In electrodynamics,the density of energy and momentum of electromagnetic field is expressed by the intensity of electronic field and magnetic field. In electrodynamics, if we have know the distribution of electronic charge and electronic current,we can derive the electronic scalar potential and magnatic vector potential from solving the Laplace Equation(but we generally can't derive the intensity of electronic field and magnetic field directly,and must through potential,because Maxwell's Equations are vector equation,can't be solved directly ), and derive the intensity of electronic field and magnetic field, by calculating the gradient of the electronic scalar potential and the rotation of magnatic vector potential . So we can see that the potential is seemly most essential,so we should firstly express the energ of electromagnetic field with the two potentials. So the situation will be similar to the Lagrangian dynamics,which is only the equation of energy,and has a clear and high opinion of dynamics of material system. Perhaps someone can derive the Maxwell's Equations from the equation of energy. And it gives us a similar picture with electromagnetic field equation , Schrodinger Equation and the field equation in General Relativity. So it is a very deep way of calculating and thinking.It can enlighten us to think the identity of the nature.