# E field calculations for continuous charge distributions

1. Oct 25, 2015

### kiwibird4

so I was reviewing my textbook on calculating electric field when we can assume a continuous charge distribution and they said three useful tools are
(1) making use of symmetry
(2) expressing the charge dq in terms of charge density lambda
(3) and checking the answer at the limit of large r which serves as an indication of the correctness of the answer -- if result does not check at large r, your result has to be wrong (giancoli textbook).

Anyway, I understand the usefulness of 1 and 2 but do not fully understand what number 3 is talking about or how to check the answer in that way? Can anyone explain further what they are referring to or maybe give an example

2. Oct 25, 2015

### axmls

For example, if you're calculating the electric field around a ring or disc or sphere or something, then you should expect that for $r$ that are very far away, since the object will look essentially like a point, the electric field equation should look like coulomb's law for a point charge.