(*) Impulse is defined as the change in this quantity with respect to time, which is conserved in inelastic collisions.   
By Poynting's theorem, the density of this value can be calculated for traveling electromagnetic waves to be epsilon nought E cross B.  For a charged particle, its canonical form includes a term proportional to the vector potential.  In quantum mechanics its operator is given by minus i times h-bar del, and by Noether's theorem the conservation of this quantity follows from translation invariance of space. (*) Impulse is defined as the change in this quantity with respect to time, which is conserved in inelastic collisions.   For 10 points, name this quantity, sometimes symbolized by p, which can be expressed as mass times velocity.
By Poynting's theorem, the density of this value can be calculated for traveling electromagnetic waves to be epsilon nought E cross B.  
Linear {Momentum} [prompt on {p} before mentioned]