Dear sir/lady I have a question about the magnrtic force of steady curent . In Biot - Savart law to evalute of B (magnetic Field ) , below the Integral we have to do a cross product Idl'*(r-r')/|r-r'|^3 that r and r' are the vector position of the field and source . How we can evaluate this Integral if the vector r-r' is zero . ? for example if we have a U shape Incomplete circuit and put a metal bar as the fourth side to complete it , when a stationary current I circualtes in square , a force will exert on the bar . if we would like to evaluate the force exerted by three sides on the fourth one , we will encounter this problem for the toppest and the lowest point of fourth one , on corners .Because vector r-r' =0 and B approches to infinity . Please help me .