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The image-method certificate for a point charge above a grounded plane

Physical scope

The ideal electrostatic problem of a point charge held above an infinite grounded conducting plane, read in the framework's SI conventions: on the vacuum half-space the image potential is harmonic off the source, has the point-charge distributional source, vanishes on the plane, decays like |x|^-2, and is unique in the stated continuous-boundary and decay class; the plane's induced density is the idealization's bookkeeping normal-derivative reading. The item asserts no finite-plane edge effects, no conductor thickness, no dynamics, no self-force and no measurement, and the half-space conductor and noncompact sheet are explicitly outside the bounded-conductor model and the declared source types. The induced density is explicitly smooth on the whole plane, has absolutely convergent total charge −q, and the full-space point-plus-plane divergence identity is verified with local tests as a separate mathematical idealization. Matching singularities allow any remainder of order o(∣x−p∣−1).

Statement

Assume the Axiom of Countable Choice (ACω) inherited from the Newtonian, polar and distribution interfaces (The Axiom of Countable Choice (ACω)). Let

H:={x∈R3:x3>0},Π:={x3=0},p:=ae3 (a>0),

Here e3:=(0,0,1) is the third coordinate unit vector, a carries metres, and the coordinates are SI numerical readings in the named frame. Let q∈R be a charge parameter (coulombs) and ε0>0 the framework constant (Framework-local SI units, the vacuum constants and the exact/measured status of their values). Define, with arbitrary values on the exceptional sets,

φI(x):=q4πε0(1∣x−p∣−1∣x+ae3∣)(x∈H∖{p}),EI:=−∇φI.

The map EI is a spatial field of The electric and magnetic fields as fixed-time spatial vector fields in the fixed-time reading, with the units of Framework-local SI units, the vacuum constants and the exact/measured status of their values. Then:

(a) [Candidate field] φI∈C∞(H∖{p}) and ΔφI=0 pointwise on H∖{p}: the first term is the Coulomb potential of p, whose n=3 fundamental solution is harmonic off its pole (The Laplace fundamental solution is harmonic off its pole, The localized Newtonian potential and the ideal point-charge Coulomb field(d)), and the second term is C∞ and harmonic on the whole of H because its pole −ae3 lies outside H; EI is C∞ on H∖{p}.

(b) [Distributional point source] φI∈Lloc1(H) and

−ΔTφI=qε0 δpin D′(H),

where TφI is the regular distribution of φI (Regular distribution from a locally integrable function, Locally integrable functions embed in distributions) and δp the Dirac distribution at p (Dirac delta and its derivatives); moreover the field satisfies div⁡EI=(q/ε0)δp and curl⁡EI=0 in D′(H), where the divergence and curl of EI are the distributional derivatives of its components (Distributional derivative). Thus (EI,0) is a pure electrostatic model on H with the fixed-time point source qδp, the static reading of the stationary point charge of Distributional Maxwell equations with point and stationary-sheet sources(b) (The static and pure electrostatic regimes and their fixed-time reading(e)).

(c) [Boundary trace and decay] φI extends continuously to H‾∖{p} with φI=0 on Π (the two denominators are equal there), and for ∣x∣≥2a the cancellation bound holds:

∣φI(x)∣≤2∣q∣ aπε0∣x∣2.

The bound follows by applying the mean value theorem along the segment from −ae3 to ae3 to the function y↦∣x−y∣−1, whose gradient is bounded by (∣x∣−a)−2 there (The mean value theorem, as the case g(x)=x of Cauchy's: for f continuous on [a,b] with a<b and differentiable on (a,b) there is c∈(a,b) with f(b)−f(a)=f′(c)(b−a), The chain rule for total derivatives: D(g∘f)(a)=Dg(f(a))∘Df(a), Continuity and derivatives of positive-base real powers, For a natural n≥1 the function x↦xn is differentiable everywhere with derivative ι(n) x n−1; for n=0 it is the constant 1, with derivative 0; for a natural n≥1 the function x↦x−n is differentiable at every x≠0 with derivative −ι(n) x−n−1; consequently every polynomial function is differentiable at every real, with the derivative computed term by term, The Euclidean inner product ⟨x,y⟩=∑k<nxkyk on Rn). In particular φI(x)→0 as ∣x∣→∞ in H.

