A2L Item 213
- Description: Indicate the electric field due to a uniformly charged rod at a point along its axis some distance from its end.
- Goal: Reasoning regarding electric fields due to distributed charges
- Source: 283-395 Electric field from a rod, on its axis.
- Keywords: Distributed Charges, Electromagnetism, Electric Field
The question for students:
A rod of length L and charge +q
(uniformly distributed) is positioned along the x-axis, as shown to the
right. What is E at point P, a distance a from the origin?
1.  |
2.  |
3.  |
4.  |
5.  |
Commentary for teachers:
Answer
(3) It is worthwhile having students examine their choice for the limiting case a->0. Students are inclined to immediately start a formal calculation rather than think about the problem long enough to figure out what they really need to know. In this case all but two of the answers can be ruled out because they do not limit appropriately as the point P moves toward the origin. If a»L the field should drop off as from a point charge. The only answer meeting both these requirements is