Side-by-side comparison
| Parameter | Electric Field | Magnetic Field |
|---|---|---|
| Source | Static or moving charge (∇·D = ρ_v) | Moving charge or current (∇×H = J + ∂D/∂t) |
| Force on charge | F = qE — acts on both stationary and moving charge | F = qv×B — acts only on moving charge |
| SI unit of field | V/m (Volt per metre) | A/m (Ampere per metre) for H; Tesla for B |
| Energy storage | w_e = ½εE² (J/m³) | w_m = ½μH² (J/m³) |
| Field lines | Start on +ve charge, end on –ve charge | Always closed loops, no magnetic monopole |
| Shielding material | Conductor (Faraday cage); copper foil | High-μ material (MuMetal, µ_r ≈ 20000) |
| Typical magnitude (practical) | 10³–10⁶ V/m in capacitor gaps | 10 µT–1 T in transformers and MRI coils |
| Boundary condition (tangential) | E_t1 = E_t2 (continuous across interface) | H_t1 – H_t2 = J_s (surface current dependent) |
| Related capacitor/inductor | Capacitor (stores ½CV²) | Inductor (stores ½LI²) |
| Duality | E analogous to H in dual problems | H analogous to E in dual problems |
Key differences
Electric field originates from charge (including stationary charge) and exerts force qE regardless of whether the charge is moving. Magnetic field requires current or a time-varying E; its force qv×B only acts on charges in motion — a stationary electron in a pure B field feels nothing. Energy densities are ½εE² and ½μH²; in free space at the same frequency, the ratio E/H = 377 Ω, the wave impedance. MuMetal shields B effectively; a copper Faraday cage kills E.
When to use Electric Field
Use electric field analysis when designing capacitor values in an LM7805 bypass network, calculating breakdown voltage across a PCB gap, or solving electrostatic shielding problems.
When to use Magnetic Field
Use magnetic field analysis when designing transformer cores, calculating inductance of a toroid, or predicting EMI from a high-current bus bar in a 400 V inverter.
Recommendation
For circuit design, choose the field type that matches your energy storage element — E for capacitors, B for inductors. On EMT theory papers, know both boundary conditions and energy density formulas; examiners split marks between them.
Exam tip: University papers almost always include a boundary condition question — know that tangential E is continuous while tangential H is discontinuous by the surface current density J_s.
Interview tip: A core-company interviewer will ask you to explain why a stationary charge creates an electric field but not a magnetic field, and expects the answer tied to the Lorentz force law F = q(E + v×B).