PSEB / CBSE CLASS 12 — PHYSICS — SET-2
MM : 70 Time : 3 Hr
Section A: 1 Mark Questions (MCQs) 1 × 20 = 20 Marks
i) The SI unit of electric potential is
(a) J/C (b) N (c) C (d) J
ii) Two identical point charges +q and +q are separated by distance d. The electric field at the midpoint is
(a) zero (b) along the line joining the charges (c) perpendicular to the line joining charges (d) infinite
iii) A capacitor of capacitance 5 μF is charged to 50 V and disconnected. If the plate separation is halved, the new potential difference (neglecting fringing) becomes
(a) 25 V (b) 50 V (c) 100 V (d) 200 V
iv) The resistivity of a conductor depends mainly on
(a) length (b) temperature (c) area of cross-section (d) applied voltage
v) In a series circuit containing resistors R1, R2 and R3, the current through each resistor is
(a) same (b) different (c) zero in the largest resistor (d) maximum in the largest resistor
vi) A straight wire of length 0.4 m carrying current 2 A is placed perpendicular to a magnetic field of 0.3 T. The magnitude of magnetic force on the wire is
(a) 0.24 N (b) 0.6 N (c) 1.5 N (d) 0.02 N
vii) The SI unit of magnetic flux is
(a) Tesla (b) Weber (c) Gauss (d) Henry
viii) The direction of induced emf in a coil is given by
(a) Fleming’s left-hand rule (b) Fleming’s right-hand rule (c) Lenz’s law (d) Biot-Savart law
ix) The self-inductance of a solenoid depends on
(a) number of turns (b) area of cross-section (c) permeability of core material (d) all of these
x) In a pure inductive AC circuit, the phase of current relative to voltage is
(a) in phase (b) current lags by 90° (c) current leads by 90° (d) 180° out of phase
xi) Which electromagnetic waves have the shortest wavelength?
(a) Infrared (b) Visible (c) X-rays (d) Radio waves
xii) The source of electromagnetic waves is
(a) stationary charges (b) uniformly moving charges (c) accelerated charges (d) permanent magnets
xiii) TV signals are normally transmitted using
(a) Gamma rays (b) X-rays (c) Radio waves (d) Ultraviolet rays
xiv) In an ideal transformer, the ratio of secondary to primary voltages equals the ratio of turns. If primary = 200 turns and secondary = 50 turns, and primary voltage is 240 V, secondary voltage is
(a) 30 V (b) 60 V (c) 480 V (d) 120 V
xv) The electric flux through a closed surface that encloses no net charge is
(a) zero (b) depends on surface shape (c) depends on field at surface (d) infinite
Section B: True/False (xvi → xx) 1 × 5 = 5 Marks
xvi) The electrostatic potential is the same at all points inside a hollow charged conductor. (True/False)
xvii) Increasing the cross-sectional area of a conductor increases the drift velocity for the same current. (True/False)
xviii) The magnetic field inside a long solenoid is directly proportional to its number of turns per unit length. (True/False)
xix) In a pure capacitive AC circuit, the current leads the voltage by 90°. (True/False)
xx) Electromagnetic waves do not require any material medium for propagation. (True/False)
Section C: 2 Mark Questions (Q.2 → Q.8) 7 × 2 = 14 Marks
Q.2 (a) Define electric dipole moment and give its SI unit.
OR
(b) Write the dimensional formula of capacitance.
Q.3 A parallel plate capacitor of capacitance 8 μF is connected to a 150 V battery. Calculate (i) the charge stored, (ii) the energy stored.
Q.4 (a) Define resistivity and state its SI unit.
OR
(b) A wire of length 1.5 m and cross-sectional area 0.5 mm² has resistance 3 Ω. Find the resistivity of the material.
Q.5 State Kirchhoff’s Voltage Law and give one simple example of its application.
Q.6 Name two practical applications of an oscilloscope in electrical measurements.
Q.7 Write two differences between electric field lines and magnetic field lines.
Q.8 A copper wire (number density of electrons = 8.5 × 10²8 m⁻³, charge of electron = 1.6 × 10⁻¹⁹ C) carries a current of 4 A. If its cross-sectional area is 1 mm², calculate the drift velocity of electrons.
Section D: 3 Mark Questions (Q.9 → Q.15) 7 × 3 = 21 Marks
Q.9 Derive the expression for the energy stored in a capacitor and show U = ½ C V².
Q.10 State Ohm’s law. Sketch and explain the V–I graph for an ohmic conductor.
Q.11 Explain the working principle and use of a meter bridge for finding an unknown resistance. (Give a neat labelled diagram.)
Q.12 (a) State Ampere’s circuital law. Use it to find the magnetic field inside a long solenoid of n turns per unit length carrying current I.
OR
(b) A straight conductor of length 0.4 m carrying 6 A is placed perpendicular to a magnetic field of 0.25 T. Calculate the force on it.
Q.13 (a) What are ferromagnetic materials? Give two examples and one application.
OR
(b) A circular coil of radius 0.08 m has 120 turns and carries a current of 1.5 A. Calculate the magnetic field at its center.
Q.14 (a) Define self-induction. Derive an expression for the energy stored in an inductor.
OR
(b) An AC source of emf E = 150 sin(200πt) is connected to an inductor of 40 mH. Find the peak value of current if reactance of inductor is XL.
Q.15 Draw a labelled diagram of a step-up transformer and explain briefly how it works.
Section E: Comprehension Question (Q.16) 5 × 1 = 5 Marks
Q.16 Read the passage and answer briefly:
A coil rotating in a uniform magnetic field experiences a change of magnetic flux linkage and thus an emf is induced in it (Faraday’s law). The direction of induced current is such as to oppose the change in flux (Lenz’s law). In AC generators, mechanical rotation produces alternating emf. In circuits containing inductors and capacitors, voltage and current may show phase differences.
(a) State Faraday’s law of electromagnetic induction.
(b) What does Lenz’s law state?
(c) Name the device in which mechanical rotation is used to generate AC.
(d) Which circuit element opposes rapid change in current?
(e) In an AC generator, how does the polarity of induced emf change with time?
Section F: 5 Mark Questions (Q.17–Q.18) 2 × 5 = 10 Marks
Q.17 (a) What is the neutral axis of an electric dipole? Derive the expression for the electric field on the axial line of an electric dipole.
OR
(b) Derive the expression for the capacitance of a parallel plate capacitor and show it equals ε₀A/d (vacuum). Where is the energy stored?
Q.18 (a) Using Gauss’s law, derive the expression for the electric field due to an infinite uniformly charged plane sheet.
OR
(b) Derive the expression for the capacitance of a parallel plate capacitor with dielectric slab of constant K filling the space between plates. What happens to capacitance if the slab fully fills the space?
