Himachal Hpbose Class 12 Physics — Real Previous-Year Papers

A 4.5 cm needle is placed 12 cm away from a convex mirror of focal length 15 cm. The location of the image is (a) formed at 6.67 cm behind the mirror. (b) formed at 67 cm behind the mirror. (c) formed at 70 cm same side of the mirror. (d) formed at 5.57 cm same side of the mirror.

Let the point P be at distance r from the centre of the dipole on the side of the charge q, as shown in Figure, then. (a) E(+q) = q / [4πε₀ (r + a)²] , directed along P̂ (b) E(+q) = q / [4πε₀ (r - a)²] , directed along P̂ (c) E(+q) = -q / [4πε₀ (r + a)³] , directed along P̂ (d) None of these

The resistance of a wire is 'R' ohm. If it is melted and stretched to 5 times its original length, its new resistances will be (a) 5 R (b) R/5 (c) 25 R (d) R/25

To convert a galvanometer into a voltmeter, (a) a high resistance is connected in parallel (b) a low resistance is connected in series (c) a low resistance is connected in parallel (d) a high resistance is connected in series

In an a.c. circuit having pure inductor, current (a) leads the voltage by an angle of π/2 (b) leads the voltage by an angle of π (c) lags the voltage by an angle of π/2 (d) lags the voltage by an angle of π

The work function of caesium metal is 2.14 eV. When light of frequency 6 × 10¹4 Hz is incident on the metal surface, photoemission of electrons occurs. What is the Stopping potential? (a) 34 V (b) 3.4 V (c) 340 V (d) 0.34 V

The energy equivalent of 1 mg of substance is (a) 9 × 10¹0 J (b) 9 × 10¹3 J (c) 1.35 × 10¹4 J (d) 3 × 10¹3 J

A solenoid of length 0.5 m has a radius of 1 cm and has 500 turns. It carries current of 5 A. What is the magnitude of the magnetic field inside the solenoid? **OR** Out of an ammeter, voltmeter and galvanometer, which one has lowest and highest resistance? Justify your response.

Apply the Kirchhoff's loop rule to the closed loop ABCA of the network shown in the figure, and write the relevant equation.

Describe two important properties of paramagnetic substances.

Verify the laws of photoelectric emission using the photoelectric equation.

Define one atomic mass unit. Find the energy equivalent of one atomic mass unit, first in Joules and then in MeV.

Two large, thin metal plates are parallel and close to each other. On their inner faces, the plates have surface charge densities of opposite signs and of magnitude 17.0 × 10^-22 C/m². What is E⃗ : (a) in the outer region of the first plate, (b) in the outer region of the second plate, and (c) between the plates?

Write a short note on microwaves and infrared waves.

Define electric energy and power. Where does the power come from? Equation P = V²/R has an important application to power transmission. How can power loss be minimised in the transmission cables connecting the power stations to homes and factories.

What are the drawbacks of Rutherford's nuclear model of the atom? State and explain the postulates of Bohr's model of the atom.

What is meant by self-induction? Define self inductance. Derive an expression for the self inductance of a long solenoid.

Derive the laws of refraction of light on the basis of 'Huygens' wave theory of light.

[Case study] Consider the charges q1 and q2 initially at infinity and determine the work done by an external agency to bring the charges to the given locations. Suppose, first the charge q1 is brought from infinity to the point r1⃗. There is no external field against which work needs to be done, so work done in bringing q1 from infinity to r1⃗ is zero. From the definition of potential, work done in bringing charge q2 from infinity to the point r2⃗ is q2 times the potential at r2⃗ due to q1. (Figure: two point charges q1 and q2 joined by a line of length r12.) (i) To bring q2 from infinity to r2. The work done in this step is (a) W2 = 1/(4πε₀) × 2q1q2/r12³ (b) W2 = 1/(4πε₀) × q1q2/r12³ (c) W2 = 1/(4πε₀) × q1q2/r12² (d) W2 = 1/(4πε₀) × q1q2/r12

[Case study, continued] The charges q1 and q2 produce a potential, which at any point P will be (a) V1,2 = 1/(4πε₀) × (q1/r1P² + q2/r2P²) (b) V1,2 = 1/(4πε₀) × (q1/r1P + q2/r2P) (c) V1,2 = 1/(4πε₀) × (q1/r1P - q2/r2P) (d) V1,2 = 1/(4πε₀) × (2q1/r1P + 3q2/r2P)

[Case study, continued] Let us calculate the potential energy of a system of three charges q1, q2 and q3 located at r1⃗, r2⃗, r3⃗ respectively as shown in the figure (triangle with sides r12, r13, r23). To bring q1 first from infinity to r1⃗, no work is required (W1 = 0). (iii) The work done in bringing q3 from infinity to the point r3 is - (a) W3 = 1/(4πε₀) × (2q1q3/r13 + 2q2q3/r23) (b) W3 = 1/(4πε₀) × (q1q2/r12 + q1q3/r13) (c) W3 = 1/(4πε₀) × (q1q3/r13 + q2q3/r23) (d) W3 = 1/(4πε₀) × (q1q3/r12 - q2q3/r23)

[Case study, continued] The total work done in assembling the system of three charges q1, q2 and q3 at the given locations is given by (a) U = 1/(4πε₀) × (q1q2/r12 + q1q3/r13 + q2q3/r23) (b) U = 1/(4πε₀) × (q1q2/r12 - q1q3/r13 + q2q3/r23) (c) U = 1/(4πε₀) × (q1q2/r12 + q1q3/r13 - q2q3/r23) (d) U = 1/(4πε₀) × (q1q2/r13 + q1q3/r22 + q2q3/r12)

Using phasor diagram, derive an expression for the impedance of a series LCR-circuit. What do you mean by the resonance condition of a series LCR-circuit? Calculate its resonant frequency. **OR** A series LCR circuit connected to a variable frequency 230 V source. L = 5.0 H, C = 80 μF, R = 40 Ω. (a) Determine the source frequency which drives the circuit in resonance. (b) Obtain the impedance of the circuit and the amplitude of current at the resonating frequency. (c) Determine the rms potential drops across the three elements of the circuit. Show that the potential drop across the LC combination is zero at the resonating frequency.

(i) If f = 0.5 m for a glass lens, what is the power of the lens? (ii) The radii of curvature of the faces of a double convex lens are 10 cm and 15 cm. Its focal length is 12 cm. What is the refractive index of glass? (iii) A convex lens has 20 cm focal length in air. What is its focal length in water? (Refractive index of air-water = 1.33, refractive index for air-glass = 1.5) **OR** By stating assumptions and new cartesian sign convention, derive the lens maker's formula for a convex lens. Using the formula, determine the radius of curvature required to manufacture a double-convex lens to be made from glass of refractive index 1.55, given that both faces have the same radius of curvature and the focal length of the lens is 20 cm.

(i) What are intrinsic semiconductors? (ii) C, Si and Ge have same lattice structure. Why is C insulator while Si and Ge are intrinsic semiconductors? (iii) Using a schematic two-dimensional representation of Si or Ge structure showing covalent bonds at low temperature, explain how are charge carriers generated in an intrinsic semiconductor?