The Uncertainty Principle from Wave Packets · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Why pinning down waves spreads them

Science · Quantum & Modern Physics · ages 19-20
Name ______________________   Date ____________
  1. A sharper microscope can prepare a state with ΔxΔp below ħ/2.

    Circle one:   True   False

  2. For finite position and momentum standard deviations along one axis, which relation holds?

    • ΔxΔp is greater than or equal to ħ/2.
    • ΔxΔp is exactly equal to ħ in every state.
    • ΔxΔp is less than or equal to ħ/2.
  3. Why does a localised packet involve a spread of possible momenta?

    • Its detector always supplies a random heating force.
    • Its different wave numbers have different momenta.
    • Its position density directly gives its momentum values.
  4. A hypothetical state has a nucleus-sized Δx instead of an atom-sized Δx. What follows from the uncertainty relation?

    • Its momentum lower bound becomes much smaller.
    • Its momentum lower bound remains unchanged.
    • Its momentum lower bound becomes much larger.
  5. Use ħ = 1.056e-34 J s. If Δx = 2.00e-10 m, the momentum lower bound is a × 10^-25 kg m/s. What is a?

    Answer: ______________

  6. A state has ΔxΔp = 0.80ħ. Which conclusion follows from the position-momentum uncertainty relation?

    • It must be a minimum-product state.
    • It satisfies the stated lower bound.
    • It must violate the stated lower bound.
  7. Δx is halved. What is the new momentum lower bound divided by the old momentum lower bound?

    Answer: ______________

  8. Let L > 0. State A has Δx = 4L and Δp = ħ/L. State B has Δx = 2L and Δp = ħ/(2L). Because both spreads shrink, B violates ΔxΔp ≥ ħ/2.

    Circle one:   True   False

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Answer key

For grown-ups. Fold this page away before handing over the rest.

Why pinning down waves spreads them W1-mt_DPYnoGksRd-s1

  1. False · Improved equipment may reduce instrument error, but it cannot remove the prepared state's spread bound.
  2. ΔxΔp is greater than or equal to ħ/2. · The relation is ΔxΔp ≥ ħ/2. Equality and larger products are allowed.
  3. Its different wave numbers have different momenta. · A localised packet combines wave numbers. Since p = ħk, those components have different momenta.
  4. Its momentum lower bound becomes much larger. · The nucleus-sized Δx is much smaller. Since Δp_min = ħ/(2Δx), its momentum lower bound is much larger.
  5. 2.64 · Δp_min = ħ/(2Δx) = 1.056e-34/(4.00e-10) = 2.64e-25 kg m/s, so a = 2.64.
  6. It satisfies the stated lower bound. · The bound is 0.50ħ. The product 0.80ħ exceeds it, so it satisfies the inequality without attaining equality.
  7. 2 · The lower bound is inversely proportional to Δx. Halving Δx doubles this bound, so the ratio is 2.
  8. False · A has product 4ħ and B has product ħ. Both exceed ħ/2. Actual spreads can both decrease when the starting product is above the bound.
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