Flux across a plane curve
Problem 12.428 · medium
Find the flux of \( \displaystyle \mathbf F = \left\langle - y, x \right\rangle \) across the circle of radius 3, counterclockwise (outward normal).
- With r(t) = <3*cos(t), 3*sin(t)>, the normal n ds = ⟨y′, −x′⟩ dt (to the right of the direction of travel), so flux = ∫ (P y′ − Q x′) dt.
- \[ \frac{d}{d t} 3 \sin{\left(t \right)} \left. - y \right|_{\substack{ x=3 \cos{\left(t \right)}\\ y=3 \sin{\left(t \right)} }} - \frac{d}{d t} 3 \cos{\left(t \right)} \left. x \right|_{\substack{ x=3 \cos{\left(t \right)}\\ y=3 \sin{\left(t \right)} }} = 0 \]P y′ − Q x′.✓ Proved
- \[ \int\limits_{0}^{2 \pi} 0\, dt = 0 \]Integrate.✓ Proved
Answer \( 0 \)
Lines: 2 proved, 1 not checked. The answer was also checked a second way, without looking at the solution. The reviewers disagree about how one step is explained; every verdict is in the receipt. How we verify
The full receipt
| Line | Status | Checked by | Detail |
|---|---|---|---|
| 1 | Not checked | — | a sentence; read, not computed |
| 2 | ✓ Proved | sympy 1.14.0 | simplify(lhs - rhs) reduced to 0 |
| 3 | ✓ Proved | sympy 1.14.0 | simplify(lhs - rhs) reduced to 0 |
| answer, a second way | ✓ Checked independently | sympy 1.14.0 + mpmath 1.3.0 | F · n summed over a 20,000-segment polyline |
Reviewers
gpt-oss:20b: passqwen3.6:27b-mlx: fail (error) — The normal vector formula used, <y', -x'>, points inward for a counterclockwise parametrization, whereas the problem specifies the outward normal. Additionally, the flux integral formula derived (P y' - Q x') corresponds to the inward flux, leading to the incorrect sign (though the magnitude is correct, the reasoning is flawed regarding direction).
Every verdict on record (6)
qwen3.6:27b-mlx: fail (error) 2026-10-18 — The normal vector formula used, <y', -x'>, points inward for a counterclockwise parametrization, whereas the problem specifies the outward normal. Additionally, the flux integral formula derived (P y' - Q x') corresponds to the inward flux, leading to the incorrect sign (though the magnitude is correct, the reasoning is flawed regarding direction).gpt-oss:20b: pass 2026-10-18qwen3.6:27b-mlx: fail (error) 2026-10-16 — The normal vector formula used, <y', -x'>, points inward for a counterclockwise parametrization, whereas the problem specifies an outward normal. Additionally, the flux integrand formula P y' - Q x' corresponds to the inward normal; the correct outward flux integrand is P x' + Q y' (or equivalently -P y' + Q x'). Using the wrong sign leads to the incorrect conclusion that the flux is 0, when it should be 18pi.gpt-oss:20b: pass 2026-10-16qwen3.6:27b-mlx: fail (error) 2026-10-15 — The normal vector formula is incorrect for the outward normal. For a counterclockwise parametrization, the outward normal is <x', y'>, not <y', -x'> (which points inward). Consequently, the flux integral should be P x' + Q y', not P y' - Q x'. Although the final answer happens to be correct due to symmetry, the method described is wrong.gpt-oss:20b: pass 2026-10-15
Proved: SymPy reduced the difference between the two sides to zero. Checked independently: a separate
method, named above, confirmed it. Checked numerically: the two sides agree at every sampled point, which is evidence,
not proof. Reviewed: a model or a person read it; that is all a sentence can have. Solution by
generator:structured/flux_plane, checked 2026-10-18 with SymPy 1.14.0.