Electric Charges 05 - Practice


Yesterday we kinda struggled a bit with efield lines.... however, we quickly realized that a real-world particle traveling through an efield would not strictly follow the efield lines. For example the real world particle travelling in this animation is slowly deflected into a path similar to the efield lines but it does not directly follow them.

Here's an interesting thing to chew on.... what type of particle WOULD follow the classic efield lines shown in THIS animation

What is different between those two scenarios...which is to say, why won't a real world particle follow electric field lines?


Oh hey, while we're at it... what are the UNITS for an E-field?

OBJECTIVE:  Practice Practice Practice




  • Test Charge: A mathematical construct-- a charge that does not exert any influence on surrounding particles but IS influenced by other electric fields.
  • Conductors - materials where electrons can roam
  • Insulators - materials that keep their electrons close to home
  • coulomb - a unit of electrical charge (see below)


  • e-mass = 9.1 x 10-31 kg
  • charge of an electron = 1.60 x 10-19 coulombs (C)
  • ke = 8.987 x 109 Nm2/C2


      1. Electrical Force: Fe = (keq1q2)/r2
      • Electrical Field: E = Fe / qo where qo  = a positive test charge
      1. #1 and #2 (above) work well together to tell us about the strength of the electrical field at some distance (r) away from a point source:

        E = (keq)/r2

        Please keep in mind that E is a vector and that we will frequently need to componentize 'r' in terms of unit vectors i and j


Let's get someone up to dissect the long, thin rod problem....


Now let's take a few moments to quiz each other on efields. Using one of our animations, sketch out an efield. Now show that animation to your partner and have them describe the relative ratio of the charges and the charge types of each.

"Rubrics stifle creativity"Seth D.


Problems 5, 11, 21, 49 et al? (please choose one other problem that illustrates a skill or concept you think is helpful for your team to review.

Do that problem and then <somehow> share the results with your team sometime today. Oh and please keep the time-honored science concept of GOLDILOCKS front and center.

That is to say that the problem you select shouldn't be so easy it's irrelevant, or so hard that it is obscure... we want to benefit the entire team on this.