(d) [Uniqueness in the declared class] let ψ:H∖{p}→R be C2 and harmonic on H∖{p}, continuous on H‾∖{p}, with ψ=0 on Π, ∣x−p∣(ψ(x)−φI(x))→0 as x→p and ψ(x)→0 as ∣x∣→∞ in H. Then ψ=φI on H∖{p}. Equivalently, the Dirichlet problem Δψ=0 on H∖{p}, ψ=0 on Π, with the leading singularity of the point charge at p and decay at infinity, has exactly one solution in the stated class.

(e) [Physics reading: ideal grounded plane in the fixed-time reading of Laboratory geometry: a restricted inertial chart, its spatial slices and fixed-time fields] the ideal grounded conducting half-space {x3<0} with the field EI on H, E≡0 below Π and the induced density σ:=ε0 n⋅E+∣Π=−ε0 ∂3φI∣Π (using the normal n=e3 from the conductor into the vacuum; σ∈C∞(Π)) is a model of the same equilibrium structure as The ideal electrostatic conductor model: it is a pure electrostatic field on H with the point source at p and the equipotential boundary condition φI=0 on Π; the pair (EI,0) satisfies the static electrostatic equations with source qδp on H, and the induced density is explicitly σ(x1,x2)=−qa/[2π(x12+x22+a2)3/2], is absolutely integrable over the plane, and has total charge ∫Πσ dS=−q. However, the half-space conductor is not a compact conductor body: the plane is not a compact embedded hypersurface, Π and {x3<0} are not among the geometric objects of The ideal electrostatic conductor model, the density σ is not a declared stationary-sheet source of Distributional Maxwell equations with point and stationary-sheet sources(c), and the noncompact plane pairing is not an adopted compact-sheet source of that definition. As a separately specified mathematical idealization, extending EI by zero below the plane gives div⁡E~=(qδp+σδΠ)/ε0 and curl⁡E~=0 on R3, where (σδΠ)(η)=∫R2σ(x′)η(x′,0)dx′. This verified identity does not extend the framework postulate to arbitrary noncompact sheets. The clause is the explicitly declared idealization whose exact mathematical content is (a)-(d).

(f) [Scope] the statement concerns the ideal point charge above an ideal grounded plane in the declared decay class; it excludes finite-plane edge effects, conductor thickness, transients, moving charges, radiation, self-force and any measurement; no uniqueness is asserted outside the class of (d), and the point-source singularity is never evaluated.

Facts & Assumptions

Given: The half-space H, the plane Π, the point p=ae3, a>0, the charge parameter q, the constant ε0>0, and the candidate φI and field EI of the statement.

[F1]

The n=3 fundamental solution is Φ(z)=1/(4π∣z∣), smooth and harmonic on R3∖{0}; the Coulomb point-charge branch of The localized Newtonian potential and the ideal point-charge Coulomb field(d) identifies the point-charge potential φqp=(q/ε0)Φ(⋅−p) as locally integrable with distributional Laplacian −ΔTφqp=(q/ε0)δp on R3 and field Eqp=−∇φqp satisfying div⁡Eqp=(q/ε0)δp, curl⁡Eqp=0 in D′(R3) (The Laplace fundamental solution is harmonic off its pole, The localized Newtonian potential and the ideal point-charge Coulomb field, The Laplacian of a C2 function and of a C2 vector field, Dirac delta and its derivatives).

[F2]

A C1 function on an open set represents its regular distribution and its distributional first derivatives are the regular distributions of its classical derivatives; distributional differentiation is linear and the Laplacian of a classical harmonic function therefore represents the zero distribution (Regular distribution from a locally integrable function, Locally integrable functions embed in distributions, Distributional derivative, Distribution, Local finite order characterization of distributions, If u,v are differentiable on [a,b] with u′,v′ integrable, then ∫abuv′=u(b)v(b)−u(a)v(a)−∫abu′v, Distributional differentiation is continuous and commutes, Test function space d of an open set).

[F3]

Local integrability in dimension three: the radial integral of ∣z∣−1 over a ball is finite by polar coordinates, finite sums and scalar multiples of locally integrable measurable functions are locally integrable, as are their products with locally bounded measurable functions, and continuous functions are Borel measurable (Polar coordinates decompose Lebesgue measure into r^{n-1} dr d sigma, The polar surface set function on the unit sphere, Integral over a measurable subset, Monotonicity and nonnegative homogeneity of the nonnegative integral, Continuous functions on Euclidean spaces are Borel measurable, Agreement with the existing polar sphere measure, The closed form for the volume of the unit n-ball).

