Coulomb-gauge source separation
In Coulomb gauge ∇·A=0, the vacuum scalar potential satisfies ∇²φ=−ρ/ε₀ while the vector potential obeys (∇²−c⁻²∂_t²)A=−μ₀J_T, J_T=J−ε₀∇∂_tφ and ∇·J_T=0.
Key knowledge
In Coulomb gauge ∇·A=0, the vacuum scalar potential satisfies ∇²φ=−ρ/ε₀ while the vector potential obeys (∇²−c⁻²∂_t²)A=−μ₀J_T, J_T=J−ε₀∇∂_tφ and ∇·J_T=0. With vanishing spatial boundary value the scalar potential uses the instantaneous Coulomb integral, yet the complete electric field retains causal propagation.
Values and equations
∇·A=0; ∇²φ=−ρ/ε₀; (∇²−c⁻²∂_t²)A=−μ₀J_T; J_T=J−ε₀∇∂_tφ
- A
- Coulomb-gauge vector potential T m
- φ
- Scalar potential V
- ρ
- Charge density C m⁻³
- J,J_T
- Current density and divergence-free part A m⁻²
- ε₀
- Vacuum permittivity F m⁻¹
- μ₀
- Vacuum permeability H m⁻¹
- c
- Vacuum light speed m s⁻¹
- t
- Time s
Conditions and scope
- Conserved smooth vacuum charge/current sources on all space, adequate spatial decay, consistently specified wave initial/radiation conditions and the Coulomb-gauge constraint.
- φ is a gauge-dependent auxiliary quantity: its instantaneous change is cancelled appropriately by −∂_tA in the physical field. Solving only the Poisson constraint and ignoring vector evolution does not describe time-dependent electromagnetic propagation.
Sources · 1
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
Evidence and review
Maxwell equations: gauge and transverse current notes (2018): Printed pages 5–7, equations 30–38: Coulomb Poisson constraint, instantaneous integral, A wave equation and transverse source, with causal cancellation paragraph. Reverses the wave-operator sign and expresses equation 35 as J_T=J−ε₀∇φ̇; continuity verifies ∇·J_T=0. Excludes its alternate label 'Landau' for the Lorenz condition and the extra 4π factors later printed in equations 71–74, which are not used.
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