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The total energy of an… (Class 12 Physics SQP 2023-24, Q24)

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PhysicsCBSE Class 12 · Sample paper, session 2023-24 · 3 marks · Short answer

CBSE Class 12 Physics sample paper 2023-24: question 24

From the official CBSE sample question paper for 2023-24, section C, question 24, for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q24 · 3 marks · Short answerAtoms

The total energy of an electron in the first excited state of the hydrogen atom is about –3.4 eV.
a. What is the kinetic energy of the electron in this state?
b. What is the potential energy of the electron in this state?
c. Which of the answers above would change if the choice of the zero of potential energy is changed?

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2023-24.

Hints

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

Hint 1

In a hydrogen atom the kinetic energy equals the magnitude of total energy.

Hint 2

Potential energy is twice the kinetic energy in magnitude and negative; only potential energy depends on the reference zero.

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. For an electron in a Bohr orbit: and (so ).

    1 mark

  2. (a)
    (b)

    1 mark

  3. (c) The kinetic energy does not change. The potential energy, and hence the total energy, depend on the chosen zero of potential energy, so their values would change.

    1 mark

Answer: (a) 3.4 eV; (b) −6.8 eV; (c) K stays the same; U and E change

Where the marks are

  • Uses , (1 mark)
  • K = 3.4 eV and U = −6.8 eV (1 mark)
  • States K unchanged, U and total energy change (1 mark)

Revise the chapter

Practise this chapterBoard-style questions from Atoms, one at a time, marked against the answer key and marking points.Back to the full paperSample paper 2023-24: 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.