JEE Main · 2025 · Shift-IImediumATOM-212

Given below are two statements: Statement (I): It is impossible to specify simultaneously, with arbitrary precision,…

Structure of Atom · Class 11 · JEE Main Previous Year Question

Question

Given below are two statements:

Statement (I): It is impossible to specify simultaneously, with arbitrary precision, both the linear momentum and the position of a particle.

Statement (II): If the uncertainty in the measurement of position and uncertainty in measurement of momentum are equal for an electron, then the uncertainty in the measurement of velocity is hπ×12m\geq \sqrt{\dfrac{h}{\pi}} \times \dfrac{1}{2m}.

In the light of the above statements, choose the correct answer from the options given below:

Options
  1. a

    Statement I is true but Statement II is false.

  2. b

    Both Statement I and Statement II are true.

  3. c

    Statement I is false but Statement II is true.

  4. d

    Both Statement I and Statement II are false.

Correct Answerb

Both Statement I and Statement II are true.

Detailed Solution

🧠 The "Aha!" Moment Heisenberg's Uncertainty Principle is the "speed limit" of information in the universe. It isn't a failure of our tools, but a fundamental property of nature. Statement I is the direct law. Statement II is a specific mathematical case where we try to balance the scales perfectly between position and momentum.

🗺️ The Proof for Statement II

  1. The Principle: ΔxΔph4π\Delta x \cdot \Delta p \ge \frac{h}{4\pi}
  2. The Condition: If uncertainties in position and momentum are equal (Δx=Δp\Delta x = \Delta p): (Δp)2h4π    Δph4π=12hπ(\Delta p)^2 \ge \frac{h}{4\pi} \implies \Delta p \ge \sqrt{\frac{h}{4\pi}} = \frac{1}{2} \sqrt{\frac{h}{\pi}}
  3. Link to Velocity: Since momentum uncertainty is Δp=mΔv\Delta p = m \Delta v: mΔv12hπ    Δv12mhπm \Delta v \ge \frac{1}{2} \sqrt{\frac{h}{\pi}} \implies \Delta v \ge \frac{1}{2m} \sqrt{\frac{h}{\pi}} Both statements are mathematically and conceptually flawless.

The Equal Uncertainty Trap If you ever see a problem where Δx=Δp\Delta x = \Delta p, remember that the uncertainty in velocity will always involve a square root of Planck's constant (hh). It’s a common "numerical shortcut" in competitive physics.

⚠️ Watch the Pi! h/4π\sqrt{h/4\pi} simplifies to 12h/π\frac{1}{2}\sqrt{h/\pi}. Don't let the placement of the 1/21/2 throw you off; it's simply the square root of the 1/41/4 that sits inside the original principle.

Answer: (b)\boxed{\text{Answer: (b)}}

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