Tamil Nadu Class 12 Physics Model Question Paper 1 — Samacheer Kalvi

Paper Pattern (70 Marks)

Part I — Choose the Correct Answer (1 mark each)

1. The force between two point charges Q₁ and Q₂ separated by distance r in vacuum is F. If the distance is halved and each charge is doubled, the new force is:

• (a) F (b) 4F (c) 8F (d) 16F

2. The drift velocity of electrons in a conductor is of the order of:

• (a) 10⁸ m/s (b) 10⁴ m/s (c) 10⁻⁴ m/s (d) 10⁻⁸ m/s

3. A magnetic field exerts no force on:

• (a) a moving electron (b) a moving proton (c) a stationary charge (d) a current-carrying wire

4. The phenomenon of electromagnetic induction is based on:

• (a) Ampere's law (b) Faraday's law (c) Gauss's law (d) Coulomb's law

5. In a step-down transformer, the turns ratio N₁:N₂ is:

• (a) > 1 (b) = 1 (c) < 1 (d) = 0

6. Which of the following has the shortest wavelength?

• (a) Gamma rays (b) X-rays (c) Ultraviolet (d) Visible light

7. The critical angle for total internal reflection depends on:

• (a) frequency only (b) the refractive index of the media (c) wavelength only (d) intensity

8. In Young's double slit experiment, if the slit separation is doubled, the fringe width:

• (a) halves (b) doubles (c) remains the same (d) quadruples

9. The photoelectric effect proves the:

• (a) wave nature of light (b) particle nature of light (c) dual nature of light (d) neither

10. The ground state energy of hydrogen is:

• (a) +13.6 eV (b) +3.4 eV (c) −13.6 eV (d) −3.4 eV

11. In an n-type semiconductor, the majority carriers are:

• (a) electrons (b) holes (c) protons (d) neutrons

12. The output of a NAND gate is 0 only when:

• (a) both inputs are 0 (b) both inputs are 1 (c) one input is 1 (d) one input is 0

13. Which wave propagation mode is used by FM radio?

• (a) Ground wave (b) Sky wave (c) Space wave (d) All three

14. At resonance in an LCR circuit, the impedance equals:

• (a) XL (b) XC (c) R (d) √(XL² + XC²)

15. The SI unit of magnetic flux is:

• (a) Tesla (b) Weber (c) Henry (d) Ampere

Part II — Very Short Answer (2 marks each)

1. State Gauss's law in electrostatics and write its mathematical form. 2. Define resistivity. On what factors does it depend? 3. State Lenz's law. What conservation law is it based on? 4. What is displacement current? Who introduced this concept? 5. Define stopping potential in the photoelectric effect. 6. What is a universal gate? Name two universal gates.

Part III — Short Answer (3 marks each)

1. Derive the expression for the electric field at an axial point due to an electric dipole. 2. Explain Kirchhoff's laws with a suitable circuit and equations. 3. Derive the expression for the force per unit length between two parallel current-carrying conductors. 4. Explain the working of a half-wave rectifier with a circuit diagram and output waveform. 5. Describe Young's double slit experiment and derive the formula for fringe width. 6. State and explain Einstein's photoelectric equation. Define work function. 7. Write the truth tables for AND, OR, NOT, NAND, NOR gates. 8. Explain the three modes of wave propagation with frequency ranges. 9. Define self-inductance. Derive the self-inductance of a solenoid.

Part IV — Long Answer (5 marks each; internal choice)

Q1 (a): State Coulomb's law in vector form. Explain the principle of superposition of forces. Find the net force on a charge placed at the vertex of an equilateral triangle with two equal charges at the other vertices.

OR

Q1 (b): Derive the expression for the electric potential at a point due to an electric dipole. Discuss the special cases.

Q2 (a): Derive the condition for balance of a Wheatstone bridge using Kirchhoff's laws. Explain how it is used in a metre bridge to find an unknown resistance.

OR

Q2 (b): Explain the principle and working of a potentiometer. Describe how it is used to (i) compare EMFs of two cells and (ii) determine the internal resistance of a cell.

Q3 (a): Derive the expression for the energy of an electron in the nth orbit of hydrogen atom using Bohr's model. Calculate the wavelength of the first line of the Balmer series. (Given: RH = 1.097 × 10⁷ m⁻¹)

OR

Q3 (b): Describe the construction and working of a p-n junction diode. Explain forward and reverse bias characteristics with I-V graphs.

Answer Key Hints

• Q1(a): F_net on the central charge involves vector addition using geometry of equilateral triangle
• Q2(a): Apply KCL at junctions and KVL for two loops; balance when P/Q = R/S
• Q3(a): En = −13.6/n² eV; for Balmer series: 1/λ = RH(1/4 − 1/n²)