Coulomb's law and electrostatic potential
The electric field from a point charge Q in free space is E = Q/(4πε₀r²) directed radially. Potential is V = Q/(4πε₀r), defined relative to a zero reference. E = −∇V relates field to potential. Superposition applies for multiple charges.
Check yourself: Is your reference potential at infinity, and does the field point from high to low potential?
Gauss's law and divergence
Gauss's law in integral form states ∮ D·dS = Q_enc. In differential form, ∇·D = ρ_v. Choose a Gaussian surface that exploits symmetry so D is constant on the surface and either parallel or perpendicular to dS. Without symmetry, use superposition or the differential form.
Check yourself: Is D constant in magnitude and aligned with the surface normal everywhere on your Gaussian surface?
Biot–Savart law and Ampère's law
The Biot–Savart law gives the magnetic field from a current element: dB = μ₀ I dl × r̂ / (4πr²). Ampère's law ∮ H·dl = I_enc simplifies calculations when current symmetry allows H to be factored out of the integral. The right-hand rule determines field direction.
Check yourself: Does your Ampèrian loop lie on a surface where H is constant and tangential?
Maxwell's equations and boundary conditions
Maxwell's four equations unify electrostatics, magnetostatics, and time-varying fields. At a boundary between two media, the tangential component of E and the normal component of B are continuous (in the absence of surface charge and current). Boundary conditions determine reflection and transmission of waves.
Check yourself: Have you matched both tangential and normal components at the interface?
Uniform plane wave propagation
In a lossless medium, a uniform plane wave has E and H perpendicular to each other and to the propagation direction. The intrinsic impedance is η = √(μ/ε), and the phase velocity is v_p = 1/√(με). In free space, η₀ = 120π ≈ 377 Ω and v_p = c.
Check yourself: Are E, H, and the propagation direction mutually orthogonal and right-handed?