Peter
Project Title
A Common Misconception
regarding Field Lines
Author
IP file and Exploration
by Peter Rapp '99

Issue:
    This project started as an exploration to see if we could use the computer to draw field lines. After several unsuccessful attempts with Microsoft Excel, I decided to employ Interactive Physics. Even though it couldn’t draw the exact field lines, I learned that I had accidentally gone along with one of the big misconceptions about field lines.


Solution: 

     One of the problems with both the IP and Excel simulations was that the positive test particles would immediately fly off of the screen once the simulation started. Looking at the formula, this makes sense.

Vo=45m/s; s=?m; t=6.6sec; a=-10m/s^2

The two equations are the same. Don't be confused by the different notations with k and epsilon zero, just use whatever you feel more comfortable with.

     When r is very small, the resulting electrostatic force from that charge is tremendous, rendering the force from the other charge negligible. We circumvented this issue by placing the test charges on an evenly spaced grid with interactive physics. By turning on the trace feature, we can see the movement of these charges over time. We noticed one important error with this method. Depending on the mass of the test particles, field lines were distorted to various degrees.

     Contrary to popular belief, field lines do not show the movement of a small test particle, but instead show the instantaneous direction of the net force on the test particle. In actuality, the particle will move in a path outside of the real field lines. Think momentum. Once the particle gets going, the net force will only try to change the direction, but the more momentum it builds up, the harder it is to change that direction.

     Possible solution: Make a tight grid of test charges with large masses. The vectors show net force: not quite field lines, but they do show the field. Here is what I came up with. The snapshot above can be recreated by downloading the "Vectors.IP" file below.


Resources Window
Interactive Physics is a powerful simulation program that is available from MSC Working Knowledge. IP documents are playable with Interactive Physics v1.2 or later. Earlier versions may be compatable.
Text versions of the equation work are made with Microsoft Equation Editor and are embedded in Microsoft Word documents.
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