Electric Field Lines

Related Physics Education Research

The article Confusion by representation: On student's comprehension of the electric field concept by S. Tornkvist, K.-A. Pettersson and G. Transtromer (AJP 61 (4), April 1993) illustrates students difficulties with the electric field line representation. In the first part they measure students' abilities to recognize errors in field lines (closed loops, kinks, and crossings) and explain them. In the second part they give the results of interviews which focus on students understanding of the links between field lines, force and motion.

Overview of Worksheet

The goals of the worksheet are threefold:

Given a set of charges and corresponding electric field lines, students can identify incorrectly drawn field lines and explain why the lines are incorrect.

Given a small set of charges, student can qualitatively sketch correct field lines.

Given a set of field lines, a charged test particle and its initial velocity, students can qualitatively sketch the path of the particle.

Approximate Time

This is two hours at a minimum, though it can be broken into smaller pieces if necessary.

Required Student Background

The students need to be familiar with the Coulomb force and the definition of the electric field as the force divided by the test charge.

Connections to the Tutorials

Students will need to recall their work on "Motion in 2-dimensions" to be able to infer the motion of a particle given the electric field.

Required Equipment

This worksheet uses two computer simulations available from Physics Academic Software: "EMfield" by Trowbridge and Sherwood and "Electric Field Hockey" by Chabay. Colored pencils and large sheets of white paper are also useful.

Evaluation of Worksheet

This worksheet has been used with four different classes; we listened to student difficulties as they ran through the worksheet and modified it accordingly.

Summer 1999: Initially 17/37 students believed that electric field lines that were closed circles were incorrect, but none could give the correct reason. On the final, 31/37 noted that they were incorrect and could give the correct reasoning. This compares favorably with approximately 50% of the students in the Tornkvist study who could give the correct reasoning on a final exam. Also on the final, 18/37 could give the correct reasoning for the unphysicality of kinked field lines; for the Tornkvist study less than 10% of the students got the correct answer.