The 15 one-mark questions from the March 2024 12th Standard Physics public exam, in paper order, with the correct option marked and a short explanation.
Q1
The principle based on which a solar cell operates is :
- A. Photovoltaic actionCorrect
- B. Diffusion
- C. Carrier flow
- D. Recombination
Explanation. A solar cell is a p-n junction that converts light energy directly into electrical energy; this is the photovoltaic action.
Q2
If the amplitude of the magnetic field is \(3\times10^{-6}\) T, then the amplitude of the electric field for a electromagnetic wave is :
- A. 600 \(Vm^{-1}\)
- B. 100 \(Vm^{-1}\)
- C. 900 \(Vm^{-1}\)Correct
- D. 300 \(Vm^{-1}\)
Explanation. \(E_0 = cB_0 = 3\times10^{8}\times3\times10^{-6} = 900\ Vm^{-1}\).
Q3
A toaster operating at 240 V has resistance of 120 Ω. Its power is :
- A. 240 W
- B. 400 W
- C. 480 WCorrect
- D. 2 W
Explanation. \(P = V^2/R = 240^2/120 = 57600/120 = 480\) W.
Q4
Atomic number of H - like atom with ionization potential 122.4 V for n = 1 is :
- A. 3Correct
- B. 4
- C. 2
- D. 1
Explanation. For a hydrogen-like atom, ionization energy from n = 1 is \(13.6Z^2\) eV. So \(13.6Z^2 = 122.4\), \(Z^2 = 9\), Z = 3.
Q5
The transverse nature of light is shown in :
- A. scattering
- B. interference
- C. polarisationCorrect
- D. diffraction
Explanation. Only transverse waves can be polarised; interference and diffraction occur for longitudinal waves too. So polarisation shows the transverse nature of light.
Q6
In an oscillating LC circuit, the maximum charge on the capacitor is Q. The charge on the capacitor when the energy is stored equally between the electric and magnetic field is :
- A. \(\frac{Q}{\sqrt{2}}\)Correct
- B. \(\frac{Q}{2}\)
- C. Q
- D. \(\frac{Q}{\sqrt{3}}\)
Explanation. Total energy \(\frac{Q^2}{2C}\). When the electric energy is half of it, \(\frac{q^2}{2C} = \frac{1}{2}\cdot\frac{Q^2}{2C}\), so \(q = \frac{Q}{\sqrt{2}}\).
Q7
A bar magnet of length 'ℓ' and magnetic moment '\(P_m\)' is bent in the form of an arc as shown in figure. The new magnetic dipole moment will be :
- A. \(\frac{2}{\pi}P_m\)
- B. \(P_m\)
- C. \(\frac{1}{2}P_m\)
- D. \(\frac{3}{\pi}P_m\)Correct
Explanation. Arc angle 60° = π/3, so \(\ell = r\pi/3\) and \(r = 3\ell/\pi\). The two ends and the centre form an equilateral triangle, so the new pole separation is \(r = 3\ell/\pi\). New moment \(= m\cdot\frac{3\ell}{\pi} = \frac{3}{\pi}P_m\).
Q8
The speed of light in an isotropic medium depends on :
- A. the nature of propagation
- B. its intensity
- C. the motion of the source w.r.t. medium
- D. its wavelengthCorrect
Explanation. The refractive index of a medium, and hence the speed of light in it, varies with wavelength (dispersion). It does not depend on intensity or the motion of the source.
Q9
In a series RL circuit, the resistance and inductive reactance are the same. Then the phase difference between voltage and current in the circuit is :
- A. \(\frac{\pi}{6}\)
- B. \(\frac{\pi}{4}\)Correct
- C. Zero
- D. \(\frac{\pi}{2}\)
Explanation. \(\tan\phi = X_L/R = 1\), so \(\phi = \pi/4\).
Q10
Two identical conducting balls having positive charges \(q_1\) and \(q_2\) are separated by a center to center distance 'r'. If they are made to touch each other and then separated to the same distance, the force between them will be :
- A. more than beforeCorrect
- B. less than before
- C. zero
- D. same as before
Explanation. After contact each ball has \(\frac{q_1+q_2}{2}\). Since \(\left(\frac{q_1+q_2}{2}\right)^2 \ge q_1q_2\), the force becomes more than before (it stays the same only if \(q_1 = q_2\)).
Q11
Three capacitors are connected in triangle as shown in figure. The equivalent capacitance between the points A and C is :
- A. 4 μF
- B. 2 μF
- C. 8 μF
- D. 6 μFCorrect
Explanation. Between A and C, the 4 μF capacitor AC is in parallel with AB and BC in series (4×4/8 = 2 μF). Equivalent = 4 + 2 = 6 μF.
Q12
The threshold wavelength for a metal surface whose photoelectric work function is 3.313 eV :
- A. 6000 Å
- B. 4125 Å
- C. 2062.5 Å
- D. 3750 ÅCorrect
Explanation. \(\lambda_0 = \frac{hc}{\phi_0} \approx \frac{12400}{3.313}\) Å ≈ 3750 Å.
Q13
Light transmitted by Nicol prism is :
- A. plane polarisedCorrect
- B. partially polarised
- C. elliptically polarised
- D. unpolarised
Explanation. A Nicol prism removes the ordinary ray by total internal reflection and transmits only the extraordinary ray, which is plane polarised.
Q14
The gravitational waves were theoretically proposed by :
- A. Albert EinsteinCorrect
- B. Conrod Rontgen
- C. Edward Purcell
- D. Marie Curie
Explanation. Albert Einstein predicted gravitational waves in 1916 from his general theory of relativity.
Q15
The value of forbidden energy gap for Si at room temperature is :
- A. 1.1 V
- B. 0.7 eV
- C. 1.1 eVCorrect
- D. 0.7 V
Explanation. The forbidden energy gap is an energy, measured in eV; for silicon at room temperature it is about 1.1 eV (0.7 V is its barrier potential).
About this paper
These are the Part I one-mark multiple-choice questions from the March 2024 12th Standard Physics public examination conducted by the Tamil Nadu Directorate of Government Examinations. The answers and explanations are prepared by TN Online Test for revision. Each question links to the textbook chapter it comes from, so you can go back to that chapter's notes and MCQs.
