EM Energy Density

This formula tells you how much electromagnetic energy is packed into each cubic metre of space when electric and magnetic fields are present. The two terms come from the field energy stored in the electric field, which grows with E^2, and the magnetic field, which grows with B^2; the constants b50 and bc0 set the scale so the units come out as joules per cubic metre. It is a field-theory result: rather than thinking of energy as belonging to charges or currents alone, Maxwell's theory assigns energy directly to the fields themselves.

Use this when you need the energy density of an electromagnetic wave, the field energy in a capacitor or in free space, or when converting between field strength and stored energy. It is especially useful in advanced electromagnetism, optics, and radiation problems.

• Forgetting that the electric term uses E squared, not E itself
• Using the wrong permeability or permittivity constant, or mixing up b50 and bc0
• Treating the formula as valid only for waves, when it also applies to static field energy densities