Fire a beam of alpha particles at gold foil and watch how the atom's positive charge bends their paths, just as Rutherford did to discover the nucleus, and notice which model can bounce a particle almost straight back.

How do I explore?

• Use the Model toggle to switch between the Nuclear model (positive charge packed into a tiny point) and the Plum-pudding model (positive charge spread through the whole atom).
• Set Beam size to choose how many alpha particles to fire, and Alpha speed to choose how fast they launch. Faster alphas bend less and get closer to the charge.
• Turn on Watch one to follow a single alpha in slow motion and read its kinetic and potential energy as it climbs toward a nucleus.
• Press Play to fire the beam. Press Reset to clear the detector and start over.

What should I do/notice?

• Where do most alphas land on the detector ring? Where do the rare ones land?
• Does the plum-pudding model ever bounce an alpha almost straight back? Does the nuclear model?
• What does a backscatter tell you about how the positive charge is packed inside an atom?
• In Watch one, where along the path does an alpha slow down the most, and where does it speed back up?

What about the data?

When a run ends, the sim automatically records one row for each detected alpha. Each row gives its Deflection angle, Impact parameter (how far off-center it was aimed), Closest approach, Outcome (Passed straight, Deflected, or Backscattered), Initial KE, Min KE, and Model. Nuclear and plum-pudding runs add to the same table, so you can compare them side by side. Try a graph of Deflection angle against Impact parameter, or a histogram of Deflection angle.

Accessibility

Press Tab to focus the sim, then Tab again to move between the model toggle, beam size, alpha speed, and watch-one controls. Press Enter to change the focused control. Press Space to fire or pause the beam, and R to reset. The controls also work with a mouse or touch.

Fire a beam of alpha particles at gold foil and watch how the atom's positive charge bends their paths, just as Rutherford did to discover the nucleus, and notice which model can bounce a particle almost straight back.

Standards

HS-PS2-4 Use mathematical representations of Newton's Law of Gravitation and Coulomb's Law to describe and predict the gravitational and electrostatic forces between objects.

HS-PS3-5 Develop and use a model of two objects interacting through electric or magnetic fields to illustrate the forces between objects and the changes in energy of the objects due to the interaction.

Design intent

• Each alpha follows a real path bent by Coulomb repulsion from the positive charge in the foil, so students see Coulomb's law act rather than read it.
• The model toggle sets up a fair comparison. The nuclear model concentrates charge in a tiny point and produces rare backscatter; the plum-pudding model spreads charge out and never does.
• Watch one follows a single alpha and shows kinetic energy trading with electric potential energy as it climbs toward a nucleus and back.

Discussion prompts

• Where do most alphas land, and where do the rare ones land?
• Does the plum-pudding model ever backscatter? Does the nuclear model?
• What does backscatter tell you about how the positive charge is arranged in an atom?
• For one alpha, how does its lowest kinetic energy compare to its starting kinetic energy, and where along the path does that happen?

Data format

One Collect records one row per detected alpha when a run ends: Deflection angle (°), Impact parameter, Closest approach, Outcome (Passed straight, Deflected, or Backscattered), Initial KE, Min KE, and Model. Runs accumulate in one table, so nuclear and plum-pudding results sit side by side. Try a histogram of Deflection angle, or plot Deflection angle against Impact parameter.

Model details

Deflection angle is a true geometric angle. Distances and energies are shown in the sim's own relative units, not real-world femtometres or MeV, and energies are scaled so a medium-speed alpha starts at 100. The relationships are still honest: the closest an alpha gets to a nucleus grows with the nuclear charge and shrinks as the alpha moves faster, and an alpha aimed nearly at a nucleus (a small impact parameter) is deflected the most. The impact parameter is how far off-center the alpha's incoming path is from the nucleus it comes closest to. Each path is computed step by step with a method that keeps the total energy steady over the whole flight, and the repulsion is capped very close to the center so the force never shoots to infinity at a single point.

A gold nucleus is far heavier than an alpha and barely moves when hit, so the nuclei are held in place. The foil is thin enough that an alpha meaningfully interacts with only one nucleus, the one it comes closest to. No electrons are drawn, because electrons are far too light to bend an alpha's path. In the plum-pudding model the positive charge is spread evenly through the whole atom instead of packed into a point. Inside that spread-out charge the push on an alpha is weak and grows gently from the center outward, and only outside the atom does it follow the usual inverse-square falloff (force weakens with the square of the distance). That weak inside push is why the plum-pudding model produces only small deflections and never backscatters.