Electric and Magnetic Fields near a Transmission Line

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This Demonstration shows the fields in the region near a transmission line consisting of a pair of cylindrical conductors. The electric and magnetic fields and the Poynting vector distribution are shown by colors and arrows. The sinusoidal frequencies of the voltage and current are assumed to be sufficiently low for the electric and magnetic fields of the slowly changing configurations to be analyzed by assuming static behavior. You can vary the gap and cylinder diameter, as well as the voltage, current, and power factor of the transmission line.

Contributed by: Y. Shibuya (October 2012)
Open content licensed under CC BY-NC-SA



Snapshot 1: an electric field distribution

Snapshot 2: a magnetic field distributions

Snapshot 3: a Poynting vector distribution

The electric field of parallel cylinders is calculated assuming parallel line charges with separation , where and are the gap length and cylinder diameter. The magnetic field is simulated by that of parallel line currents flowing along the cylinder axes (i.e. a uniform current density is assumed, ignoring the skin effect). The electric and magnetic vectors are in the plane of observation perpendicular to the cylinders. The average of the Poynting vector, given by , is directed along the axial direction of conductors.

The electric and magnetic field intensities (shown in colors for the peak values) are similarly distributed on the observation plane, but the directions are very nearly mutually perpendicular. The Poynting vector's intensity distribution in space decreases rapidly with the distance from the conductor.

The power of the transmission line is actually carried in a thin region on the outside of the conductors, not within the lines themselves.


[1] J. A. Stratton, Electromagnetic Theory, New York: McGraw–Hill, 1941.

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