Two parallel wires with current running through them exert a force on each other. Each one creates a magnetic field in which the other runs. By Fleming's Left Hand Rule, that means a force. By Newton's Third Law they are equal and opposite forces. It is hard to show bu long parallel foil strips just about do the trick. Here current goes in at the top, down one side and back up the other side. The foil strips repel. The theory is here:
http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/wirfor.html
