OpenStax College Physics for AP® Courses, Chapter 22, Problem 15 (Test Prep for AP® Courses)

This is College Physics Answers with Shaun Dychko. I've drawn an end view of both these parallel wires that are beside each other. We have current going into the page the way I've drawn it. So if you want to figure out the magnetic field created by the wire A let's say, and here's wire B, the magnetic field created by wire A you determine by putting your thumb in the direction of the current which is into the page. Then your fingers will curl in the direction of the magnetic field. So it's going to create a field that is circular directed downwards at the position of wire B, and that goes around like this. So the next question is what force will this magnetic field exert on the current-carrying wire B? To figure that out, we put our thumb in the direction of the current of wire B which is into the page. Our fingers point downwards in the direction of the magnetic field at wire B. Your palm is pointing in the direction of the force which will be to the left. So that's going to be a force towards wire A and that's an attractive force. You could do the same analysis by figuring out the magnetic field due to wire B and find the force on wire A and you'll discover that it is to the right. So the force is attractive. The answer is C.