[F4]

Mean value theorem and derivative calculus: for g(y):=∣x−y∣−1 with x fixed, g is C∞ off x and ∇yg(y)=(x−y)/∣x−y∣3, so ∣∇yg(y)∣=∣x−y∣−2; differentiation with respect to x instead gives ∇x∣x−y∣−1=−(x−y)/∣x−y∣3; the mean value theorem bounds ∣g(p)−g(−p)∣ by ∣p−(−p)∣sup⁡∣∇g∣ on the segment (The mean value theorem, as the case g(x)=x of Cauchy's: for f continuous on [a,b] with a<b and differentiable on (a,b) there is c∈(a,b) with f(b)−f(a)=f′(c)(b−a), The chain rule for total derivatives: D(g∘f)(a)=Dg(f(a))∘Df(a), Continuity and derivatives of positive-base real powers, For a natural n≥1 the function x↦xn is differentiable everywhere with derivative ι(n) x n−1; for n=0 it is the constant 1, with derivative 0; for a natural n≥1 the function x↦x−n is differentiable at every x≠0 with derivative −ι(n) x−n−1; consequently every polynomial function is differentiable at every real, with the derivative computed term by term, Directional derivatives and partial derivatives of a map U⊆Rm→Rn, Sums, scalar multiples, products and quotients: (f+g)′(c)=f′(c)+g′(c), (αf)′(c)=αf′(c), (fg)′(c)=f′(c)g(c)+f(c)g′(c), and (f/g)′(c)=(f′(c)g(c)−f(c)g′(c))/g(c)2 when g(c)≠0, The Euclidean inner product ⟨x,y⟩=∑k<nxkyk on Rn, The finite and reverse triangle inequalities for a norm; and for n≥1 every norm N on Rn satisfies N(x)≤C∥x∥1 and is Lipschitz, hence continuous, for d2, Ck Euclidean maps and diffeomorphisms).

[F5]

The half-space uniqueness lemma: a function continuous on H‾∖{p}, C2 and harmonic on H∖{p}, zero on Π, sub-Coulomb at p and vanishing at infinity in H is identically zero (Uniqueness for harmonic functions on a punctured half-space with zero trace, decaying at the pole and at infinity).

[F8]

For 0<r<R there is a smooth function χ:R3→[0,1] equal to one on B‾r(p) and supported strictly inside BR(p) (Compactly supported scaled Euclidean bumps).

Proof

technique · direct
1.1F1F2

Clause (a). Write φI=(q/ε0)Φ(⋅−p)−(q/ε0)Φ(⋅+ae3) [F1]. The first term is smooth and harmonic off its pole p; the second has its pole at −ae3, which lies outside the closed half-space H‾, so it is C∞ on H and harmonic there by [F1] and linearity of the Laplacian [F2]; hence φI∈C∞(H∖{p}), ΔφI=0 there, and EI=−∇φI is C∞ on H∖{p}.

1.2F1F2F3F7

Local integrability and distributional identities. φI is locally integrable on H: ∣x−p∣−1 is locally integrable in dimension three and ∣x+ae3∣−1 is continuous on H, and sums and scalar multiples of locally integrable measurable functions are locally integrable [F3]. Its regular distribution is TφI=(q/ε0)TΦ(⋅−p)−(q/ε0)TΦ(⋅+ae3) [F2, F3]. On H the second term is C∞ with classically vanishing Laplacian, so its distributional Laplacian is the regular distribution of the zero function [F2]; the first term has −ΔT=(q/ε0)δp on R3, hence on the open subset H [F1]. Summing gives −ΔTφI=(q/ε0)δp in D′(H). For the field, EI=Eqp−Eq−p where Eq−p:=−∇((q/ε0)Φ(⋅+ae3)) is smooth on H with div⁡Eq−p=0, curl⁡Eq−p=0 there [F2]; componentwise TEI,i=−∂iTφI on H by the same representation and [F2], so the distributional divergence and curl of EI are those of the sum, giving div⁡EI=(q/ε0)δp and curl⁡EI=0 in D′(H) [F1, F2]. This is (b); taking it together with [F7] reads (EI,0) as the pure electrostatic model on H with the fixed-time point source qδp, as asserted.

1.3F4

Clause (c). On Π one has ∣x−p∣=∣x+ae3∣ for every x with x3=0, so the bracket in φI vanishes and φI extends continuously (indeed C∞ away from x=p∉Π) with value 0 on Π. For the decay, fix x with ∣x∣≥2a and apply the mean value theorem to g(y):=∣x−y∣−1 on the segment from −ae3 to ae3: since every point y of that segment satisfies ∣x−y∣≥∣x∣−a≥a>0, one has ∣∇g(y)∣=∣x−y∣−2≤(∣x∣−a)−2 and therefore ∣g(p)−g(−p)∣≤2a(∣x∣−a)−2 [F4]. Multiplying by ∣q∣/(4πε0) gives ∣φI(x)∣≤∣q∣a/(2πε0(∣x∣−a)2)≤2∣q∣a/(πε0∣x∣2) because ∣x∣≤2(∣x∣−a) for ∣x∣≥2a. This is (c).

