User define interaction force

Hello

Now I am using charged particle tracing module and ,as you know, the defult interaction among particle set by comsol is Coulomb force.

I want to change Coulomb interaction to other types of interaction, for example, the force inversely proporitonal to particle distance.

I know that I can put the desired expression in "user define force", however, I have no idea to write it down. That is, I dont know the syntax. Whenever I tried, it appeared that the syntax was wrong but I couldn't find any information regarding syntax.

I really look forwad to your helping me. Thanks.