Algodoo

this is a long chain of pendulums, constrained to each other via spring, that can emulate the way charged particles, like electrons and positrons, interact. There are 4 controls for adding a particle, which rotate one of either end of the chain, and once the end has made one full rotation, a particle is emitted and propagetes down the chain. "loops" and "lumps" are attracted to one another, but repel their own kind. In some rare cases, a loop and a lump may pair together and slowly decay away as an oscillation in the chain, slowly releasing energy, akin to electromagnetic radiation, out into space.

A to add a lump to left end
D to add a loop to left end
left to add a loop to right end
right to add a lump to right end