Electromagnetic momentum density

The momentum density stored by the electric and magnetic fields is given by

⃗ 𝐠 = 𝜖 0 𝜇 0 ⃗ 𝐒 = 𝜖 0 ⃗ 𝐄 \times ⃗ 𝐁

where $⃗ 𝐒$ is the Poynting vector. This quantity is required to state conservation of momentum in electrodynamics via the electromagnetic momentum equation.