where Π Π = 1 2 Re ⁡ ⁡ [ E ∗ ∗ × × H ] is the time averaged Poynting vector. If W a > 0 energy is absorbed within the sphere, otherwise energy is being created within the sphere. We will not consider this case here. If the host medium is non-absorbing, the energy must be absorbed by the particle. We decompose the total field into incident and scattered parts E = E i + E s , and the same for the magnetic field H . Thus, we can decompose W a into the three terms W a = W i − − W s + W ext , where