Nuclear Models: the Liquid Drop and the Semi-Empirical Mass Formula · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

The nucleus as a charged drop

Science · Quantum & Modern Physics · ages 20-21
Name ______________________   Date ____________

  1. Why does the surface term reduce binding energy?

    • The surface spins faster than the interior
    • The surface holds all the protons
    • Surface nucleons have fewer neighbours pulling on them

  2. Which five terms make up the semi-empirical mass formula?

    • Volume, surface, Coulomb, asymmetry, pairing
    • Kinetic, spring, friction, charge, spin
    • Proton, neutron, electron, photon, gluon

  3. The Coulomb push grows as protons are added, since each new proton repels all the existing ones.

    Circle one:   True   False

  4. The Coulomb repulsion spreads over the nuclear radius. What softens it?

    • Adding more protons to the nucleus
    • A larger radius spreading the same charge
    • Cooling the nucleus down

  5. For a fixed mass number, how do you find the most stable charge?

    • Take the highest Z available
    • Take the charge nearest to zero
    • Take the bottom of the parabola in Z

  6. Why does fission of a heavy nucleus release energy?

    • The products weigh more than the parent
    • Fission removes all the neutrons
    • The lighter products bind tighter per nucleon toward iron

  7. A student picks the highest Z in an isobar chain as most stable. What is the mistake?

    • Higher Z always means more neutrons too
    • Isobars all have identical mass exactly
    • Stability sits at the parabola bottom, not at the edge

  8. Why does fission of a light nucleus not release energy?

    • Light nuclei have no binding energy at all
    • Light products would bind looser per nucleon, away from iron
    • Fission only works on elements past uranium
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Answer key

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

The nucleus as a charged drop W1-mt_-cTGNdUJLz-s1

  1. Surface nucleons have fewer neighbours pulling on them · Missing neighbours at the skin mean lost attraction.
  2. Volume, surface, Coulomb, asymmetry, pairing · Each term pictures one feature of the charged drop.
  3. True · That mutual repulsion is what the Coulomb term pays for.
  4. A larger radius spreading the same charge · The same push spread over a bigger drop costs less.
  5. Take the bottom of the parabola in Z · The formula minimum is the tightest bound nuclide.
  6. The lighter products bind tighter per nucleon toward iron · Sliding up the curve toward iron frees the difference.
  7. Stability sits at the parabola bottom, not at the edge · Coulomb cost grows with Z, so the winner sits in the middle.
  8. Light products would bind looser per nucleon, away from iron · Down the low side of the curve, joining pays instead of splitting.
Worksheet · LightMySky