cbse paper 3

8/10/2019 Cbse Paper 3

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JOSHI CLASSESA PREMIER INSTITUTE FOR QUALITY EDUCATION

Test (+2)

JC

Joshi Classes, SCO5, Top Floor, Sector 20/D, Chandigarh. M:- 08968081307, Ph:- 0172-4604567 Page 1

Duration: 3 Hr 09/02/2014 Max. Marks: 70

One Marks Questions:

Q 1. Sketch the electric lines of force for two positive charges Q1and Q2(Q1> Q2) separated by distance d.

Q 2. Mention two properties of the alloys from which permanent magnets are made.

Q 3. Two bulbs whose resistances are in the ratio of 1 : 2, are connected in parallel to a source of constant

voltage. What will be the ratio of power dissipated in the bulbs?

Q 4. A light of wavelength 5000 falls on a metal surface of work function 1.9 eV. What will be kinetic energy of

photoelectrons ejected?

Q 5. Draw a graph showing the variation of stopping potential with frequency of incident radiations in relation to

photoelectric effect. Deduce an expression for the slope of this graph using Einsteins photoelectric equation.

Q 6. Compare the radii of two nuclei with mass number 1 and 27 respectively.

Q 7. What will be angular separation between two consecutive maxima in interference pattern on increasing the

separation between the plane of the slits and the screen to twice?

Q 8. What the termdq

dtstands for in case of displacement current?

Two Marks Questions:

Q 9. Two points charges +4 C and 6 C are separated by a distance of 20 cm in air. At what point on the line

joining the two charges in the electric potential zero?

Q 10. An indictor of inductive reactance 50 and a 50 resistor are connected in series with a 220 V. 50 Hz

source. What is the ratio of the voltage across the resistor to the source voltage?

Q 11. Show that during the charging of a parallel plate capacitor, the rate of change of charge on each plate equals

0times the rate of change of electric flux (E) linked with it. What is the name given to the termE

0

d

dt

?

Q 12. The critical angle of total internal reflection for a sapphire surrounded by air is 34.4. Calculate the polarising

angle for sapphire if the light is incident from air.

OR

Draw a labeled ray diagram showing the formation of image of a distant object in an astronomical telescope.

Q 13. Obtain the expression for the maximum kinetic energy of the electrons emitted from a metal surface in terms

of the frequency of the incident radiation and the threshold frequency.

Q 14. A radio broadcast is transmitted using amplitude modulation at a carrier frequency of 680 kHz. Explain themeaning of each of the italicized words.

Q 15. A magnetic oscillating in a horizontal plane has a time period of 3 seconds at a place where the angle of dip is

30and 4 seconds at the another place, where the dip is 60. Compare the resultant magnetic field at the

two places.

Q 16. State one property of nuclear forces. Prove that the density of matter in nuclei is independent of mass

number A.

Three Marks Questions:

Q 17.n identical cells each of emf E and internal resistance r are connected in series to a resistor R.

(a)

Deduce an expression for the internal resistance r of one cell in terms of the current I flowing through

the circuit.


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JOSHI CLASSES Test (+2)


(b) How does the internal resistance of the cell vary with temperature?

Q 18. Gurpreet wanted to give a gift to his mother on her birthday. He thought of giving gift which could help her

out in every day household work and decided for an induction heater. But he was not sure whether it would

be economical, safe and effective. He discussed it with her fried Meenakshi. Meenakshi explained that

induction heater is safer and more effective than heater having coil.

(a)

Discuss the type of nature that Gurpreet has.(b) What is the principle of induction heater?

Q 19. Three rays of light red (R), green (G) and blue (B) are incident on the face

AB of a right-angled prism ABC. The refractive indices of the material of the

prism for red, green and blue wavelengths are 1.39, 1.44 and 1.47 respectively.

Trace the path of the rays through the prism. How will the situation change if

these rays were incident normally on one of the faces of an equilateral prism?

Q 20. A double convex lens made of glass of refractive index 1.5 has both radii of curvature of magnitude 20 cm.An

object 2 cm high is placed at 10 cm from the lens. Find the position, nature and size of the image.

Q 21. The output of a 2-input AND gate is fed to a NOT gate. Draw the logic circuit of the combination of gates.Give its logic symbol and write down its truth table.

Q 22. When a deuteron of mass 2.0141 u and negligible kinetic energy is absorbed by a lithium 63 Li nucleus of

mass 6.0155 u, the compound nucleus disintegrates spontaneously into two alpha particles, each of mass

4.0026 u. Calculate the energy in joules carried by each alha particle.

(1 u = 1.66 1027kg).

Q 23. A battery of emf E, and internal resistance r, gives a current of 0.5 A with an external resistor of 12 ohm

an a current of 0.25 A with an external resistor of 25 ohm. Calculate (a) internal resistance of the cell and,

(b) emf. of the cell.

