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Relation between E and B
Intermediate
Magnitudes related by c.
E = cBIn a plane EM wave E₀ = cB₀ and the fields are in phase. B is tiny: E = 300 V/m gives B = 1 μT.
Sunlight has E₀ ≈ 1000 V/m.
Both fields carry equal energy.
In more detail
In an electromagnetic plane wave in vacuum, the magnitudes of the electric and magnetic fields are related by E = cB, and they are in phase. Their directions are perpendicular to each other and to the propagation direction.
Real-world uses
- Engineers use E = cB to convert between field measures in radio and optics.
- Light detectors respond to electric fields, which are linked to magnetic fields.
- Engineers calculating the strength of a radio signal use the relation E equals c times B to find the magnetic field from the electric field.
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Frequently asked questions
What is the direction of propagation?
Given by the cross product of E and B.
Why is the ratio c?
Because the field equations of Maxwell impose this relation for travelling waves.
Are E and B always equal in size?
Not in units where they differ, but in SI units their magnitudes in a plane wave are related by a factor of c.
Related topics in AC Circuits & EM Waves
EM wave properties · Speed of light · EM wave equation · Energy of EM waves · Poynting vector · Intensity of EM wave