Force between Two Current-Carrying Wires

[Peter G.]

Hi,

I always had difficulty with understanding how to determine whether the force between two current-carrying wires is attractive or repulsive. I know that if the currents are traveling in the same direction the force is attractive and repulsive if the currents are traveling in opposite directions, but, why?

Can anyone maybe direct me to a website that explains this well or just provide an explanation?

Thanks in advance!

 

[TSny]