Desmos Activity: Electric Field Due to Point Charges

Two of my main goals when teaching students about electric fields are getting them to understand superposition and vector addition.  I usually do this by having them sketch several examples, but it is difficult for me to provide good feedback.  So I decided to create my own Desmos activity: Electric Field Due to Point Charges.  The basic idea is that there is a green region, and you have to maneuver charges until the electric field vector falls entirely within that region.  (By the way, Desmos is quickly becoming one of my favorite teaching tools.  The activity builder allows you to create scaffolded activities that are still interactive.)

Screen Shot 2016-01-19 at 11.36.32 AM

Screenshot from Desmos activity: Electric Field Due to Point Charges

I tried to use design elements to make the activity feel as natural as possible.  For example, positive charges are red and negative charges are blue.  Each type of charge produces an electric field of its corresponding color, but the combination of a red charge and a blue charge will produce a purple electric field.  The process of design is still pretty new to me, though, so if you have any suggestions please let me know in the comments.