The simulation shows the electric field near two charged particles.
If the particle is drawn in red, it has a positive charge.
If the particle is drawn in blue, it has a negative charge.
You should be able to click-and-drag the particles to move them around the screen, and use the sliders to change the charges.
Simulation first posted on 8-3-2015. Written by Andrew Duffy
Last revised 3-2-2016 by Andrew Duffy
This work by Andrew Duffy is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.
This simulation can be found in the collection at http://physics.bu.edu/~duffy/classroom.html.