? Why?

21. Explain briefly, with the help of circuit diagram, how a p-n

junction diode work as half wave rectifier?

22. Draw a simple demodulation circuit and explain its working.

Section D

23. Muthuswami a resident of Kundakulam was all set to leave

everything and shift to another place in view of the decision of

Govt. to start nuclear thermal power plant at Kundakulam.

His granddaughter Prachi, a science student was really upset

on the ignorant decision of her grandfather. She could finally

convince him not to shift, since adequate safety measures to

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3

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avoid any nuclear mishap have already been taken by the

Govt. before starting nuclear thermal plants.

a. What is the value displayed by Prachi in convincing her

grandfather?

b. What is the principle behind working of nuclear reactor?

c. Why is heavy water used as moderator?

Section E

24. a. A parallel plate capacitor of capacitance C is charged to a

potential difference V and then the battery is disconnected.

Now a dielectric slab of the dimensions equal to the spacing

between the plates is inserted between the plates. What are

the changes, in the capacitance, charges, potential

difference, electric field and the energy stored?

b. A parallel plate capacitor of capacitance C is charged to a

potential V. It is then connected to another uncharged

capacitor having the same capacitance. Find out the ratio of

the energy stored in the combined system to that stored

initially in the single capacitor.

OR

a. Derive an expression for the maximum torque acting on an

electric dipole, when held in a uniform electric field.

b. Consider two hollow concentric spheres, S1 and S2, enclosing

charges 2Q and 4Q respectively as shown in the figure.

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5


1. Find out the ratio of the electric flux through them.

2. How will the electric flux through the sphere S1 change if

a medium of dielectric constant 'εr' is introduced in the

space inside S1 in place of air? Deduce the necessary

expression.

25. State Biot-Savart law and give the mathematical expression

for it. Use this law to derive the expression for the magnetic

field due to a circular coil carrying current at a point along its

axis. How does a circular loop carrying current behave as a

magnet?

OR

a. Draw a labelled diagram of a moving coil galvanometer.

Describe briefly its principle and working.

b. Answer the following :

1. Why is it necessary to introduce a cylindrical soft iron

core inside the coil of a galvanometer?

2. Increasing the current sensitivity of a galvanometer may

not necessarily increase its voltage sensitivity. Explain,

giving reason.

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5

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26. a. With the help of ray diagram, explain the phenomenon of

total internal reflection.

b. Obtain lens makers formula using the expression

Here the ray of light propagating from a rarer medium of

refractive index to a denser medium of refractive index

is incident on the convex side of spherical refracting surface

of radius of curvature R.

OR

a. Deduce the expression for the refractive index of glass in

terms of angle of prism and angle of minimum deviation.

b. Out of blue and red light, which is deviated more by a

prism? Give reason.

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5


1. By connecting a suitable high resistance in series to the coil of a

galvanometer.

2. The drift velocity will become half of the initial value and resistance

will become double.

3. 200 m

4. Zero

5. Reverse biasing

6.

The constancy of binding energy per nucleon in the range 30 ≤A≤170

shows that a nucleon in the nucleus is not attracted by all the

remaining nucleons. It is attracted only by its some close neighbours

and hence the nuclear force is short ranged.

ANSWERS


7. a. When magnet moves towards the coil, the current flows in clockwise

direction and

b. When magnet moves away from the coil, the current flows in anti-

clockwise direction.

8.

9. a. Use the relations:

b. For real image object distance is – 20 cm and for virtual image object

distance is – 10 cm.

OR

Use the relation

We get i = 60°


10. Use the relation:

We get, X = 8.16 ohm

If the galvanometer and cell are interchanged at the balance point of

the bridge, the galvanometer will not show any deflection as the

condition for a balanced bridge is still satisfied.

11. a. Since Lorentz force on the free electrons in the conductor is zero, no

potential difference is produced across the ends of the conductor and

hence no induced current will be developed.

b. Using the relations:

We get, M = 9.6 × 10-24 Am2

12. a.

b. Both the charges are of opposite sign or nature.

The point where electric field intensity is zero is closer to the

smaller charge.

13. a. No change in the focal length of a spherical mirror on increasing the

wavelength of light used.

b. The index of refraction of the medium is


14. a. The input resistance of a transistor used in its common emitter

configuration is defined as the ratio of small change in base-emitter

voltage to the small change produced in base current at constant

collector-emitter voltage.

b.

