1. FIRST PRE BOARD EXAMINATION 20 L4-L5
sutsJECT - PHYSTCS
CLASS. xII
Time: Three Hours Max. Marks:70
I Generrl Instructions -"t
(il All questions are compulsory.
(ii) There are 26questions in total. Questions 1 to 5 carry one mark each.
[ii| Questions 6 to 10 carry two marks each, questions ]"L to 22 carry three marks each, question
23 carry four marks and questi ons 24 to 26 carry 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. You have to attempt only one of the given choices in such questions.
(") Use of calculators is not permitted. I .
(viJ You may use thef.q-Uowj$pl{ysical,i6jista whbiever necessary:
h = 6.6x 10-34Js e= 1.6x 10-1eC ps=4n xL0-7TmA-1
difference is applied across its ends?
The figure given below shgw5 the block diagram of
Identify the element labelled'X'and write its function.
or*,? {z
a generalized communication system,
A: l, both B ,'and B* decay processes, the mass number of a nucleus remains same whereas the
;)l How does the random motion of free electrons in a conductor get affected when a potential
Define the term'coherent sources'which are required to produce interference pattern i'
) n th";'-o/,'l*n-uYoung's double slit experiment. f r *
l
€.:9 +
L,iforrnlio::
Sar:ic*
DJ-Phy-xll-7/2050

2. atomic number Z increases b.y one in p- decay and decreases by one in B* decay, Explain, giving
reason.
-8: X-rays fall on a photosensitive surface to cause photoelectric emission. Assuming that the work
t
d function of the surface can be neglqcted; find the relation between the de-Broglie wavelength
L
*y;(r)oftheelectronsemittedtotheeribrgy(E)oftheincidentphotons.
ffi*opoint charges q and - zqarekept'd'distance apart. Find the location of the point relative
wF'<r
to charge 'q' at which potential due to this system of charges is zero. &.
oR'
./-/
/O"electric dipole is placed in a uniform electric field with its dipole moment parallel to the
field. Find (i) the work done in turning the dipole till its dipole momgnt points in the direction
opposite to .. [iiJ the orientation of the dipole for which the torque acting on it becomesMC. 9o
maximum.
7 ldentify the equivalent gate represented by the circuit shown in the fig1r.re. Drew,its logic
symbol and write the truth table.
8. Out of the two rnagnetic materials, 'A' has relative permeability slightly greater than unity
while 'B' has less than unity. Identiff the nature of the niaterials A' and 'B'. Will lhe.!5
susceptibilities be positive or negative? i
How does the resolving power of an astronomical telescope change on -
(i) decreasing the wavelength of light used, and [ii) decreasing the diameter of its object lens?
9.
/At
fOJ' Catcutate the half-liie period of a radioactive substance if its activity drops to * th of its initial
tl
,t'2*
value in 30'years.
,--
yY- ' . r2-- (7
DJ-Phy-xll-7/2050

3. 11.6rt Write the formula for the velocity of light in a material'mediun of relative permittivity t r
and relative magnetic permeability p..
r'"*tl
qrrq r LrqLry L rrrqErrrvLre yLr'ruqvrrrLJ tf,r,
{g1i.ro ,*.,+
giiJ Unrictr constituent radiation of electromagnetic spectrum is used : (a) in radar (b) to 'dd'
photograph internal parts human body (c) for taking photograph during foggy condition (d)
1{*
for treatment of cancer?
tlVV
.L2. (a) State the underlying principle of a potentiometer. Why is it necessary to (i) use a long wire,
(ii) have uniform area of cross-section of the wire and (iii) use a driving cell whose emf is
taken to be greater than the emfs of the primary cells?
(bJ In a potentiometer experiment, if the area of the cross-section of the wire increases
uniformly from one end to the other, draw a graph showing how potential gradient would vary
as the length of the wire increases from one end. :-: ---- :.-: -----
L3. Two long straight parallel conductors 'a' and 'b', carrying steady currents Ia ancl Ir arg
:i
separated by a distance d. W-rite the magnitude and direction of the magnetic field produced by
the conductor 'a' at the points along the conductor 'b'. If the currents are flowing in the same
direction, what is the nature and magnitude of the force between the two conductors?
(b) Show with the help of a diagram how the force between the two conductors would change
the opposite directions.
14.In the electric network shown in the figure, use Kirchhoffs rules to calculate the power
. consumed b1a the resistance R = 4Q.'
El:12Itr
3
t-- DJ.Phy-xu-22050

