Goal: Reason regarding electric fields
Source: 283-10, E at origin due to charged rods
All of the configurations shown below consist of charged rods of the
same length L. The magnitude of the total charge is also the same for
each rod. The total charge in each rod is distributed uniformly.
For which
configuration(s) is the electric field vector at the origin in the
positive x direction?
- 2 only
- 1 and 3 only
- 5 only
- 4 only
- 1 and 5 only
- None of the above
Commentary:
Answer
(4) Because they are given lots of examples involving point charges,
spheres and rings, students often miss the fact that there are many
situations for which the direction of the field can be deduced even
though determining the value or formal expression for the field is way
beyond them.