Model Question Paper (70 marks)
A full paper in the pattern of the current Karnataka 2nd PUC Physics board paper, with every question taken from Chapter 1, “Electric Charges and Fields”: charge and its properties, Coulomb’s law and superposition, electric field and field lines, flux, the electric dipole, and Gauss’s law with its applications. All questions are new – not the textbook examples or exercises. Write it on paper with a timer before you look at the solutions.
II PUC · Model Question Paper · Physics (33)
Chapter 1 · Electric Charges and Fields
The pattern follows the current Karnataka 2nd PUC Physics board paper.
General instructions:
- All parts are compulsory. Answer the required number of questions in each section.
- Answers without a relevant diagram or figure, wherever necessary, will not carry any marks.
- Direct answers to numerical problems without detailed solutions will not carry any marks.
- For Part A, questions answered for the first time alone are evaluated.
Use: 1/(4πε0) = 9 × 109 N m2 C−2 · ε0 = 8.854 × 10−12 C2 N−1 m−2 · e = 1.6 × 10−19 C · π = 3.14
PART – A
I. Pick the correct option among the four given options for ALL of the following questions:15 × 1 = 15
1.
The SI unit of electric flux is
(a) N m2 C−1(b) N C−1(c) C m(d) C m−2
2.
When a glass rod is rubbed with silk, the rod becomes positively charged because
(a) protons move from the silk to the rod(b) electrons move from the rod to the silk(c) electrons move from the silk to the rod(d) positive charge is created on the rod
3.
The electric field due to an infinitely long, straight, uniformly charged wire at a distance r from it varies as
(a) r(b) 1/r(c) 1/r2(d) 1/r3
4.
The force between two point charges is F. If one of the charges is doubled and the distance between them is halved, the force becomes
(a) F/2(b) 2F(c) 4F(d) 8F
5.
At large distances r, the electric field of an electric dipole varies as
(a) 1/r(b) 1/r2(c) 1/r3(d) 1/r4
6.
The direction of the dipole moment of an electric dipole is
(a) from +q to −q along the axis(b) from −q to +q along the axis(c) perpendicular to the axis(d) along the equatorial line
7.
The torque on an electric dipole in a uniform electric field is maximum when the angle between p and E is
(a) 0°(b) 45°(c) 90°(d) 180°
8.
An electric dipole placed in a uniform electric field experiences
(a) a net force but no torque(b) both a net force and a torque(c) neither a force nor a torque(d) a torque but no net force
9.
A closed surface encloses charges +2q and −q. A charge +3q lies outside it. The net electric flux through the surface is
(a) 4q/ε0(b) q/ε0(c) 3q/ε0(d) zero
10.
The electric field at a point inside a uniformly charged thin spherical shell is
(a) zero(b) constant but not zero(c) proportional to r(d) proportional to 1/r2
11.
The magnitude of the electric field due to an infinite plane sheet of uniform surface charge density σ is
(a) σ/ε0(b) 2σ/ε0(c) σ/(2ε0)(d) σ/(2ε0r)
12.
Which of these statements about electrostatic field lines is not true?
(a) They start on positive charges and end on negative charges.(b) Two field lines never cross.(c) They form closed loops.(d) They are crowded where the field is strong.
13.
The value of 1/(4πε0) in SI units is
(a) 9 × 109 N m2 C−2(b) 8.854 × 10−12 N m2 C−2(c) 9 × 10−9 N m2 C−2(d) 6.67 × 10−11 N m2 C−2
14.
Which of the following cannot be the charge on a body?
(a) 3.2 × 10−19 C(b) 4.8 × 10−19 C(c) 2.4 × 10−19 C(d) 1.6 × 10−18 C
15.
During a thunderstorm, the safest place among these is
(a) under a tall tree(b) in an open field(c) inside a car with a closed metal body(d) on top of a hill
II. Fill in the blanks by choosing the appropriate answer from those given in the bracket: (N C−1, zero, induction, polar, conservation)5 × 1 = 5
16.