OR

State the principle of potentiometer. Draw a circuit diagram used to compare the emf of two primary cells.

Write the formula used. How can the sensitivity of a potentiometer be increased?

Q 24. Devices A and B are connected independently to a variable frequency alternating voltage source as shown.

The current in A is ahead of the applied voltage whereas it lags behind the voltage in B.

(a) Identify the devices A and B.

(b) How will the current in each of these devices change on decreasing the frequency of the applied voltage?

Give reason to support your answer in each case.

Q 25. 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 modulating signal at the receiving station.


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Four Marks Questions:

Q 26. (a) In only one of the circuits given below the lamp L lights. Which circuit is it? Given reason for your

answer.

(b) Explain briefly why the output signals of a common-emitter amplifier differ in phase by 180.

Five Marks Questions:

Q 27. An electric dipole is held in a uniform electric field.

(a) Show that no translator force acts on it.

(b) Derive an expression for torque acting on it.

(c) The dipole is aligned parallel to the field. Calculate the work done in rotating it through 180.

OR

(a) Derive an expression for the energy stored in a parallel plate capacitor with air as the medium between

its plates.

(b) Air is replaced by a dielectric medium of dielectric constant k. How does it change the total energy of the

capacitor?

Q 28. Figures shows an equiconvex lens (of refractive index 1.50) in contact with a

liquid layer on top of a plane mirror. A small needle with its tip on the principal

axis is moved along the axis until its inverted image is found at the position of the

needle. The distance of the needle from the lens is measured to be 45.0 cm. the

liquid is removed and the experiment is repeated. The new distance is measured

to be 30.0 cm. What is the refractive index of the liquid?

OR

Discuss construction, principle and working of a compound microscope. Derive an expression for its

magnifying power.

Q 29. (a) With the help of a labeled diagram, explain the principle and working of a moving coil galvanometer.

(b) Two parallel coaxial circular coils of equal radius R and equal number of turns N, carrying equal current

I in the same direction and are separated by a distance 2R. Find the magnitude and direction of the net

magnetic field produce at the mid-point of the line joining their centres.

OR

(a) With the help of a schematic sketch of a cyclotron explain its working principle. Mention its two

applications. What is the important limitation encountered in accelerating a light elementary particle such

as electron to high energies?

(b) A article of mass m and charge q moves at a right angles to a uniform magnetic field. Plot of graph

showing the variation of the radius of the circular path describes by it with the increase in its (i) charge,

(ii) kinetic energy, where, in each case other factors remain constant. Justify your answer.


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Solutions

Q 1.

Q 2. (i) High coercivity.

(ii) High residual magnetism (retentivity)

Q 3. As voltage is constant

P =2V

R

1 1

2 2

P R 2

P R 1

P1: P2= 2 : 1

Q 4. Ek=hc

0

=34 8

7

6.62 10 3 10

5 10

J 1.9eV

=196.62 3 10

5

J 1.9eV

=19

19

19.86 10

5 1.6 10

eV 1.9eV

Ek= 2.4825eV 1.9eV = 0.5825eV

Q 5. The variation of stopping potential with frequency is as shown:

The slope of graph is given by:

Slope =h

e

Q 6.

13

1 1

2 2

R A 1

R A 3

Q 7. As =Y

;D d

it remains same.

Q 8. Rate of variation of charge.

Q 9. Let potential difference is zero at point P, then

1 2

0 1 0 2

q q

4 r 4 r


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OR

Q 13. According to Einstein, light incident on metal surface is used in two ways:

(i)

Overcoming work function(ii) Providing max. K.E., i.e.,

E = 0+ K.E.max

hv = hv0+2

max

1mv

2

is required relation.

Q 14. It is a process in which original signals are superimposed over a carrier wave in such a way that amplitude of

the modulated wave varies with the amplitude of the modulating signal whereas, its frequency is same as

that of the carrier wave.

Carrier frequency is the frequency of the carrier wave. It must be much greater than the highest frequencycomponent of the message signal.

Q 15. As T2= 42H

I

B M

1

2

2

H2 1 1 1

1 H 2 2 2

BT B cos B cos 30

T B B cos B cos 60

1

2

3 B16

19 B

1

2

B 16

B 9 3

Q 16. Properties of nuclear forces:

(i) They are charge independent.

(ii) They are short range forces.

As r = r0A1/3

Density =3 3

3 0 0

A 3A 3

4 4 r A 4 rr

3

which is independent of mass number.


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Q 17.

(a) Net emf = nE

Net resistance = nr + R

I =nE

nr R

Internal resistance of each cell is

nE = IR + Irn

nE IR

In

= r

So, r is terms of I will beE R

I n = r

(b) If temperature is increased, internal resistance increases.

