1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

I Integration to find Electric Field at a point P -- dQ or dq?

  1. Feb 24, 2016 #1
    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?
     
  2. jcsd
  3. Feb 24, 2016 #2
    Hi.

    A couple of things-

    1. The use of dQ vs dq typically depends on the charge given to a particular body. Say, for example, you give a charge Q to a ring. The use of dQ is simply, because you are considering a very very small portion of charge out of the total Q. For a total charge q, you may use dq. This is no hard-and-fast rule, however.

    2. When you're saying dQ(or dq) you are actually referring to an infinitesimally small part, rather than a 'change in'.

    3. Electric fields of continuous distributions are not typically in the format of kQ/(r∧2) or q(I hope I've cleared that).
    They are usually found by integrating, or by the use of symmetry arguments.

    Hope this helps,
    Qwertywerty.
     
  4. Mar 10, 2016 #3
    Hi,
    Q and q are can be used interchangably. You need to consider what kind of system you are dealing with. There is no such thing q is used for a continuous system and Q is used for other kind of systems (discrete etc.). So if you know what is the difference between q (or Q) and dq (or dQ) the rest is as a matter of convention. I hope it is clear.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted