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Sample Paper (CBSE)

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

SP/1-C ©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. Draw the geometrical shape of equipotential surfaces due to a

single isolated charge.

2. What are permanent magnets? Give one example.

3. When unpolarised light passes from air to a transparent

medium, under what condition does the reflected light get

polarized?

4. With what purpose was famous Davisson and Germer

experiment with electrons performed?

5. How is a sample of an n-type semiconductor electrically

neutral, though it has an excess of negative charge carriers?

Section B

6. What is effect on the resolving power of a microscope when:

a. The diameter of the objective lens is decreased?

b. The wavelength of illuminating radiations is decreased?

7. A transformer has an efficiency of 80 % and works at 100 volt

and 4 kW. If the secondary voltage is 240 volt, calculate the

primary and secondary currents.

8. Two cells each of e.m.f. E and internal resistances r1 and r2 are

1

1

1

1

1

2

2


connected in series to an external resistance R. What is the

value of R such that the potential difference at the first cell is

zero?

9. A tangent galvanometer has 100 turns of wire of mean

diameter 10 cm. Calculate the reduction factor of the

galvanometer at a place where the horizontal component of

earth’s magnetic field is 0.32 × 10-4 tesla.

10. a. Find the average value of current of the a.c. voltage V = V0

sinωt over the time interval between t = 0 and t = π/ω.

b. The figure shows two long coaxial solenoids, each of length .

The area of cross section of the outer solenoid is and it

has number of turns per unit length. The corresponding

values for the inner solenoids are and . Show that

.

Section C

11. a. Out of radio waves, gamma rays, visible light, microwave,

ultraviolet rays and infrared rays, which electromagnetic

wave has the maximum frequency which has the minimum

frequency?

2

2

2


b. A plane electromagnetic wave travels in vacuum along the

Y-direction. Write the

1. ratio of the magnitudes and

2. the direction of its electric and magnetic field vectors.

12. a. Write the nuclear decay process for β-decay of .

b. The half life period of radioactive element A is the same as

the mean life time of another radioactive element B.

Initially both of them have same number of atoms. The

radioactive element B decays faster than A. Explain why?

13. What is space wave propagation? Give two examples of

communication system, which use space wave mode.

14. The threshold frequency of a metal is . When the light of

frequency is incident on the metal plate, the maximum

velocity of electrons is . When the frequency of incident

radiation is increased to , the maximum velocity of electrons

emitted is . Find the ratio of to .

15. What is the maximum possible number of spectral lines

observed when the hydrogen atom is in its second excited

state? Justify your answer. Calculate ratio of the maximum

and minimum wavelengths of the radiations emitted in this

process.

16. Calculate the work done to dissociate the system of three

3

3

3

3

3


charges placed on the vertices of a triangle of sides 10 cm as

shown in figure. Given: q = 1.6 × 10-10 C.

17. a. A bulb and a capacitor are connected in series to a source of

alternating current. What will happen on increasing the

frequency of the a.c.?

b. For circuits used for transporting electric power, a low

power factor implies larger power loss in transmission.

Explain.

c. Power factor can often be improved by the use of a capacitor

of appropriate capacitance in the circuit. Explain.

18. In a single slit diffraction pattern, how is the width of central

bright maximum changed, when

a. The slit width is decreased,

b. The distance between the slit and the screen is increased

and

c. Light of smaller wavelength is used.

Justify your answer.

3

3

3


19. In Young’s double slit experiment, using light of wavelength

400 nm, interference fringes of width X are obtained. The

wavelength of light is increased to 600 nm and the separation

between the slits is halved. If one wants the observed fringes

width on the screen to be same in the two cases, find the ratio

of the distance between the screen and the plane of the

interfering sources in the two arrangements.

20. Write the symbol and truth table of an AND gate. Explain how

this gate is realized in practice by using two diodes?

21. Draw the circuit of n-p-n transistor as an amplifier in common

emitter configuration and explain its working.

22. Due to economic reasons, only the upper sideband of an AM

wave is transmitted, but at the receiving station, there is a

facility for generating the carrier. Show that if a device is

available which can multiply two signals then it is possible to

recover the modulation signal at the receiving station.

Section D

23. Tarun was preparing a project for science exhibition. He

required a capacitance of 2 μF having a capacity to operate

under 2 kV potential. He went to a shop to purchase it.

Shopkeeper was having only 1 μF capacitors of 500 V rating.

Tarun calculated minimum number of capacitors of 1 μF so

3

3

3

3


that he could arrange them to form a capacitor of 2μF value.

a. What values do you judge in Tarun?

b. Show the calculations done by Tarun.

Section E

24. a. State the working principle of a potentiometer. With the

help of the circuit diagram, explain how a potentiometer is

used to compare the e.m.f.’s of two primary cells. Obtain the

required expression used for comparing the e.m.f.’s.

b. Write two possible ways to increase the sensitiveness of a

potentiometer.

OR

a. State Kirchhoff’s rules for an electric network. Using

Kirchhoff’s rules, obtain the balance condition in terms of

the resistance of four arms of Wheatstone bridge.

b. The resistance in the left gap of a meter bridge is 20 Ω and

the balance point is 40 cm from the left end. Find the value

of the unknown resistance.

25. a. Derive the expression for the torque on a rectangular

current carrying loop suspended in a uniform magnetic

field.

b. Write down the basic difference between the atom or

molecule of a diamagnetic and a paramagnetic material.

Give reason why the elements of even atomic number are

4

5

5


more likely to be diamagnetic?

OR

A straight thick long wire of uniform cross section of radius a

is carrying a steady current I. Use Ampere’s circuital law to

obtain a relation showing the variation of the magnetic field

(Br) inside and outside the wire with distance r, (r≤a) and (r>a)

of the field point from the centre of its cross section. Plot a

graph showing the nature of this variation.

26. a. State the principle, which helps us to determine the shape

of the wave front at a later time from its given shape at any

time. Apply this principle to show that a spherical/plane

wave front continues to propagate forward as a

spherical/plane wave front.

b. Derive Snell’s law of refraction by drawing the refracted

wave front corresponding to a plane wave front incident on

the boundary separating a rarer medium from denser

medium.

OR

a. Derive the mirror formula which gives the relation

between , and . What is the corresponding formula for a

thin lens?

b. Calculate the distance between the convex lens and

concave mirror, so that a real image of an object at O, 15 cm

in front of a convex lens of focal length 10 cm be formed at

5

5

5


the same point O after reflection from the mirror. The

radius of curvature of the mirror is 20 cm. Will the image be

inverted or erect?

5


1.

2. The materials which retain their ferromagnetic properties for long time

at room temperature are called as permanent magnets.

Example: Steel

3. The ray of light should be incident on the refracting medium at

polarizing angle.

4. To demonstrate the wave nature of electrons.

5. The penta-valent impurity atoms added to the semiconductor and the

atoms of the semiconductor are neutral.

ANSWERS


6. The resolving power of a microscope is given by:

a. The resolving power of a microscope decreases when the diameter of

the objective lens is decreased.

b. Resolving power will increase on decreasing the wavelength of light.

7. Use the relation:

We get, Ip = 40 A and Is = 13.3 A

8. The electric current flowing through the circuit is given by,

Potential drop across first cell is zero. That is,

Solving we get,

9. Using the relation,

We get, K = 0.025 A

10. a.

b. The self inductances of the two solenoids and their mutual

inductance is given by,


Solving we get,

Or

11. a. Gamma rays have maximum frequency and radio waves have

minimum frequency.

b. 1.

, where c is the speed of electromagnetic wave.

2. If an electromagnetic wave is propagating along Y-direction, then

the electric and magnetic field vectors vary sinusoidally along X-

direction and Z-direction respectively.

12. a.

b.

That is decay constant of element Y is greater than that of X hence

element Y decays faster than X.

13. When the radio waves from the transmitting antenna reach the

receiving antenna either directly or after reflection from the ground or

in troposphere, the wave propagation is called space wave propagation.

Examples: TV transmission and satellite communication.

14. Using the Einstein’s photoelectric equation,


15. For the second excited state n = 3. Thus for n = 3 to n = 2 and n = 1, two

transitions takes place and from n = 2 to n = 1, one transition takes

place. Thus the maximum of 3 spectral lines are observed.

Thus,

16. Using the formula:

We get, W = - 2.304 × 10-8 J

17. a. The bulb will glow brighter because the capacitive reactance

decreases when frequency is increased and hence the current

through the bulb increases.

b. We know that, , where is the power factor. At a

given voltage if power factor is small then to supply a given power

has to be increased accordingly. Thus power loss in transmission

would increase as .

c. Power factor is given by,


In order to improve power factor This can be achieved by using a

capacitor of appropriate capacitance in the circuit with given value of

so that .

18. The width of central bright maximum in a single slit diffraction pattern

is given by:

a. The width of central maximum is inversely proportional to the slit

width, thus the width of central maximum will increase with

decrease in slit width.

b. The width of central maximum is directly proportional to the

distance between the slit and screen, thus the width of central

maximum will increase with increase in the distance between the

slit and the screen.

c. Since the width of central maximum is directly proportional to the

wavelength of light, thus the width of central maximum will

decrease when light of smaller wavelength is used.

19. Using the relation:

We get,

20.


Truth table:

Circuit for AND gate using two diodes:

21.

22. For the transmission of the upper sideband, the transmitted signal is


The filter signal blocks the high frequency and allows the low

frequency to pass through it. Thus

can be recovered at the receiving station.

23. a. Value for money and ability to apply scientific calculations.

b. Total potential difference across each row = 2000 V

Potential difference across each capacitor = 500 V

Number of capacitors in series, n =2000/500 = 4

or n = 4

Capacitance of capacitors in series is given by:

If there are ‘m’ rows of capacitors, for an equivalent capacitance of

2μF, we have

Thus, total number of capacitors required are, N = m × n

N = 32

i.e., 8 rows of capacitor in parallel with4 capacitors in each row.

24. a. Potentiometer is based on the principle that the potential drop

across a part of a wire is directly proportional to the length of that

part provided the wire has uniform cross- sectional area and

constant current is passed through it.


If an emf is balancing against the length , then,

Similarly, if emf is balancing against length , then,

Thus, we have

b. By decreasing its potential gradient which can be achieved by:

1. Increasing the length of potentiometer wire.

2. By reducing the current in the potentiometer wire circuit with

the help of rheostat if the length of the potentiometer wire is

fixed.

OR

a. Kirchhoff's Rules: There are two types of Kirchhoff’s rules:

1. Junction Rule: At any junction, the sum of the currents entering the

junction is equal to the sum of currents leaving the junction. That is,

the algebraic sum of the currents into any junction is zero.


2. Loop Rule: The algebraic sum of changes in potential around any

closed loop involving resistors and cells in the loop is zero. That is,

around any closed loop, the sum of the potential drops equals the

sum of the potential rises.

A Wheatstone bridge is an electric circuit which is used to measure an

unknown electrical resistance by balancing the resistances in the two

branches of a bridge circuit, one branch of which includes the unknown

resistance.

When no current flows through the galvanometer, the bridge is said to

be balanced.

Applying Kirchhoff's junction rule, we get:

This is the balancing condition for Wheatstone bridge.

b. Using

We get, S = 30 Ω

25. a.


Torque is the product of force and moment arm.

b. The resultant magnetic moment of an atom or molecule of a diatomic

material is zero whereas that of a paramagnetic material is non-

zero.

The magnetic moment of the electrons of the atoms of elements of even

atomic number cancel each other and hence such elements are more

likely to be diatomic material.

OR

Magnetic field inside the wire is give by,


That is

Magnetic field outside the wire is give by,

That is

The variation of magnetic field due to the wire with distance r is given

by,

26. a. Huygens’ Principle helps us to determine the shape of the wave

front at a later time from its given shape at any time. According to

Huygens’ Principle:

1. Each point on the primary wave front acts as a source of

secondary wavelets, sending out disturbance in all directions in a

similar manner as the original source of light does.

2. The new position of the wave front at any instant is the envelope

of the secondary wavelets at that instance.


b. Snell’s law is

OR

a. The mirror formula is:

The corresponding formula for a thin lens is:


b. Using the lens formula, we get, v = 30 cm. The convex lens forms the

image at the centre of curvature of the concave mirror.

Thus or

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