Physics Question Paper - December 2025
Answer Key
Note: This answer key provides solutions for the questions in the provided physics question paper. For questions requiring diagrams or detailed derivations, brief explanations are provided.
SECTION 4.1 (5 × 1 = 5 marks)
1. Pick out the unit of magnetic flux in SI system.
Answer: (c) Weber
Weber (Wb) is the SI unit of magnetic flux. Tesla meter (Tm²) is also a valid unit but Weber is the standard SI unit.
2. The polarization of light proves that light is a:
Answer: (b) Transverse wave
Only transverse waves can be polarized. Longitudinal waves cannot be polarized as their oscillations are along the direction of propagation.
3. The speed of electromagnetic waves in a medium is less than that in ______.
Answer: vacuum (or free space)
Electromagnetic waves travel fastest in vacuum (c = 3×10⁸ m/s). In any material medium, their speed decreases due to interaction with matter.
4. The total electric flux for the set of charges (-q, +2q, -3q, +5q) kept in a closed surface inside water (ε_r = 80) is
Answer: (c) \(\frac{+3q}{80 ε_0}\)
Net charge inside surface = (-q) + (+2q) + (-3q) + (+5q) = +3q
By Gauss's law: Φ = Q_net/(ε) = (3q)/(ε_r ε_0) = (3q)/(80 ε_0)
By Gauss's law: Φ = Q_net/(ε) = (3q)/(ε_r ε_0) = (3q)/(80 ε_0)
5. A concave lens is also known as a ______ lens.
Answer: Diverging lens
A concave lens diverges parallel rays of light, hence it's called a diverging lens.
6. Temperature coefficient of resistivity for a conductor is ______.
Answer: positive
For conductors, resistivity increases with temperature, so the temperature coefficient is positive.
7. The efficiency of a transformer is the ratio of output power to input power.
Answer: (a) True
Efficiency = (Output Power)/(Input Power) × 100% for any device, including transformers.
SECTION B (5 × 2 = 10 marks)
8. (a) Define an electric field. (1 mark)
Answer: Electric field at a point is defined as the electric force experienced by a unit positive test charge placed at that point.
(b) Write ANY TWO properties of electric field lines. (1 mark)
Answer:
- Electric field lines start from positive charges and end at negative charges.
- They never intersect each other.
- The density of field lines indicates the strength of the electric field.
9. (a) Define electric potential. (1 mark)
Answer: Electric potential at a point is the work done in bringing a unit positive charge from infinity to that point against the electric field.
(b) Write down the expression for electric potential due to electric dipole and find the values of potential along equatorial and axial lines. (1 mark)
Answer:
- Electric potential due to a dipole: V = (1/(4πε₀)) (p·cosθ/r²), where p is dipole moment
- Along equatorial line (θ = 90°): V = 0
- Along axial line (θ = 0° or 180°): V = ± (1/(4πε₀)) (p/r²)
10. Derive the expression for the force per unit length between two parallel current-carrying conductors.
Answer:
Consider two parallel conductors separated by distance d, carrying currents I₁ and I₂.
Magnetic field due to first conductor at the location of second: B₁ = μ₀I₁/(2πd)
Force on second conductor of length L: F₂ = I₂L × B₁ = I₂L × (μ₀I₁/(2πd))
Force per unit length: F/L = (μ₀I₁I₂)/(2πd)
The force is attractive if currents are in the same direction, repulsive if in opposite directions.
11. (a) State Gauss law in magnetism. (1 mark)
Answer: The net magnetic flux through any closed surface is zero. Mathematically: ∮B·dA = 0
(b) A bar magnet is cut into two pieces as shown below. Redraw the diagram with correct markings of the magnetic poles: (1 mark)
Answer: When a bar magnet is cut into two pieces, each piece becomes a complete magnet with both North and South poles. The broken ends become opposite poles.
12. (a) State Snell's law of refraction. (1 mark)
Answer: n₁ sin θ₁ = n₂ sin θ₂, where n₁ and n₂ are refractive indices of the two media, θ₁ is angle of incidence, and θ₂ is angle of refraction.
(b) Show that light ray bends towards normal when it travels from rarer to denser medium. (1 mark)
Answer: When light travels from rarer (lower n) to denser (higher n) medium, according to Snell's law: sin θ₁/sin θ₂ = n₂/n₁ > 1. Thus sin θ₁ > sin θ₂, so θ₁ > θ₂. Hence the ray bends towards the normal.
13. Define self-inductance. What is 1 Henry of self-inductance?
Answer:
Self-inductance (L) of a coil is defined as the ratio of the induced emf to the rate of change of current in the coil: ε = -L(dI/dt)
1 Henry of self-inductance is defined as the inductance when a rate of change of current of 1 ampere per second induces an emf of 1 volt in the coil.
14. Match the following for A.C. circuit:
| A | B |
|---|---|
| (a) Purely inductive | (q) current lags behind voltage |
| (b) Purely capacitive | (p) current leads the voltage |
| (c) Purely resistive | (s) voltage and current are in phase |
| (d) Series LCR | (r) current may lag, lead or be in phase with voltage |
SECTION C (6 × 3 = 18 marks)
15. (a) What is total internal reflection? (1 mark)
Answer: Total internal reflection is the phenomenon where a light ray traveling from a denser medium to a rarer medium is completely reflected back into the denser medium at the interface, provided the angle of incidence exceeds the critical angle.
(b) What are the conditions to achieve total internal reflection? (2 marks)
Answer:
- Light must travel from a denser medium to a rarer medium (n₁ > n₂).
- The angle of incidence must be greater than the critical angle (θ > θ_c), where θ_c = sin⁻¹(n₂/n₁).
16. (a) Define Lorentz force. (1 mark)
Answer: Lorentz force is the combined force experienced by a charged particle moving in both electric and magnetic fields: F = q(E + v × B).
(b) Compute the magnitude of the magnetic field of a long, straight wire carrying a current of 1 A at distance of 1 m from it. (2 marks)
Answer:
Using Ampere's law for a long straight wire: B = (μ₀I)/(2πr)
Given: I = 1 A, r = 1 m, μ₀ = 4π × 10⁻⁷ Tm/A
B = (4π × 10⁻⁷ × 1)/(2π × 1) = (4π × 10⁻⁷)/(2π) = 2 × 10⁻⁷ T