Modern Physics
Iron-56 binding energy
Stable iron peak — read binding energy per nucleon B/A.
Iron-56 binding energy — interactive Modern Physics simulation. Stable iron peak — read binding energy per nucleon B/A. Free browser-based virtual physics lab with live SI measurements and a guided tutorial.
Binding energy
Stable iron peak — read binding energy per nucleon B/A.
Iron-56 sits near the peak of binding energy per nucleon B/A on the nuclear stability curve. Fusion of light nuclei and fission of heavy ones both release energy by moving toward iron. Read B(Z,A) from the liquid-drop model: B/A is maximal near Fe, explaining why stars stop fusing past iron and why fission fragments are neutron-rich.
- B/A peak at Fe-56
- Mass defect
Investigation brief
Plan the question before you open the lab
The brief mirrors the prerendered page: driving question, competing predictions, variable roles, governing laws, setup, analysis and extension prompts remain visible and in this order.
Driving question
The semi-empirical mass formula (liquid-drop model) estimates a nucleus's total binding energy from its proton count Z and mass number A. Holding Z fixed at 26 (iron) and changing only A, does the model predict binding energy increasing steadily with every added nucleon?
Predictions to weigh
- Yes — more nucleons always means more total binding energy.
- No — the binding energy depends on a balance of competing terms (volume, surface, Coulomb, asymmetry) that shift differently as A grows, so it doesn't simply increase with every added nucleon.
- Binding energy always decreases as A increases.
Variable roles
What you set:
- Mass number A
What you measure:
- Total binding energy E_B (MeV)
How the investigation runs
- Open the iron-binding preset and press Reset. This preset models iron with Z = 26 protons.
- Enable the binding-energy readout.
- Set the mass number A for each trial (holding Z = 26 fixed) and record the total binding energy.
Governing equation
Mass–Energy Equivalence (Binding Energy) — B = Δmc²
A nucleus's mass is less than the sum of its separate protons and neutrons — that missing mass Δm converts to binding energy via B = Δmc², the energy holding the nucleus together. The value of Δm itself (and so B) comes from the semi-empirical mass formula's competing volume, surface, Coulomb, and asymmetry terms, modeling the nucleus as a liquid drop.
What the printable worksheet asks students to work out
- For one trial, compute E_B = a_vA − a_sA^(2/3) − a_cZ(Z−1)/A^(1/3) − a_a(A−2Z)²/A using a_v = 15.75, a_s = 17.8, a_c = 0.711, a_a = 23.7 (MeV), Z = 26. Compare to the table.
- Your three trials rise and then fall — E_B climbs from A = 56 to A = 80, then drops again by A = 110. Explain how the volume term +a_vA, which grows with every nucleon added, is eventually overtaken by the surface and asymmetry terms, and why −a_a(A−2Z)²/A penalises a nucleus whose neutron count runs far ahead of its proton count.
Where this shows up beyond the lab
- This same liquid-drop model explains why very heavy nuclei (large A) release energy through fission (splitting) while very light nuclei release energy through fusion (combining) — both processes move nuclei toward the peak binding-energy-per-nucleon region near iron. Explain why iron sits near this peak.
- This is a simplified version of the semi-empirical mass formula (it omits the pairing term for even/odd nucleon counts), so its predictions won't exactly match a real nuclear mass table. Why might physicists still find a simplified model like this useful for teaching, even knowing it's not perfectly accurate?
- AP Physics 2 — Unit 15: Modern Physics
- IB Physics — E.3 Radioactive decay
- General High School Physics — Modern physics intro
- NGSS High School Physics — Wave-particle duality of light
- Welcome to Iron Binding
- Select the nuclide
- Press Play
- Peak binding per nucleon
- Open the Properties panel
- You did it!
Open the interactive simulation to build the scene, press Play, and explore with live measurements and a guided tutorial.