Physics Model Figures

Physics Model Figures (model diagram + function curves)

A standard two-panel layout for physics teaching figures: a conceptual model diagram on the left (charges, field lines, vectors, test points) beside a function-curve plot on the right (e.g. $E(r)$, $V(r)$). Side-by-side, equal or near-equal width, shared visual style.

This skill captures the pattern already implemented in this repo (src/point_charge_field.py, src/magnetic_field_loop.py). Reuse that shape rather than reinventing it.

When to use

• A physics illustration where a model/scene and one or more quantitative curves belong together (field + potential, spring + force/energy curve, charge distribution + E/V profile, wave + amplitude, orbit + r(t), …).
• Standalone single-panel model diagrams (see "single-panel variant" below).

Do NOT use for pure data plots (no model diagram) — those are plain matplotlib.

The recipe

1. Pick an output preset from src/_viz/output.py. Default per AGENTS.md: transparent PNG. For textbook serif style use Presets.PNG_TEXTBOOK (transparent PNG + serif) or Presets.SVG_TEXTBOOK (vector).

   from _viz.output import Presets
   SPEC = Presets.PNG_TEXTBOOK
   

2. Build a horizontal 2-panel figure — left = model, right = curves:

   fig = SPEC.figure()
   ax_model, ax_curves = fig.subplots(1, 2)
   fig.subplots_adjust(left=0.07, right=0.98, top=0.92, bottom=0.10, wspace=0.18)
   

3. Model panel (set_aspect("equal"), set_axis_off()) — draw with matplotlib.patches (Circle, FancyArrowPatch) and Line2D. Draw field/force vectors as arrowed line segments; mark test points with small white-filled circles; add a small loc="lower right" legend built from Line2D handles.
4. Curve panel — plot the governing function(s). Multiple curves that share compatible units may share one y-axis with a combined label (e.g. "E (N/C) / V (V)"); if units/ scales differ a lot, use a secondary axis (ax.twinx()). Add light grey grid (color="#dddddd").
5. Annotations: marked points as white-filled circles with a short label; legend loc="upper right" (so it won't cover the falling right tail of a $1/r$-type curve); sub-title via ax.set_title(...), no suptitle.
6. Save through the spec so format/padding/transparency stay consistent:

   path = SPEC.save(fig, OUT_DIR / "my_figure")
   

Conventions (from AGENTS.md)

• Default output is transparent-background PNG unless another format is explicitly requested.
• Reuse Presets.* from src/_viz/output.py; do not hard-code figsize/dpi/facecolor in each script.
• All annotation text in English; serif fonts for textbook style.
• No inter-panel separator lines, no overall title — only per-panel subtitles.
• Legend entries list the function/expression only (e.g. E = kq/r²), not the numeric constants.

Single-panel variant

If only a model diagram is needed (no function curves), drop the second panel and use a Presets.SVG_DOC-style square or near-square figure; see src/magnetic_field_loop.py for a 3D example (it adds an Arrow3D subclass so FancyArrowPatch renders under matplotlib's 3D projection sort).

Reference implementation

src/point_charge_field.py is the canonical example of the two-panel pattern. Copy it as a starting point and swap in the target physics:

• the constants block (K, Q, …)
• draw_model(ax) — replace the charge/field-line geometry
• draw_curves(ax) — replace the function(s) and y-axis label/range

Gotchas

• 3D arrows: FancyArrowPatch added to a 3D axes raises 'FancyArrowPatch' object has no attribute 'do_3d_projection' on matplotlib ≥ 3.11. Use the Arrow3D subclass that calls proj_transform (see src/magnetic_field_loop.py).
• Legend occlusion: $1/r^n$ curves fall steeply on the right — keep the function legend at upper right, not lower right.
• Transparent background requires transparent=True (the presets' default); passing facecolor overrides transparency and lays down an opaque rectangle.