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Poynting vector

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description Poynting vector Overview

In electromagnetism, the Poynting vector represents the directional energy flux of an electromagnetic field. Named after the English physicist John Henry Poynting, it is mathematically defined as the cross product of the electric field vector and the magnetic field vector. The resulting vector points in the direction of wave propagation and its magnitude quantifies the rate of energy transfer per unit area. It is a fundamental concept used to calculate the intensity of electromagnetic radiation, including radio waves.

help Poynting vector FAQ

What does the direction of the Poynting vector mean?

The vector points in the direction that electromagnetic energy flows locally. In a simple plane wave, it points along the wave's propagation direction and is perpendicular to both the electric and magnetic fields.

How is the Poynting vector calculated?

In SI units, the instantaneous Poynting vector is S = E × H. Its units are watts per square metre, so its magnitude represents electromagnetic power crossing a unit area.

How is the Poynting vector related to light intensity?

For a periodic electromagnetic wave, the time-averaged Poynting vector gives the average intensity. This is why laser irradiance and radio-wave power density can both be expressed in watts per square metre.

Can the Poynting vector show energy flow in a circuit?

Yes. Around a powered wire or transmission line, the surrounding electric and magnetic fields produce a Poynting vector that carries energy toward the load, rather than describing energy as moving only inside the conductor.

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