Q 18. (a) Love and affection for her mother. Free to express her feelings to her friends.

(b) Induction heater is based on the principle of electromagnetic induction i.e., whenever magnetic flux

changes rapidly it produces induced current which develops heat.

Q 19.

sin45

sin90

= ag

ag=1

2

ga= 2

ga= 1.414

As Gand Bare greater than 1.414, the green and blue rays of light get internally reflected.

For red light

sin r =g

a

sin45

= agsin 45

r = sin11.39

2

79

All the rays will get internally reflected.


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Q 20. Given: = 1.5, R1= 20 cm, R2= 20cm, h0= 2 cm, u = 10 cm

1 1

v u = (1)

1 2

1 1

R R

1 2

1 1 1 11

v u R R

1 1 2 1 0.5

0.5v 10 20 10 10

1 1

v 20

v = 20 cm

I

O

h v

h u

hI=20

10

2 = 4 cm

Thus, (i) Position of image is at 20 cm from the lens on the same side. (ii) Image is virtual. (iii) Image is of

4 cm height.

Q 21.

The truth-table is as shown:

A B Y = A.B Y

0

0

1

1

0

1

0

1

0

0

0

1

1

1

1

0

Q 22. m(1H2) = 2.0141 u

m(3Li6) = 6.0155 u

m(2He4) = 4.0026 u

6 2

3 1Li H 2He4+ 2He

4+ Q

Mass defect m = (2.0141 + 6.0155) 2 (4.0026)

m = 0.0244 um = 0.0244 1.66 1027kg

= 4.0504 1029kg

Q = m c2

= 4.0504 1029(3 108)2

= 3.645 1012J

Half of this energy is carried by each of the electrons.

Q 23.


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V = E Ir I1R1= E Ir.

0.5 12 = E 0.5 r

6 = E 0.5 r

Multiply both sides by 2

12 = 2E r ..(i)

V = E Ir

I2R2= E Ir

0.25 25 = E 0.25 r

6.25 = E 0.25 r

Multiply both sides by 4

25 = 4E r ..(ii)

Then from equations (i) and (ii), we get

13 = 2E

E = 6.5 volt

from equation (i),

r = 2E 12

= 2 6.5 12 = 1

OR

Principle of potentiometer:

Fall of potential across any portion of a wire is directly proportional to the length of that portion provided the

wire is of uniform area of cross-section and current passing through it is constant.

Circuit diagram to compare e.m.f. of two primary cells:

Formula used, 1 1

2 2

where 1and 2are balancing lengths for the cells of e.m.f. 1and 2respectively.

We can increase the sensitivity of a potentiometer by reducing the value of potential gradient.

i.e., K =V

We can do so by

(i) Increasing the length of potentiometer wire.

(ii) By joining a resistance in series with the driving cell.

Q 24. (a) Device A is a capacitor.

Device B is an inductor.


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(b) On decreasing the frequency of applied voltage.

Current through device A decreases because

XC1

vand i

c

1

X

current through device B increases because XLv and i

L

1

X

Q 25. Let, for simplicity, the receiving signal be

AIcos(c+ m)t

The carrier wave is Accos ct at the receiving station.

By multiplying the two signals, we get

AIACcos (c+ m)t cos ct

= I CA A

2[cos (2 c+ m)t + cos mt]

By using 2 cos A cos B = cos (A + B) + cos (A B)

If this signal is passed through a low pass filter, we can record the modulating signal I CA A2

cos mt.

Q 26. (a) In circuit (b) lamb L lights because only in this circuit, input circuit is forward biased and output circuit is

reverse biased.

(b)

Positive half of input a.c. signal makes input circuit more forward biased. As a result emitter current and

hence collector current increases. Increase in collector current increase the potential drop across RL,

which makes the output voltage V0less positive or more negative.

This way when input signal goes through its positive half cycle the amplified output signal goes through

negative half cycle.

This is why, input and output signals are 180out of phase.

Q 27. Torque experience by an electric dipole:

(a) Consider an electric dipole of dipole of moment p p q 2

held at an angle in a uniform electric field

E as shown.

The force experienced by the charges q and q are qE and qE respectively. Hence, Net force =

qE qE 0.


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However, the forces qE and qE, being equal in magnitude and opposite in direction, constitute a couple.

(b)

Here, Torque, = (qE) (AC)

= qE AB sin

= q (2) E sin

= pE sin

Vectorially p E

(c) Work done W = pE(cos 2cos 1)

= pE (cos 180cos 0)

W = 2pE

OR

(a) Consider a capacitor of capacitance C. Let it be charged gradually. At any instant time if the charge on

the capacitor is q.

