Hey everyone, So I've been learning about electric fields and all that good stuff. So now the question arises, what is the electric field at a point P away from an object (say a rod or a ring) at a distance D away? So we know E=Kq/r2 or E=KQ/r2 . The difference between q and Q: q the charge for a small particle while Q is the charge of a continuous object, say a rod, ring, disk, etc. Now, when we're asked the questions aforementioned sometimes our book uses dE=kdq/r2 and sometimes it uses dE=kdQ/r2 where d signifies "change in". My question is, why do sometimes they use dq and sometimes dQ? How would I know which to use? My view on it is, say a disk has a charge Q (charge of the whole disk), and that disk is changing with respect to a change in r which still gives u a circle which means that circle still has a charge of Q since it's a continious object. On a rod I would say you use dq because you can take a piece of the rod to be infinitesimally small and make a point (since you consider a rod to be "linear"). Am I correct or am I viewing things a wrong way?