PSEB / CBSE CLASS 12 PHYSICS – SET 3
Maximum Marks: 70 Time: 3 Hours
Section A: 1 Mark Questions (20 × 1 = 20 Marks)
MCQs (i–xv):
i) The electrostatic potential due to a point charge at distance r is proportional to
(a) 1/r (b) 1/r² (c) r (d) r²
ii) Two capacitors 4 μF and 6 μF are connected in series. Their equivalent capacitance is
(a) 2.4 μF (b) 10 μF (c) 12 μF (d) 24 μF
iii) The SI unit of electric dipole moment is
(a) C/m (b) C·m (c) N·m (d) J/C
iv) A copper wire of resistance 5 Ω is stretched to double its length. New resistance will be
(a) 5 Ω (b) 10 Ω (c) 20 Ω (d) 25 Ω
v) The drift velocity of electrons in a conductor is directly proportional to
(a) number density of electrons (b) current (c) resistivity (d) temperature
vi) When a 12 Ω and a 6 Ω resistor are connected in parallel, the equivalent resistance is
(a) 18 Ω (b) 4 Ω (c) 2 Ω (d) 8 Ω
vii) The magnetic field at the center of a circular loop is proportional to
(a) current × radius (b) current / radius (c) radius / current (d) radius²
viii) A solenoid produces maximum magnetic field when
(a) its length is increased (b) its turns are increased (c) current is decreased (d) radius is increased
ix) The SI unit of magnetic permeability is
(a) H/m (b) Wb/m² (c) A/m (d) T·m
x) Lenz’s law is based on the principle of conservation of
(a) charge (b) energy (c) mass (d) momentum
xi) In a purely capacitive AC circuit, the current is
(a) in phase with voltage (b) leads voltage by 90° (c) lags voltage by 90° (d) zero
xii) In a step-down transformer, the secondary voltage is
(a) more than primary (b) equal to primary (c) less than primary (d) infinite
xiii) The electromagnetic wave with the longest wavelength is
(a) Gamma rays (b) X-rays (c) Radio waves (d) Ultraviolet rays
xiv) Which of the following is not a property of electromagnetic waves?
(a) They are longitudinal (b) They carry energy (c) They are transverse (d) They travel with speed of light
xv) The electric flux through a closed surface enclosing charge q is
(a) q/ε₀ (b) qε₀ (c) q²/ε₀ (d) zero
True/False (xvi–xx):
xvi) Electric lines of force never intersect each other. (True/False)
xvii) Current through a conductor is due to drift of electrons. (True/False)
xviii) The magnetic field inside a toroid is uniform. (True/False)
xix) An inductor allows AC to pass through easily. (True/False)
xx) Electromagnetic waves transport only electric energy. (True/False)
Section B: 2 Mark Questions (Q.2 → Q.8) 7 × 2 = 14 Marks
Q.2 (a) Define electric flux and write its SI unit.
OR
(b) A parallel plate capacitor of area 0.2 m², separation 0.01 m, is filled with air. Find its capacitance. (ε₀ = 8.85 × 10⁻¹² C²/N·m²)
Q.3 A dipole of moment 3 × 10⁻²⁹ C·m is placed in a uniform field of 10⁵ N/C. Find the maximum torque on it.
Q.4 (a) State Ohm’s law and conditions under which it is valid.
OR
(b) A wire of cross-section 2 mm² carries a current of 4 A. If number density of electrons is 8 × 10²⁸ m⁻³, calculate drift velocity. (e = 1.6 × 10⁻¹⁹ C)
Q.5 (a) State Kirchhoff’s current law.
OR
(b) Three resistors 2 Ω, 4 Ω, 6 Ω are in series. Find the total resistance.
Q.6 Write two applications of moving coil galvanometer.
Q.7 State Faraday’s second law of electromagnetic induction.
Q.8 Write any two properties of electromagnetic waves.
Section C: 3 Mark Questions (Q.9 → Q.15) 7 × 3 = 21 Marks
Q.9 Derive expression for capacitance of a parallel plate capacitor with dielectric of constant K.
Q.10 Explain the variation of resistivity of a semiconductor with temperature.
Q.11 Explain the construction and principle of a potentiometer. How can it be used to compare emf of two cells?
Q.12 (a) State Ampere’s circuital law. Derive expression for magnetic field due to a straight conductor of infinite length.
OR
(b) A wire of length 0.2 m carries a current of 8 A and is kept perpendicular to a magnetic field of 0.5 T. Calculate force on it.
Q.13 (a) Define paramagnetic materials. Give two examples and one use.
OR
(b) A circular coil of radius 0.05 m, 200 turns, carries current 1 A. Find magnetic field at center.
Q.14 (a) What is mutual induction? Derive expression for emf induced in terms of mutual inductance.
OR
(b) An AC source of emf E = 220 sin(314t) is connected to a 100 mH inductor. Find peak current.
Q.15 Draw labelled diagram of AC generator and explain its principle.
Section D: Comprehension Question (Q.16) 5 × 1 = 5 Marks
Read the passage and answer:
A charge moving in a magnetic field experiences a force perpendicular to both velocity and field, given by F = q(v × B). This is the basis of motion of charged particles in cyclotron and mass spectrometer. A current-carrying coil in a magnetic field experiences torque, used in measuring devices.
(a) Write expression for force on a moving charge.
(b) On what principle does cyclotron work?
(c) Which rule gives direction of force on current-carrying conductor?
(d) State the expression for torque on a coil of N turns carrying current I in field B.
(e) Name one instrument that works on this principle.
Section E: 5 Mark Questions (Q.17–Q.18) 2 × 5 = 10 Marks
Q.17 (a) Derive expression for electric potential at a point due to a dipole.
OR
(b) Derive expression for energy stored in a capacitor and explain in what form it is stored.
Q.18 (a) Derive expression for electric field due to a uniformly charged spherical shell using Gauss’s law.
OR
(b) Derive expression for capacitance of a parallel plate capacitor when a dielectric slab partially fills the space between plates.