A B y’ y’’ Y

0 0 1 1 0

1 0 0 1 1

0 1 1 0 1

1 1 0 0 1

15. Use the formula:

The percentage increase in the transmission range of the TV tower is

10 %.

16.

a. I1 = 0.0294, I2 = 0.2647 and

b. Potential difference across R = 1.4705 V


17. a. The average power consumed in a series LCR circuit connected to

a.c. source in which the phase difference between voltage and

current in the circuit is φ is given by

b. The quality factor of a resonant LCR-circuit is defined as the ratio of

the voltage drop across inductor to the applied voltage.

18. Use the formula:

a. In medium A lens will behave as a diverging lens.

b. In medium B lens will behave as a converging lens.

OR

a. Fringe width will increase as D increases.

b. Fringe width will increase as d decreases.

c. The colours of white light will produce interference pattern. The

central bright fringe is white in colour and the fringes closest on

either side of it is blue and farthest is red.

19. Resolving power of a microscope is defined as the reciprocal of the least

separation between two close objects so that they appear just separated

when seen through the microscope.

a. Resolving power of a microscope will increase on decreasing the

wavelength of light.


b. Resolving power of a microscope will decrease on decreasing the

diameter of its object lens.

20. a. The metal A has larger threshold wavelength as it has smaller

threshold frequency.

b. The metal A gives out electrons having larger kinetic energy for the

same wavelength of incident radiation because it has smaller

threshold frequency and hence smaller work function.

c. If the distance between the light source and metal A is halved, the

kinetic energy of electrons emitted will remain unchanged because

the change in intensity of the incident radiation does not affect the

kinetic energy of the emitted electrons.

21.


When an alternating voltage is applied across a diode in series

with a load resistance, a pulsating output voltage appears across

the load only during the half cycles of the a.c. when the diode is

under forward bias. The corresponding circuit is called a half-

wave rectifier circuit as shown in the image.

When the secondary of the transformer supplies a.c. voltage

across the terminals A and B, then in the positive half-cycle of

a.c. there is a current through the load RL when the diode is

under forward bias.

In the negative half-cycle of a.c. there is no current through the

load RL when the diode is under reverse bias.

The output voltage is in the form of varying pulses restricted to

only one direction.

22.

Working of Demodulation Circuit: The current flows through the

diode only during the positive half cycle of the AM wave. The

envelope of the rectified wave is the information signal (fm). The


signal fm is separated from the carrier wave using the RC filter and

given to an amplifier. The output voltage of amplifier is connected to

a loud speaker which produces the original sound waves.

23. a. Awareness, social responsibility

b. Controlled fission chain reaction

c. Ordinary water absorb neutron at a fast rate. To overcome this

difficulty, Heavy water is used as a moderator which has negligible

cross sections for neutron absorption.

24. a. The capacitance increases, charge remains the same, potential

difference decreases, electric field decreases and energy stored in the

capacitor also decreases.

b. Using the formula:

We get, the ratio of the energy stored in the combined system to that

stored initially in the single capacitor as 1:2.


OR

a.

b. 1. Using Gauss theorem,

2. If S1 is filled with medium of dielectric constant 'εr', then

25. Biot Savart law states that the magnetic field produced due to a small

conductor of length dl carrying a current I at a point at distance r

is given by


Where k is the constant of proportionality and θ is the angle

between the current and the line joining the small conductor to the

observation point.

The magnetic field due to a circular coil carrying current at a point

along its axis is given by,

Where a is the radius of the circular coil, x is the distance from the

centre of the coil, n is the number of turns in the coil and I is the

current flowing in the coil.

The magnetic field pattern produced by a circular loop is similar to

that of a magnet.

OR

a.

Principle: When a current carrying coil is placed in magnetic field, it

experiences a torque.


b. 1. It is necessary to introduce a cylindrical soft iron core inside the

coil of a galvanometer to increase the sensitivity and hence magnetic

field becomes radial.

2. We know that

If the resistance of the coil is constant then voltage sensitivity is

proportional to current sensitivity. But if resistance of the coil also

increases then in that case on increasing the current sensitivity the

voltage sensitivity remains constant.

26.

a.

b.


OR

a.

b. Blue light will suffer more deviation because the refractive index of

blue light is more than red and hence the angle of deviation for blue

is more.

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