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Case Study Based… (Class 12 Physics SQP 2024-25, Q30)

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PhysicsCBSE Class 12 · Sample paper, session 2024-25 · 4 marks · Case-based

CBSE Class 12 Physics sample paper 2024-25: question 30

From the official CBSE sample question paper for 2024-25, section D, question 30, for 4 marks. Try it first; the hints and our step-by-step solution are below.

Q30 · 4 marks · Case-basedCompetency-basedDual Nature of Radiation and Matter

Case Study Based Question: Photoelectric effect
It is the phenomenon of emission of electrons from a metallic surface when light of a suitable frequency is incident on it. The emitted electrons are called photoelectrons. Nearly all metals exhibit this effect with ultraviolet light but alkali metals like lithium, sodium, potassium, cesium etc. show this effect even with visible light. It is an instantaneous process i.e. photoelectrons are emitted as soon as the light is incident on the metal surface. The number of photoelectrons emitted per second is directly proportional to the intensity of the incident radiation. The maximum kinetic energy of the photoelectrons emitted from a given metal surface is independent of the intensity of the incident light and depends only on the frequency of the incident light. For a given metal surface there is a certain minimum value of the frequency of the incident light below which emission of photoelectrons does not occur.

  1. (I)

    In a photoelectric experiment plate current is plotted against anode potential.

    1. (A)A and B will have same intensities while B and C will have different frequencies
    2. (B)B and C will have different intensities while A and B will have different frequencies
    3. (C)A and B will have different intensities while B and C will have equal frequencies
    4. (D)B and C will have equal intensities while A and B will have same frequencies.
    1 mark
  2. (II)

    Photoelectrons are emitted when a zinc plate is

    1. (A)Heated
    2. (B)hammered
    3. (C)Irradiated by ultraviolet light
    4. (D)subjected to a high pressure
    1 mark
  3. (III)

    The threshold frequency for photoelectric effect on sodium corresponds to a wavelength of 500 nm. Its work function is about

    1. (A) J
    2. (B)1 J
    3. (C) J
    4. (D) J
    1 mark
  4. (IV)

    The maximum kinetic energy of photoelectrons emitted from a surface when photons of energy 6 eV fall on it is 4 eV. The stopping potential is

    1. (A)2 V
    2. (B)4 V
    3. (C)6 V
    4. (D)10 V
    1 mark

  5. OR

    (IV)

    The minimum energy required to remove an electron from a substance is called its

    1. (A)work function
    2. (B)kinetic energy
    3. (C)stopping potential
    4. (D)potential energy
    1 mark

Figure in the paper: Graph of plate current I vs anode potential V with three curves A, B, C. A and B start rising from the same (stopping potential) point on the V axis; A saturates at a lower current than B; C starts at a more negative potential (larger stopping potential) and saturates at the same current as B. See it in the official paper.

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2024-25.

Hints

Open one hint at a time and try again before the next.

Hint 1

Saturation current depends on intensity; stopping potential depends on frequency.

Hint 2

Work function = hc/λ₀ at threshold.

Hint 3

Stopping potential in volts equals the maximum KE in eV.

Step-by-step solution

Each step shows the marks it earns, as an examiner would award them. Checked by an independent re-solve from the question paper.

  1. (I) A and B have the same stopping potential, so the same frequency, but different saturation currents, so different intensities. B and C have the same saturation current, so equal intensity, but different stopping potentials, so different frequencies. Option (D).

    1 mark

  2. (II) Zinc needs ultraviolet light (frequency above its threshold) for photoemission; heating, hammering or pressure do not supply photons. Option (C).

    1 mark

  3. (III) J. Option (A).

    1 mark

  4. (IV) eV, so V. Option (B).

    1 mark

  5. (IV) Alternative: the minimum energy needed to remove an electron from a substance is its work function. Option (A).

Answer: (I) D; (II) C; (III) A; (IV) B [alternative (IV): A]

Where the marks are

  • (I) option D (1 mark)
  • (II) option C (1 mark)
  • (III) J, option A (1 mark)
  • (IV) 4 V, option B; or alternative: work function, option A (1 mark)

Revise the chapter

Practise this chapterBoard-style questions from Dual Nature of Radiation and Matter, one at a time, marked against the answer key and marking points.Back to the full paperSample paper 2024-25: every question, by section.

The question is transcribed from the official paper; figures are described in words. The solution, hints and chapter tag are ours. Spotted an error? Write tosupport@questionbuddy.in.