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Poynting vector
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Energy flux of an EM wave.
Key relation
S = E×B/μ₀S = E × B/μ₀ gives the energy flow per unit area per unit time (W/m²), in the direction of propagation.
Worked example
Flows into a resistor from the surrounding fields.
Common mistake
Time-average of S gives intensity.
In more detail
The Poynting vector S = (1/μ0) E × B gives the power per unit area carried by an electromagnetic field. It points in the direction of energy flow.
Real-world uses
- Engineers calculate energy flow in antennas and transmission lines using the Poynting vector.
- Solar intensity is computed from Poynting flux at Earth's surface.
- Physicists use the Poynting vector to calculate the flow of energy through a region of space in a radio beam.
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Frequently asked questions
What units does S have?
Watts per square metre.
Does S always point in the direction of propagation?
Yes, for plane waves, it points along the wave direction.
Which direction does the Poynting vector point?
Along the direction the wave travels, which is the direction of energy flow.
Related topics in AC Circuits & EM Waves
EM wave equation · Relation between E and B · Energy of EM waves · Intensity of EM wave · Radiation pressure · EM spectrum