In the presence of electromagnetic field, an electrically charged object receives Lorentz force. This is a Newtonian picture; the Lorentz force gives acceleration for the object. However, as the Lagrangian and the Hamiltonian formulation are equivalent to the Newtonian one, we should have the Lagrangian and the Hamiltonian for the object from which we can derive the Lorentz force. This is the object of the second article. Also, it serves as the prerequisite to “What is a gauge theory?” even though it is not absolutely essential if one takes granted the key equations in the above article. On the other hand, the main purpose of the Poisson’s equation article is to serve prerequisites to understand how Einstein’s theory of general relativity can be reduced to Newtonian gravity as explained in “A relatively short introduction to general relativity.” “Gauss’s law expressed using divergence” is a prerequisite to the Poisson’s equation article. “Energy density of electromagnetic field” shows that electromagnetic fields carries energy and calculates their value.