The SI unit of electric field is .
17.
Charging a conductor by bringing a charged body near it, without touching, is called charging by .
18.
The total charge of an isolated system remains constant. This is the law of of charge.
19.
Molecules such as H2O which have a permanent dipole moment are called molecules.
20.
The net electric flux through a closed surface that encloses no charge is .
PART – B
III. Answer any FIVE of the following questions:5 × 2 = 10
21.
State Coulomb’s law in electrostatics and write it in vector form.
(2)22.
What is meant by quantisation of electric charge? Write its mathematical form.
(2)23.
Define electric dipole moment. Write its SI unit.
(2)24.
State Gauss’s law in electrostatics and write its mathematical form.
(2)25.
Why do two electric field lines never cross each other? Why can electrostatic field lines not form closed loops?
(2)26.
Define electric flux. Write its SI unit.
(2)27.
What is electrostatic shielding? Give one practical application.
(2)28.
How many electrons must be removed from a neutral metal sphere to give it a charge of +3.2 μC?
(2)PART – C
IV. Answer any FIVE of the following questions:5 × 3 = 15
29.
With the help of labelled diagrams, explain how two identical metal spheres on insulating stands can be charged equally and oppositely by induction.
(3)30.
Explain the three basic properties of electric charge.
(3)31.
Derive an expression for the torque acting on an electric dipole placed in a uniform electric field.
(3)32.
What is an electric field line? Draw the field lines due to (a) an isolated positive point charge, (b) an electric dipole, (c) two equal positive charges.
(3)33.
Define electric field at a point. Obtain an expression for the electric field due to a point charge.
(3)34.
Two point charges +4 μC and −6 μC are placed 20 cm apart in air. Find the magnitude of the electrostatic force between them. Is it attractive or repulsive?
(3)35.
A thin spherical shell of radius 20 cm carries a charge of 4 μC distributed uniformly. Find (a) the surface charge density, (b) the electric field at 10 cm from the centre, (c) the electric field at 30 cm from the centre.
(3)36.
A point charge of 17.7 nC is placed at the centre of a cube of side 10 cm. Find (a) the total electric flux through the cube, (b) the flux through one face. (c) Will the flux change if the side of the cube is doubled? (ε0 = 8.854 × 10−12 C2 N−1 m−2)
(3)PART – D
V. Answer any THREE of the following questions:3 × 5 = 15
37.
Derive an expression for the electric field at a point on the axial line of an electric dipole.
(5)38.
Derive an expression for the electric field at a point on the equatorial line of an electric dipole.
(5)39.
State Gauss’s law. Using it, derive an expression for the electric field due to an infinitely long, straight, uniformly charged wire.
(5)40.
Using Gauss’s law, derive an expression for the electric field due to a uniformly charged infinite plane sheet.
(5)41.
Using Gauss’s law, obtain the expression for the electric field due to a uniformly charged thin spherical shell at a point (i) outside the shell, (ii) inside the shell.
(5)VI. Answer any TWO of the following questions:2 × 5 = 10
42.
Three point charges are placed in air as shown: +2 μC at O, +3 μC at A, 30 cm from O, and −4 μC at B, 40 cm from O, with OA ⊥ OB. Find the magnitude and direction of the resultant force on the charge at O.
(5)43.
Two point charges +4 μC and +9 μC are placed 50 cm apart in air. (a) Find the point on the line joining them where the resultant electric field is zero. (b) Find the magnitude and direction of the resultant electric field at the midpoint of the line joining them.
(5)44.
An electric dipole consists of charges ±8 nC separated by 5 mm. Find (a) its dipole moment, (b) the electric field at a point on its axis 20 cm from its centre, (c) the electric field at a point on its equatorial line 20 cm from its centre. (d) If the dipole is placed in a uniform field of 5 × 104 N C−1 at 30° to it, find the torque on it. (e) What is the maximum torque?
(5)*** End of the question paper ***