# Coulomb’s Law Lab – Simulation

This summer, by taking PHS 531 – Modeling Instruction in HS Physics – Electricity & Magnetism with Michael Crofton, I’ve had a great opportunity to look in-depth at the curriculum materials available on the ASU modeling website (soon to be on the AMTA website). Like all of the rest of the modeling curriculum I’ve been exposed to, it’s very well thought-out and organized around a set of models that students can use to solve problems.

In E&M Unit 1, one of the primary mathematical models developed is Coulomb’s Law. Due to the fact that forces on a charged object can be tough to quantify with much accuracy, the experimental setup is demonstrated and a video was taken. Students are to then analyze the video (the relative positions of two charged objects) to determine how the distance between the charged particles affects the force between them.

This method works fine. The video analysis gets a little tricky, because there are two variables to worry about — distance between the particles, as well as the amount of deflection of the hanging particle. I see a few complications that could come up:

• The idea that a distance represents force might be a little muddy for my students. If the relationship being focused on is not obvious, sometimes they can’t “see the forest for the trees” when little details get in the way.
• When we use distance as a measure of force, we can figure out what type of relationship exists, but the constant of proportionality (slope) is meaningless.
• Poor accuracy in selecting points on the video (which is a little hard to see) can cause data to become unreliable, leading to an incorrect relationship. UPDATE: There is a clearer video available that would address most of this concern.

I wondered if, since the data was coming from the computer anyway, if a simulation might be just as useful for students to get this relationship. So, I spent a couple hours putting this together.

Right now, I decided to use a ‘spring’ to measure the force on the charged particle. Students can use Hooke’s law to directly calculate the force on the particle. Then they can see how r relates to the force on the particle by analyzing data with LoggerPro or excel. As a bonus, the simulation allows you to change the charge on either particle to see how that affects it. If you have questions or suggestions, let me know.

UPDATE:  I created a second version which just adds force vectors to charged particles.  No need to even worry about adding the spring relationships in–the data comes out raw and ready for graphing.