Electromagnetic energy balance
The rate of field-energy change balances outward energy flow and power transferred to charges.
Key knowledge
In vacuum classical electrodynamics, u = ε₀E²/2 + B²/(2μ₀) is field energy density and S = E×B/μ₀ is its energy flux. Their fixed-volume balance includes J·E, the signed power transferred from the field to charges per unit volume (W m⁻³). A negative J·E means charges transfer energy to the field.
Values and equations
d/dt ∫_V u dV + ∮_∂V S·n dA = −∫_V J·E dV; u = ε₀E²/2 + B²/(2μ₀); S = E×B/μ₀
- u
- Field energy density J m⁻³
- S
- Energy flux W m⁻²
- J
- Charge current density (excluding displacement current) A m⁻²
- E
- Electric field V m⁻¹
- B
- Magnetic flux density T
- ε₀, μ₀
- Vacuum permittivity and permeability F m⁻¹, H m⁻¹
- n
- Outward unit normal
- dV, dA
- Volume and area elements m³, m²
- t
- Time s
- J·E
- Signed power transferred from the field to charges per unit volume W m⁻³
Conditions and scope
- Use a fixed control volume with outward normal and fields regular enough for the integrals.
- J includes currents in that volume; material polarization and magnetization require additional energy accounting.
Selected sections only. Arbitrary material energy formulas, quantum fields, irreversible-heat assumptions and universal positive J·E are not adopted; no whole-source revalidation or human certification.
Sources · 2
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
Evidence and review
Lecture 4, Poynting theorem slide: integral energy, boundary flux and E·J work.
Fresh 2026-10-11 selected source inspection: MIT Lecture 4, L4-2–L4-4 / PDF pages 2–4 distinguishes u [J/m³], S [W/m²], E·J [W/m³] and integrated power [W]. Fitzpatrick equations 100 and 108–110 and their prose give charge-current/field energy exchange and the signed power-density interpretation. Original fixed-volume balance, outward normal, two conditions and formula are retained.
2026-10-10 · Vacuum fixed-volume balance only; dielectric energy formulas are not generalized to arbitrary media.
2026-10-11 · Author/editor checked the power-versus-energy density dimensions and selected MIT/UT source bodies. The summary, J·E signed power interpretation and J charge-current label are clarified; original equation and conditions remain exact. The new independent record/coverage adoption review is pending. Historical review 2026-10-10 remains retained with its original scope.
Entry details
Permanent link to this entry ↗- Record identity
- physics-poynting-balance
- Revision
- university-reference.1.poynting-precision-20261011.1
- Structured reference record
- University reference · University core expansion