V =q

C

To add further charge dq to the condenser amount of work done is dW =q

Cdq

Total amount of work done in giving a charge Q to the condenser

W =21 Q

2 C

Q = CV

W =1

2CV2

This work done gets stored in the condenser. Thus, energy stored in a parallel plate capacitor is

U =1

2CV2

(b) Capacitance of air filled capacitor is C0=0

A

d

where A is area of each plate d is separation between the

plates. When the space between the plates is filled with a dielectric of dielectric constant K.

Potential between the capacitor plates becomes

V = Ed 0E

KE

where, E is net electric field inside the dielectric slab.

V =0

1d

K

Where, is surface charge density on the plates of capacitor.

V =0

1d

K

C =q A

V V


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C = K 0A

d

or C = KC0

Expression for energy stored in the above case.

Suppose at any instant of time potential difference between the capacitor plates be V.

Then amount of work required to supply a charge dq to the capacitor isdW = Vdq

To supply a charge Q work done is

W =Q

0Vdq

W =Q

0

qdq

C q

VC

W =21 Q

2 C (Q = CV)

or W =

1

2 CV2

Here C = KC0.W =1

2KC0V

2

As C0=0

A,

d

and V = Ed

W =1

2K 0

d

E2d2A,

W =1

2K0E

2Ad.

Q 28. The first measurement gives the focal length f of the combination of the convex lens and the plano-convex

liquid lens. The second measurement gives the focal length f1of the convex lens. The focal length f2of the

plano-convex lens is then given by:

2

1 1 1 1

f 45 30 90

i.e., f2= 90 cm

Using the Lens makers formula for the equiconvex lens.

Here R1= R

R2= R

2 11 1 2

n n1 1 1f n R R

So1

30= (1.5 1)

1 1

R R

Which gives R = 30 cm.

for plano convex lens R1= R

R2=

The same formula applied to the plano-convex lens gives

1

90

= (n 1)1

0

30


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n 1 =30

90

n = 1 +1

3

from which the refractive index of liquid, n = 1.33

ORPrinciple: For a combination of lenses magnification get multiplied.

Working: When object is near the focus of objective image is formed

near the eye-piece. This image acts like an object for second lens, the

eyepiece. Being between the focus and optical centre of eyepiece final

image is formed at least distance of distinct vision.

Magnifying power of compound microscope is

m = e

e

A " B " /P B "tan A "B "

tan A "B"/ P B " A "B"

= A " B " A ' B '.A 'B ' AB

= mome

But mo=v

uand me= 1 +

e

D

f

m =e

v D1

u f

As u fo, v L, length of microscope.

m =o e

L D1

f f

where, L = length of microscope tube

D = least distance of distinct vision.

Magnifying power can be increased by taking objective and eyepiece of small focal lengths.

Q 29. (a) Principle: Current carrying coil placed in a magnetic field experiences a torque.

Working: suppose current i' is passing through the coil. This coil experiences a torque = NiBA sin

where N is total number of turns in the coil.

is the angle between magnetic field vector and area vector of the coil.

Counter torque is developed in the spring. If k is the restoring torque per unit angular twist, for an

angular twist of , we have

Restoring torque, = k In equilibrium =

NiBA sin = k

i =k

.NBA sin

If the magnetic field is radial, i.e., plane of the coil is parallel to the

direction of magnetic field then,

= 90

i =k

NBA. i

So on calibrating the scale linearly, we can measure the current.


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(b) Magnitude of magnetic field induction at an axial point at a distance x

from the centre of a coil of radius R, number of turns N and carrying

current i is given by

B =

2

0

3/22 2

NiR

2 R x

Resultant field B due to both the coils at the mid point P is obtained by putting x = R

B = 2

2 2

0 0

3/2 3/22 2 2

NiR NiR

2 R x 2R

B = 2B

B =

220 0

3

NiR NiWb / m

2 2 R 2 2 R

This field acts along the common axis of the

OR

(a)

Principle: A positive ion can be accelerated to a very high energy by

making it to pass through moderate electric field again and again by

making use of a magnetic field.

Perpendicular magnetic field makes the charge to move in a circular

path. Particle traces a semicircular path of radius r and Lorentzs magnetic force provides necessary

centripetal force.

Thus Bqv =2mv

r Bq =

mv

r

Bq = m( v = r)

v =Bq

2 m( = 2v)

The above expression shows that cyclotron frequency does not depend on the speed of the particle.

Use: It is used implant ions into solids and modify their properties.

Two applications:

(i) It is used to implant ions into solids and modify their properties.

(ii) It is used in hospital to produce radioactive substances.

Limitation: Light elementary particle such as electrons require unusually high frequencies (GHz)

(b) (i) As r =mv

qB

On increasing charge, radius of circular path decreases.

r =mv

qB (i)

(ii) kinetic energy (E) =1

2mv2

v =2E

m .(ii)

From equations (i) and (ii), r = 2mEqB

cbse paper 3

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