4. 15. (aJ The radius of the innermost electron orbit of a hydrogen atom is S.3 x t0-11 m. Calculate its
[b) The total energy of an electron in.the first excited state of the hydrogen atom is - 3.4 eV.
Find gut its 1i; kinetic energy and (ii) potential energy in this state.
16' (a) Deduce an expression for the frequency of revolution of a charged particle in a magnetic
field and show that it is independent of velocity or enerry of the particle.
.r'.
-{6J Draw a schematic sketch of a cyclotron and write its principle.
OR
With the help of a neat and labelled diagr4m, explain the underlying principle, and. working of
a moving coil galvanometer.
,?yfistate Gauss law. Using this Iaw to obtain an
long straight tiniformly charged wire.
exoression for the electric field due,to an infinitely
ZV€*-:Y
" )2 ,%-
a_.2fYE " -U* ^,
-4
*{ e0'5 m long metal rod PQ completes the circuit as shown in ile figure. The Area of The circuit
is perpendicular to the magnetic field of flux density 0:15 T. If the resistance of the total circuit
is 3O, calculate the force needed to move the rod in the direction as indicated with a constant
speed of Zm/s.
L
,<.
/{
,-'
5*"
xxxxx-q "(bt"
v -',
Qt',
*
q'fn
f
!
X
X
*{SUut t the graphs showing the variation of stopping potential with frequency of Incident
radiations for two photosensitive material A and ts having threshold frequencieS uo1 ) uo2
respectively. (i) Vthfch of the two materials A'or@rr. higher work function? (ii) What,
information do you get from the slope of the graphs? (iii) What does the value,of the intercept
of graph on the potential axis represent?
b 4 t)-
?;-
,a_
DJ-Phy-xll-/2050
x
x
xxx
',1
xx_x
xx.x

5. ,/.r'
.Nt"a parallel plate capacitor with air between the plates, each plate has an area of 6x10-3 m2
;;;;".;;;, ('=- c_= e.ft.{Calculatethecapacitanceofthecapacitor.
W If this capacitor is connected to 100 V supply, what would be the charge on each plate?
ffi How would charge on the plates be affected, if a 3 mm thick mica sheet of K = 6
'"<:,/
inserted between the plates while the voltage supply remains connected?
21. A slit of width 'd' is illuminated by white light. For what value of 'd' is the first minimum, for
red light of = 650 nm, located at point P? For what value of the wave length of light will the
first diffraction maxima also fall at P?
ts
22.(t) Distinguish between unpolarised and linearly polarised light.
(iil What does a polaroid consist of ? How does it produce a linearly polarised light?
(i) Explain briefly how sunlight is polarised by scattering through atmospheric particles.
Z3.When Nisha, a class XII student, came to know that rr'er parents are planning to rent out the top
/
floor of their house to a mobile company she protested. She tried hard to convince her parents
, that this rnove would be a health hazard, Ultimately her parents agreed:
p) In what way can the setting up of transmission tolver by a mobile company in a
residential colonyprove to be iniurious to health? :''
I
By objecting to this move of her parents, what values did Nisha display? cat firu , l"ealtL cotrli
Estimate the range of e.m. waves which can be transmitted by an antenna of height 20
m;(Given radius of the earth:6400 km)
${
w
DJ-Phy-x[-z2os0

6. 2a. @) A series LCR circuit is connected to an a.c. source of variable frequenry. Draw a suitable
phasor diagram and deduce the expressions for the amplitude of the cument and phase
angle. 2 "R+1'--la;
(b) Obtain the condition at resonance. Draw a plot showing the variation of current with
the frequency of a.c. source for two resistances Rr and Rz (Rr , Rz). Hence define the
quality factor, Q and write its role in the tuning of the circuit. ;
ta) Draw a Iabelled diagram of a.c. generator and state its working principle.
: '
tb)HowismagneticfluxlinkedwiththearmaturecoiIchangedinagenerator?
[c) Derive the expression for maximum value of the induced emf and state the rule that
gives the direction of the induced emf.
td) Show the variation of the emf generated versus time as the armature is rotated with
respect to the direction of the magnetic field.
25. (a)
, . ,*u briefly the processes involved in the formation of p-n junction exptaining clearly
how the depletion region is formed.
(b) Using the necessary circuit diagrams, show how the V-l characteristics of a p-n junction
How these characteristics are made use of in rectification?
I &.t^l Differentiate between three segments of a transistor on the basis of their'size and level
q)
v ofdoping.
(b)How.isatransistorbiasedtobeinactivestate?:
(c) With t[re help of necessary circuit diagram, describe briefly how n-p-n transistor in CE
,}
configuration amplifies a srnall sinusoidal input voltage, Write the expression forthe ac i
current gain.
Draw a labelled ray diagram showing the image formation of a distant object by a
refracting astronongical'telescope. Deduce the expression for its magnitying power
when the final image is formed at infinity.
OR
z6ot
DJ-Phy-x11.72050

7. tb) The sum of focal lengths of the two lenses of a refracting telescope is 105 cm. The focal
length of one iens is 20 times that of the other. Determine the total magnification of the
: Draw a ray. diagram showing the formation of the image: o-y a point objeit on the
principal axis of a spherical convex surface separating two media of refractive indices nr
.'andradiusofcurvatureof-thesurface..
{
DJ-Phy-xll-720s0