Download INDIAN SCHOOL ALWADI ALKABIR

INDIAN SCHOOL AL WADI AL KABIR
First Assessment 2015
Class: XII
Physics (042)
M.M: 70
2/6/2015
Time: 3 hrs
General Instructions:
All questions are compulsory.
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 choices in such questions.
Questions 1 to 5 are very short answer type and carry one mark each.
 Questions 6 to10 are short answer type and carry two marks each.
 Questions 11 to 22 are short answer type and carry three marks each.
Question 23 is a value based and carries four marks.
Questions 24 to 26 are long answer type and carry five marks each.
Use of calculators is not permitted.
You may use the following values of physical constants wherever necessary:
c = 3 x 108 ms-1
h = 6.626 x 10-34Js
e = 1.602 x 10-19 C
μ0 = 4π x 10-7 T m A-1
1/4π ε0 = 9 x 109 N m2 C2
Mass of neutron mn = 1.675 x 10-27 kg
Boltzmann’s constant k= 1.381 x 10-23 JK-1
Avogadro’s number NA = 6.022 x 1023 /mol-1
1. What is the unit of electric flux? A charge ‘q’ is enclosed by a spherical surface of
radius ‘R’ .If the radius is reduced to half , how would the electric flux through the
surface changes?
2. What is the work done by the field of a nucleus in a complete circular orbit of the
electron? Given radius of the orbit is 10-15m and charge of electron = 1.6 x 10-19C
3. A cell of emf ‘E’ and internal resistance ‘r’ is connected across a variable resistor ‘R’.
Plot a graph showing the variation of terminal potential difference ‘V’ with resistance
‘R’. Label in the graph, the value of p.d in terms of ‘E’ ,when R = r
4. In the fiq. q1, q2 and q3 are the 3 charges. A Gaussian surface is drawn around q1 and
q2. [i]What is the net charge which determines the electric flux through the Gaussian
surface? [ii] Which all charges produce the electric field at the point ‘P’?
5. At what temperature would the resistance of a copper conductor be double its value at
00c. Given: temp. Coefficient of resistance of copper = 4 x10-3/oc
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6. What is current density? Derive an expression for current density in terms of electric
field.
7. Derive the relation between electric current and drift velocity
OR
Derive the relation between drift velocity and electron mobility.
8.A conductor of length L and diameter ‘D’is subjected to a potential difference V.What
is the effect on the drift velocity of electrons when (i) ‘V’ is halved (ii) ‘1’ is doubled
9. Derive the expression for effective capacitance when two capacitors are connected in
series.
10.What is electrostatic shielding.It is safer to sit inside a car than standing outside during
lightning.Why?
11. What happens to the energy stored in a capacitor if after disconnecting the battery, the
distance between the plates of a charged capacitor reduced?
12. A 600µF capacitor is charged by a 300V battery[i] how much electrostatic energy is
stored in the capacitor[ii] If the capacitor is disconnected from the battery and connected
in parallel to one another 500 µF capacitor, find the electrostatic energy stored in the
system.
13.In the circuit given below, a potentiometer wire AB of length 100cm produces a
balancing length of 40cm for a cell of emf 2V.Find the unknown resistance ‘R’.
Given internal resistance is zero
14. Two point charges of 2 μC & 8 μC are placed 12cm apart. Find the position of the
point where the electric field intensity will be zero
15.Charges +q and –q are placed at points A and B respectively ,which are a distance
‘2L’ apart . C is the mid point between points A and B .Find the work done in moving a
charge +Q along the semicircle CRD
.
16.Derive an expression for the electric field due to an infinte thin nonconducting plane
sheet with surface charge density ϭ
17[a]Derive the expression for the potential energy of an electric dipole of dipole
moment ‘P’ kept in an electric field of strength ‘E’. [b]At what orientation of the electric
dipole, the dipole is said to be [i]stable equilibrium [ii] unstable equilibrium
18. Explain the basic principle of an earthed parallel plate capacitor. Derive the
expression for the capacitance of a parallel plate capacitor
19.What is the basic principle of a potentiometer? With the help of a circuit diagram
derive the relation E1/E2 = ℓ1/ℓ1
20.Two charges 20 μc and -20 μc are placed 20 cm apart at points A and B respectively,
Compute the electric field at C, 20 cm away from both A and B.
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21 Find i1,i2 and i3 in the given circuit
22. What is an electric dipole? Derive an expression to find the torque acting on an
electric dipole when it is placed in an external electric field
23. Sachin had gone to meet his grandfather who was staying in a village. In the evening,
they were both watching TV, when suddenly the lights went off. Grandfather told Sachin
that the fuse must have blown up as all their neighbors had electricity. Luckily Sachin
knew how to change a fuse. His grandfather was happy and told him that if he had been
alone, he would have had to spend the night in the dark without a fan. Sachin felt and
made up his mind to replace the fuse with a circuit breaker which uses a solenoid with a
core so that his grandfather would not have any problems in future. What values did
Sachin have? The main power supply of a house is through a 5 ampere fuse. How many
100 watt bulbs can be used in the house simultaneously at 220 volts?
24.[a] Derive an expression to find the intensity of electric field ‘E’ due to a charged
sphere at a point [a]outside the sphere[b] on the surface. Represent the relation between
‘E’ and the distance ‘r’ graphically.
[b]S1 and S2 are two hollow concentric spheres enclosing charges 2Q and 4Q respectively
as shown in the figure.
What is the ratio of electric flux through S1 andS2?
OR
[a] Derive an expression to find the intensity of electric field at a point on the equatorial
line of an electric dipole.
[b]The distance of the field point on the equatorial plane of a short electric dipole is
halved. By what factor will the electric field change for the dipole
25. Derive the expressions for the energy stored in a parallel plate capacitor
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Two identical plane metallic surfaces A and B are kept parallel to each other in air
separated by a distance of 1cm.Surface ‘A’ is given a positive potential of 10v and the
outer surface of ‘B’ is earthed.[i] What are the magnitude and direction of the electric
field between the plates[ii] What is the work done in moving a charge of 20 µc from a
point on the plate A to another point on the plate B?
OR
[a]A dielectric medium of dielectric constant ‘K’ is placed between the plates of a
capacitor. Explain and derive how[i] electric field[ii] potential.
[b]A generator is capable of building up potential difference of 15 x106 V. The dielectric
strength of the gas surrounding the electrode is 5 x107vm-1.What is the minimum radius
of the spherical shell required?
26[a]With the help of a circuit diagram for a Wheatstone bridge, obtain the condition at
which a bridge is balanced.
[b]
In the balanced condition, find the value of the unknown resistance ‘X’ if the bridge wire
has a resistance of 1ohm/cm.
OR
[a]Define relaxation time of the free electrons drifting a current in a conductor .How does
is it related to the drift velocity of free electrons? Use this relation to deduce the
expression for the electrical resistivity.
[b] Based on the above expression obtained, discuss the effect of temperature on the
resistivity of a metal
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