Rotating electric dipole (field sketch)
A point electric dipole that rotates at a constant angular frequency radiates electromagnetic waves. In the far-field radiation zone, the time-averaged Poynting flux has the familiar dipole angular pattern modulated by rotation: observers see sidebands at multiples of the mechanical frequency because the dipole moment vector is harmonically varying in time. This simulator emphasizes qualitative features—how radiation strengthens with dipole moment and frequency, and how the pattern differs from a fixed-axis Hertzian dipole—while using idealizations such as a pure rotating dipole in free space, negligible higher multipoles, and no material boundaries. Near-field reactive components and exact angular factors are simplified so the visualization stays readable. Students connect the exercise to antennas, molecular rotation spectra, and the general principle that accelerating charge (here, rotating separation of charge) sources radiation.
Who it's for: Undergraduate electromagnetism and antenna-theory students who already studied static dipoles and want a time-harmonic, rotational variant before full multipole expansions.
Key terms
- Rotating dipole
- Electric dipole moment
- Dipole radiation
- Sidebands
- Far field
- Poynting vector
- Time-harmonic source
- Angular pattern
How it works
Two point charges ±q orbit in the plane. Electric field lines are computed from Coulomb superposition at each grid point and shown as arrows. This is a near-field electrostatic picture; real radiation involves retardation and B fields — use it to motivate time-varying dipole moments before the full EM wave simulator.
Frequently asked questions
- How is this different from the fixed Hertzian dipole page?
- A linearly oscillating dipole has a dipole moment along a line, giving the classic sin² θ power pattern about that axis. A rotating in-plane dipole has two orthogonal components with a phase quadrature; the radiation field mixes those contributions and the spectrum is not a single pure tone viewed from all directions.
- Why does rotation introduce frequency structure beyond the carrier?
- The dipole moment seen along a fixed direction varies with both cos ωt and sin ωt components. Amplitude-modulated or doubly periodic currents generate harmonics in the radiated power; the simulator illustrates the idea without claiming a specific experimental bandwidth.
- Are magnetic-dipole effects included?
- No. A pure electric dipole model is assumed. Magnetic dipole radiation would require circulating currents and has a different angular dependence and scaling.
- What real systems resemble a rotating dipole?
- Symmetric rotors with displaced charge, certain molecular transitions, and simplified pedagogical models of circularly polarized emission all share the rotating-dipole cartoon, though real devices add substrate, ground planes, and ohmic loss.
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