Domain: Natural Science — Physics
Target concept: The interaction force between charges

Video generation prompt: Two identical lightweight spheres are suspended from the same support by insulating threads. A positively charged glass rod is brought into contact with each sphere in turn. The two spheres are then brought close to each other, and their mutual interaction is observed.

Evaluation guide:
(i) Target concept: Electrostatic repulsion between two identically charged conductive objects.
Minimum mechanism: Spheres must be conductive (to accept charge via contact), lightweight (so electrostatic force dominates gravity), and insulated (to retain charge); rod must carry net positive charge; charge transfer must occur via direct contact (not induction); repulsion requires both spheres to hold like charge of sufficient magnitude to overcome gravitational torque.
(ii)Storyline: Frame 1–3: Static setup: two spheres hanging vertically side-by-side from same support, threads taut, rod held away. Frame 4–6: Rod contacts first sphere (visible touch, no spark), withdraws; then contacts second sphere (separate, sequential touch). Frame 7–10: Spheres lifted and brought toward each other along horizontal plane; at close proximity, both spheres simultaneously deflect outward symmetrically, maintaining thread suspension. Perspective: fixed frontal view, full setup always in frame, no occlusion of spheres or threads.

Manual:
Experimental Procedure
1. Suspend two identical lightweight spheres from a single rigid support using two separate insulating threads.
2. Touch the positively charged glass rod to the first sphere, then remove it; touch the same rod to the second sphere, then remove it.
3. Bring the two spheres slowly close to each other.
Observation of Experimental Phenomena
Both spheres repel each other — they move apart symmetrically when brought near.
Summary of Experimental Principles
Contact charging transfers positive charge from the rod to each sphere; like charges repel, so the identically charged spheres exert mutual electrostatic repulsion.

Prompt Grounding rubric:
level_1: No two lightweight spheres are shown, no insulating threads, no shared support point, and no positively charged glass rod; or the rod never touches either sphere and the spheres are never brought close together.
level_2: The two spheres and threads appear, but they hang from two separate supports rather than one shared point; the glass rod is present but never actually touches either sphere; the spheres are moved toward each other before the rod has touched both of them; or only one sphere is ever contacted by the rod.
level_3: Two similar-looking lightweight spheres hang by insulating threads from one shared support; the charged glass rod touches the first sphere, is withdrawn, then touches the second sphere; the two spheres are then brought close together. All of these objects and steps are present and identifiable, even if fine details like exact symmetry are imprecise.
level_4: All elements of 3-score are present AND the two spheres are similar in size and shape; both threads look like the same thin, non-metallic cord rather than bare wire; a hand holds the rod and touches each sphere in a separate, distinguishable action rather than both at once; each sphere hangs still while being touched and only starts moving during the approach step.
level_5: All elements of 4-score are present AND the shared support is one visibly rigid piece (e.g., a single bar or clamp) with the two threads tied at separate but nearby points on it; the rod touches each sphere once, with a clear pause between the two contacts; the spheres are brought together along a straight, level path; no extraneous objects or occlusions (other than the hand needed to hold and guide the rod) obscure the core setup at any frame, and that hand does not itself block the view of either sphere, its thread, or the contact point.

Scientific and Causal Correctness rubric:
level_1: Neither sphere moves apart when the two are brought close after both have touched the positively charged rod; or the spheres swing toward each other; or they stay motionless when brought near despite carrying charge; or a sphere changes color, lights up, deflates, or bursts; or the rod shown is uncharged or negatively charged.
level_2: Only one sphere swings away while the other stays put when the two are brought close; the spheres bump together instead of separating; the spheres are shown repelling before the rod has touched both of them; the charging step is shown as rubbing the rod against a sphere instead of simple contact; or the spheres start moving apart before they are actually brought near each other.
level_3: After the positively charged rod has touched both spheres in turn, bringing them close causes both to swing outward at the same time; neither attraction nor motionlessness appears during the approach.
level_4: All elements of 3-score are present AND the outward swing only begins once the spheres are actually close together, not while they are still being carried toward each other; both spheres swing outward continuously and roughly symmetrically, without one twisting, tilting, or lagging noticeably behind the other; no spark, flash, or other sign of the charge leaking away is shown.
level_5: All elements of 4-score are present AND the two spheres swing outward by visibly similar angles from vertical, consistent with carrying equal charge; both threads stay taut through the swing; neither sphere keeps swinging or oscillating once the repulsion settles; nothing in the shot (attraction, sticking together, or an asymmetric response) contradicts like-charge repulsion between the two spheres.

Spatiotemporal Consistency rubric:
level_1: One or both spheres disappear and reappear in a different position; a suspension thread detaches from the support and reattaches at a different point with no visible cause; the shared support itself vanishes or is replaced partway through; the glass rod passes through a sphere's surface with no visible contact; the camera cuts to an unrelated angle; the whole scene abruptly flashes bright or goes black for one or more frames with no visible cause; or a sphere flickers in and out of visibility.
level_2: A thread appears to change its attachment point on the support between frames; a sphere rotates more than roughly a quarter turn with no rod contact or air current to explain it; the rod's position jumps abruptly from one frame to the next instead of moving smoothly toward or away from a sphere; the two spheres appear to pass through each other during the approach; or the support or backdrop shifts sideways between frames as if the whole scene had been repositioned.
level_3: The support stays in the same place, each thread remains attached at both ends, the two spheres hang straight down before the rod ever touches them, the rod's approach to and contact with each sphere plays out as one continuous motion, and each sphere's outward swing during the repulsion phase is also continuous, with no teleporting, no thread coming loose, and no spheres passing through each other.
level_4: All elements of 3-score are present AND every motion (the rod approaching or withdrawing, the spheres swinging apart) looks physically smooth rather than jerky or instantaneous; neither sphere floats free of its thread or hovers momentarily; the threads stay taut rather than visibly slackening during the repulsion; no sphere or the rod suddenly duplicates into a faint second copy or leaves a trailing blurred outline as it moves.
level_5: All elements of 4-score are present AND the camera viewpoint never pans, tilts, or zooms, and both spheres, both threads, and the support remain fully inside the frame for the entire clip; the two spheres swing outward by visually similar angles from vertical without one snapping ahead of the other; and there is no frame where the rod, a thread, or a sphere jumps to a new position or vanishes momentarily (no dropped or duplicated frames disrupting the motion).