2.1F5step 1.1step 1.3

Clause (d). Let ψ be as in the statement and put w:=ψ−φI on H∖{p}. Then w is C2 and harmonic on H∖{p} and continuous on H‾∖{p} by (a) [step 1.1]; w=0 on Π because both functions vanish there [step 1.3]; ∣x−p∣w(x)→0 as x→p by the assumed leading-singularity condition; and w(x)→0 as ∣x∣→∞ in H because both ψ and φI do. By [F5] applied to w, w=0 on H∖{p}, so ψ=φI. This is (d).

2.2F4F6step 1.2

Clause (e), first part. On the plane, n=e3 and σ=−ε0∂3φI∣Π; differentiating the two Coulomb terms, whose gradients are −(x∓p)/∣x∓p∣3 [F4], at x3=0 gives ∂3∣x−p∣−1=a/(x12+x22+a2)3/2 and ∂3∣x+ae3∣−1=−a/(x12+x22+a2)3/2, so ∂3φI=qa/[2πε0(x12+x22+a2)3/2] and hence σ(x1,x2)=−qa/[2π(x12+x22+a2)3/2], a smooth bounded function; the boundary condition φI=0 on Π is the equipotential (zero) trace of the ideal grounded plane, and (EI,0) satisfies the static equations with source qδp on H by (b) [step 1.2]. The total induced charge is ∫R2σ dλ2=−qa2π∫0∞2πr dr(r2+a2)3/2=−qa[−1r2+a2]r=0∞=−q, with the polar-coordinate computation and the improper radial primitive justified by [F6]; the identical polar computation with ∣q∣ in place of −q gives ∫R2∣σ∣ dλ2=∣q∣<∞, including q=0.

3.1F1F2F6F8step 1.2step 1.3step 2.2

Clause (e), extension identity. Extend EI by zero to {x3≤0}, obtaining E~∈Lloc1(R3) by the point-field integrability in [F1] and smoothness away from p. The plane pairing is a distribution: on tests supported in a compact box its absolute value is bounded by sup⁡∣σ∣ times the finite area of the box's planar section times sup⁡∣η∣. Choose 0<r<R<a and χ from [F8], so χ=1 near p and its support lies in H. Split an arbitrary scalar test as η1=χη, η2=(1−χ)η. For η1 the half-space identities of step 1.2 give the divergence pairing (q/ε0)η(p) and zero curl pairing; its plane trace is zero. For η2, all field/test products are smooth away from the pole and absolutely integrable on a compact box. Fubini and one-dimensional integration by parts [F2, F6] therefore give, for every i,j, −∫HEI,j∂iη2=∫H(∂iEI,j)η2+1{i=3}∫R2EI,j(x′,0)η2(x′,0)dx′. Horizontal boundary terms vanish by compact support; the vertical integral over (0,∞) has precisely the displayed lower-end term. Summing i=j gives only ∫ΠEI,3η2=ε0−1∫Πση, since div⁡EI=0 away from p. Taking the antisymmetric derivative combinations gives the sheet term e3×E+=0, because the smooth zero plane trace of φI has zero horizontal derivatives; the classical curl also vanishes away from p by step 1.2. Adding the two test pieces gives div⁡E~=(qδp+σδΠ)/ε0 and curl⁡E~=0 in D′(R3). No limit at infinity or unproved divergence theorem on a cornered truncation is used.

4.1F7step 2.1step 3.1∎

Scope. The computations above concern exactly the ideal point charge above the ideal grounded plane in the declared class: the plane is not a compact embedded hypersurface and Π, {x3<0} and the plane pairing are not among the declared conductor or compact stationary-sheet objects of The ideal electrostatic conductor model and Distributional Maxwell equations with point and stationary-sheet sources(c) [F7]; the extension identity of [step 3.1] is a verified statement about the idealization and does not extend the framework postulate to arbitrary noncompact sheets. No finite-plane edge effect, conductor thickness, transient, moving charge, radiation, self-force or measurement is asserted, the singularity at p is never evaluated, and uniqueness is claimed only in the class of (d) [step 2.1]. This is (f).

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